thodg/libgit2/docs/changelog.md

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  Date : 2019-12-13T08:53:51
  

  Merge pull request #5320 from josharian/minor-docs Minor doc improvements

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• docs/changelog.md

• v0.28 + 1
  ---------
  
  # Security Fixes
  
  - CVE-2019-1348: the fast-import stream command "feature
    export-marks=path" allows writing to arbitrary file paths. As
    libgit2 does not offer any interface for fast-import, it is not
    susceptible to this vulnerability.
  
  - CVE-2019-1349: by using NTFS 8.3 short names, backslashes or
    alternate filesystreams, it is possible to cause submodules to
    be written into pre-existing directories during a recursive
    clone using git. As libgit2 rejects cloning into non-empty
    directories by default, it is not susceptible to this
    vulnerability.
  
  - CVE-2019-1350: recursive clones may lead to arbitrary remote
    code executing due to improper quoting of command line
    arguments. As libgit2 uses libssh2, which does not require us
    to perform command line parsing, it is not susceptible to this
    vulnerability.
  
  - CVE-2019-1351: Windows provides the ability to substitute
    drive letters with arbitrary letters, including multi-byte
    Unicode letters. To fix any potential issues arising from
    interpreting such paths as relative paths, we have extended
    detection of DOS drive prefixes to accomodate for such cases.
  
  - CVE-2019-1352: by using NTFS-style alternative file streams for
    the ".git" directory, it is possible to overwrite parts of the
    repository. While this has been fixed in the past for Windows,
    the same vulnerability may also exist on other systems that
    write to NTFS filesystems. We now reject any paths starting
    with ".git:" on all systems.
  
  - CVE-2019-1353: by using NTFS-style 8.3 short names, it was
    possible to write to the ".git" directory and thus overwrite
    parts of the repository, leading to possible remote code
    execution. While this problem was already fixed in the past for
    Windows, other systems accessing NTFS filesystems are
    vulnerable to this issue too. We now enable NTFS protecions by
    default on all systems to fix this attack vector.
  
  - CVE-2019-1354: on Windows, backslashes are not a valid part of
    a filename but are instead interpreted as directory separators.
    As other platforms allowed to use such paths, it was possible
    to write such invalid entries into a Git repository and was
    thus an attack vector to write into the ".git" dierctory. We
    now reject any entries starting with ".git\" on all systems.
  
  - CVE-2019-1387: it is possible to let a submodule's git
    directory point into a sibling's submodule directory, which may
    result in overwriting parts of the Git repository and thus lead
    to arbitrary command execution. As libgit2 doesn't provide any
    way to do submodule clones natively, it is not susceptible to
    this vulnerability. Users of libgit2 that have implemented
    recursive submodule clones manually are encouraged to review
    their implementation for this vulnerability.
  
  ### Breaking API changes
  
  * The "private" implementation details of the `git_cred` structure have been
    moved to a dedicated `git2/sys/cred.h` header, to clarify that the underlying
    structures are only provided for custom transport implementers.
    The breaking change is that the `username` member of the underlying struct
    is now hidden, and a new `git_cred_get_username` function has been provided.
  
  ### Breaking CMake configuration changes
  
  * The CMake option to use a system http-parser library, instead of the
    bundled dependency, has changed.  This is due to a deficiency in
    http-parser that we have fixed in our implementation.  The bundled
    library is now the default, but if you wish to force the use of the
    system http-parser implementation despite incompatibilities, you can
    specify `-DUSE_HTTP_PARSER=system` to CMake.
  
  * The interactions between `USE_HTTPS` and `SHA1_BACKEND` have been
    streamlined. The detection was moved to a new `USE_SHA1`, modeled after
    `USE_HTTPS`, which takes the values "CollisionDetection/Backend/Generic", to
    better match how the "hashing backend" is selected, the default (ON) being
    "CollisionDetection". If you were using `SHA1_BACKEND` previously, you'll
    need to check the value you've used, or switch to the autodetection.
  
  ### Changes or improvements
  
  * libgit2 now correctly handles more URLs, such as
    `http://example.com:/repo.git` (colon but no port),
    `http://example.com` (no path),
    and `http://example.com:8080/` (path is /, nonstandard port).
  
  * A carefully constructed commit object with a very large number
    of parents may lead to potential out-of-bounds writes or
    potential denial of service.
  
  * The ProgramData configuration file is always read for compatibility
    with Git for Windows and Portable Git installations.  The ProgramData
    location is not necessarily writable only by administrators, so we
    now ensure that the configuration file is owned by the administrator
    or the current user.
  
  v0.28
  -----
  
  ### Changes or improvements
  
  * The library is now always built with cdecl calling conventions on
    Windows; the ability to build a stdcall library has been removed.
  
  * Reference log creation now honors `core.logallrefupdates=always`.
  
  * Fix some issues with the error-reporting in the OpenSSL backend.
  
  * HTTP proxy support is now builtin; libcurl is no longer used to support
    proxies and is removed as a dependency.
  
  * Certificate and credential callbacks can now return `GIT_PASSTHROUGH`
    to decline to act; libgit2 will behave as if there was no callback set
    in the first place.
  
  * The line-ending filtering logic - when checking out files - has been
    updated to match newer git (>= git 2.9) for proper interoperability.
  
  * Symbolic links are now supported on Windows when `core.symlinks` is set
    to `true`.
  
  * Submodules with names which attempt to perform path traversal now have their
    configuration ignored. Such names were blindly appended to the
    `$GIT_DIR/modules` and a malicious name could lead to an attacker writing to
    an arbitrary location. This matches git's handling of CVE-2018-11235.
  
  * Object validation is now performed during tree creation in the
    `git_index_write_tree_to` API.
  
  * Configuration variable may now be specified on the same line as a section
    header; previously this was erroneously a parser error.
  
  * When an HTTP server supports both NTLM and Negotiate authentication
    mechanisms, we would previously fail to authenticate with any mechanism.
  
  * The `GIT_OPT_SET_PACK_MAX_OBJECTS` option can now set the maximum
    number of objects allowed in a packfile being downloaded; this can help
    limit the maximum memory used when fetching from an untrusted remote.
  
  * Line numbers in diffs loaded from patch files were not being populated;
    they are now included in the results.
  
  * The repository's index is reloaded from disk at the beginning of
    `git_merge` operations to ensure that it is up-to-date.
  
  * Mailmap handling APIs have been introduced, and the new commit APIs
    `git_commit_committer_with_mailmap` and `git_commit_author_with_mailmap`
    will use the mailmap to resolve the committer and author information.
    In addition, blame will use the mailmap given when the
    `GIT_BLAME_USE_MAILMAP` option.
  
  * Ignore handling for files in ignored folders would be ignored.
  
  * Worktrees can now be backed by bare repositories.
  
  * Trailing spaces are supported in `.gitignore` files, these spaces were
    previously (and erroneously) treated as part of the pattern.
  
  * The library can now be built with mbedTLS support for HTTPS.
  
  * The diff status character 'T' will now be presented by the
    `git_diff_status_char` API for diff entries that change type.
  
  * Revision walks previously would sometimes include commits that should
    have been ignored; this is corrected.
  
  * Revision walks are now more efficient when the output is unsorted;
    we now avoid walking all the way to the beginning of history unnecessarily.
  
  * Error-handling around index extension loading has been fixed. We were
    previously always misreporting a truncated index (#4858).
  
  ### API additions
  
  * The index may now be iterated atomically using `git_index_iterator`.
  
  * Remote objects can now be created with extended options using the
    `git_remote_create_with_opts` API.
  
  * Diff objects can now be applied as changes to the working directory,
    index or both, emulating the `git apply` command.  Additionally,
    `git_apply_to_tree` can apply those changes to a tree object as a
    fully in-memory operation.
  
  * You can now swap out memory allocators via the
    `GIT_OPT_SET_ALLOCATOR` option with `git_libgit2_opts()`.
  
  * You can now ensure that functions do not discard unwritten changes to the
    index via the `GIT_OPT_ENABLE_UNSAVED_INDEX_SAFETY` option to
    `git_libgit2_opts()`.  This will cause functions that implicitly re-read
    the index (eg, `git_checkout`) to fail if you have staged changes to the
    index but you have not written the index to disk.  (Unless the checkout
    has the FORCE flag specified.)
  
    At present, this defaults to off, but we intend to enable this more
    broadly in the future, as a warning or error.  We encourage you to
    examine your code to ensure that you are not relying on the current
    behavior that implicitly removes staged changes.
  
  * Reference specifications can be parsed from an arbitrary string with
    the `git_refspec_parse` API.
  
  * You can now get the name and path of worktrees using the
    `git_worktree_name` and `git_worktree_path` APIs, respectively.
  
  * The `ref` field has been added to `git_worktree_add_options` to enable
    the creation of a worktree from a pre-existing branch.
  
  * It's now possible to analyze merge relationships between any two
    references, not just against `HEAD`, using `git_merge_analysis_for_ref`.
  
  ### API removals
  
  * The `git_buf_free` API is deprecated; it has been renamed to
    `git_buf_dispose` for consistency.  The `git_buf_free` API will be
    retained for backward compatibility for the foreseeable future.
  
  * The `git_otype` enumeration and its members are deprecated and have
    been renamed for consistency.  The `GIT_OBJ_` enumeration values are
    now prefixed with `GIT_OBJECT_`.  The old enumerations and macros
    will be retained for backward compatibility for the foreseeable future.
  
  * Several index-related APIs have been renamed for consistency.  The
    `GIT_IDXENTRY_` enumeration values and macros have been renamed to
    be prefixed with `GIT_INDEX_ENTRY_`.  The `GIT_INDEXCAP` enumeration
    values are now prefixed with `GIT_INDEX_CAPABILITY_`.  The old
    enumerations and macros will be retained for backward compatibility
    for the foreseeable future.
  
  * The error functions and enumeration values have been renamed for
    consistency.  The `giterr_` functions and values prefix have been
    renamed to be prefixed with `git_error_`; similarly, the `GITERR_`
    constants have been renamed to be prefixed with `GIT_ERROR_`.
    The old enumerations and macros will be retained for backward
    compatibility for the foreseeable future.
  
  ### Breaking API changes
  
  * The default checkout strategy changed from `DRY_RUN` to `SAFE` (#4531).
  
  * Adding a symlink as .gitmodules into the index from the workdir or checking
    out such files is not allowed as this can make a Git implementation write
    outside of the repository and bypass the fsck checks for CVE-2018-11235.
  
  v0.27
  ---------
  
  ### Changes or improvements
  
  * Improved `p_unlink` in `posix_w32.c` to try and make a file writable
    before sleeping in the retry loop to prevent unnecessary calls to sleep.
  
  * The CMake build infrastructure has been improved to speed up building time.
  
  * A new CMake option "-DUSE_HTTPS=<backend>" makes it possible to explicitly
    choose an HTTP backend.
  
  * A new CMake option "-DSHA1_BACKEND=<backend>" makes it possible to explicitly
    choose an SHA1 backend. The collision-detecting backend is now the default.
  
  * A new CMake option "-DUSE_BUNDLED_ZLIB" makes it possible to explicitly use
    the bundled zlib library.
  
  * A new CMake option "-DENABLE_REPRODUCIBLE_BUILDS" makes it possible to
    generate a reproducible static archive. This requires support from your
    toolchain.
  
  * The minimum required CMake version has been bumped to 2.8.11.
  
  * Writing to a configuration file now preserves the case of the key given by the
    caller for the case-insensitive portions of the key (existing sections are
    used even if they don't match).
  
  * We now support conditional includes in configuration files.
  
  * Fix for handling re-reading of configuration files with includes.
  
  * Fix for reading patches which contain exact renames only.
  
  * Fix for reading patches with whitespace in the compared files' paths.
  
  * We will now fill `FETCH_HEAD` from all passed refspecs instead of overwriting
    with the last one.
  
  * There is a new diff option, `GIT_DIFF_INDENT_HEURISTIC` which activates a
    heuristic which takes into account whitespace and indentation in order to
    produce better diffs when dealing with ambiguous diff hunks.
  
  * Fix for pattern-based ignore rules where files ignored by a rule cannot be
    un-ignored by another rule.
  
  * Sockets opened by libgit2 are now being closed on exec(3) if the platform
    supports it.
  
  * Fix for peeling annotated tags from packed-refs files.
  
  * Fix reading huge loose objects from the object database.
  
  * Fix files not being treated as modified when only the file mode has changed.
  
  * We now explicitly reject adding submodules to the index via
    `git_index_add_frombuffer`.
  
  * Fix handling of `GIT_DIFF_FIND_RENAMES_FROM_REWRITES` raising `SIGABRT` when
    one file has been deleted and another file has been rewritten.
  
  * Fix for WinHTTP not properly handling NTLM and Negotiate challenges.
  
  * When using SSH-based transports, we now repeatedly ask for the passphrase to
    decrypt the private key in case a wrong passphrase is being provided.
  
  * When generating conflict markers, they will now use the same line endings as
    the rest of the file.
  
  ### API additions
  
  * The `git_merge_file_options` structure now contains a new setting,
    `marker_size`.  This allows users to set the size of markers that
    delineate the sides of merged files in the output conflict file.
    By default this is 7 (`GIT_MERGE_CONFLICT_MARKER_SIZE`), which
    produces output markers like `<<<<<<<` and `>>>>>>>`.
  
  * `git_remote_create_detached()` creates a remote that is not associated
    to any repository (and does not apply configuration like 'insteadof' rules).
    This is mostly useful for e.g. emulating `git ls-remote` behavior.
  
  * `git_diff_patchid()` lets you generate patch IDs for diffs.
  
  * `git_status_options` now has an additional field `baseline` to allow creating
    status lists against different trees.
  
  * New family of functions to allow creating notes for a specific notes commit
    instead of for a notes reference.
  
  * New family of functions to allow parsing message trailers. This API is still
    experimental and may change in future releases.
  
  ### API removals
  
  ### Breaking API changes
  
  * Signatures now distinguish between +0000 and -0000 UTC offsets.
  
  * The certificate check callback in the WinHTTP transport will now receive the
    `message_cb_payload` instead of the `cred_acquire_payload`.
  
  * We are now reading symlinked directories under .git/refs.
  
  * We now refuse creating branches named "HEAD".
  
  * We now refuse reading and writing all-zero object IDs into the
    object database.
  
  * We now read the effective user's configuration file instead of the real user's
    configuration in case libgit2 runs as part of a setuid binary.
  
  * The `git_odb_open_rstream` function and its `readstream` callback in the
    `git_odb_backend` interface have changed their signatures to allow providing
    the object's size and type to the caller.
  
  v0.26
  -----
  
  ### Changes or improvements
  
  * Support for opening, creating and modifying worktrees.
  
  * We can now detect SHA1 collisions resulting from the SHAttered attack. These
    checks can be enabled at build time via `-DUSE_SHA1DC`.
  
  * Fix for missing implementation of `git_merge_driver_source` getters.
  
  * Fix for installed pkg-config file being broken when the prefix contains
    spaces.
  
  * We now detect when the hashsum of on-disk objects does not match their
    expected hashsum.
  
  * We now support open-ended ranges (e.g. "master..", "...master") in our
    revision range parsing code.
  
  * We now correctly compute ignores with leading "/" in subdirectories.
  
  * We now optionally call `fsync` on loose objects, packfiles and their indexes,
    loose references and packed reference files.
  
  * We can now build against OpenSSL v1.1 and against LibreSSL.
  
  * `GIT_MERGE_OPTIONS_INIT` now includes a setting to perform rename detection.
    This aligns this structure with the default by `git_merge` and
    `git_merge_trees` when `NULL` was provided for the options.
  
  * Improvements for reading index v4 files.
  
  * Perform additional retries for filesystem operations on Windows when files
    are temporarily locked by other processes.
  
  ### API additions
  
  * New family of functions to handle worktrees:
  
      * `git_worktree_list()` lets you look up worktrees for a repository.
      * `git_worktree_lookup()` lets you get a specific worktree.
      * `git_worktree_open_from_repository()` lets you get the associated worktree
        of a repository.
        a worktree.
      * `git_worktree_add` lets you create new worktrees.
      * `git_worktree_prune` lets you remove worktrees from disk.
      * `git_worktree_lock()` and `git_worktree_unlock()` let you lock
        respectively unlock a worktree.
      * `git_repository_open_from_worktree()` lets you open a repository via
      * `git_repository_head_for_worktree()` lets you get the current `HEAD` for a
        linked worktree.
      * `git_repository_head_detached_for_worktree()` lets you check whether a
        linked worktree is in detached HEAD mode.
  
  * `git_repository_item_path()` lets you retrieve paths for various repository
    files.
  
  * `git_repository_commondir()` lets you retrieve the common directory of a
    repository.
  
  * `git_branch_is_checked_out()` allows you to check whether a branch is checked
    out in a repository or any of its worktrees.
  
  * `git_repository_submodule_cache_all()` and
    `git_repository_submodule_cache_clear()` functions allow you to prime or clear
    the submodule cache of a repository.
  
  * You can disable strict hash verifications via the
    `GIT_OPT_ENABLE_STRICT_HASH_VERIFICATION` option with `git_libgit2_opts()`.
  
  * You can enable us calling `fsync` for various files inside the ".git"
    directory by setting the `GIT_OPT_ENABLE_FSYNC_GITDIR` option with
    `git_libgit2_opts()`.
  
  * You can now enable "offset deltas" when creating packfiles and negotiating
    packfiles with a remote server by setting `GIT_OPT_ENABLE_OFS_DELTA` option
    with `GIT_libgit2_opts()`.
  
  * You can now set the default share mode on Windows for opening files using
    `GIT_OPT_SET_WINDOWS_SHAREMODE` option with `git_libgit2_opts()`.
    You can query the current share mode with `GIT_OPT_GET_WINDOWS_SHAREMODE`.
  
  * `git_transport_smart_proxy_options()' enables you to get the proxy options for
    smart transports.
  
  * The `GIT_FILTER_INIT` macro and the `git_filter_init` function are provided
    to initialize a `git_filter` structure.
  
  ### Breaking API changes
  
  * `clone_checkout_strategy` has been removed from
    `git_submodule_update_option`. The checkout strategy used to clone will
    be the same strategy specified in `checkout_opts`.
  
  v0.25
  -------
  
  ### Changes or improvements
  
  * Fix repository discovery with `git_repository_discover` and
    `git_repository_open_ext` to match git's handling of a ceiling
    directory at the current directory. git only checks ceiling
    directories when its search ascends to a parent directory.  A ceiling
    directory matching the starting directory will not prevent git from
    finding a repository in the starting directory or a parent directory.
  
  * Do not fail when deleting remotes in the presence of broken
    global configs which contain branches.
  
  * Support for reading and writing git index v4 files
  
  * Improve the performance of the revwalk and bring us closer to git's code.
  
  * The reference db has improved support for concurrency and returns `GIT_ELOCKED`
    when an operation could not be performed due to locking.
  
  * Nanosecond resolution is now activated by default, following git's change to
    do this.
  
  * We now restrict the set of ciphers we let OpenSSL use by default.
  
  * Users can now register their own merge drivers for use with `.gitattributes`.
    The library also gained built-in support for the union merge driver.
  
  * The default for creating references is now to validate that the object does
    exist.
  
  * Add `git_proxy_options` which is used by the different networking
    implementations to let the caller specify the proxy settings instead of
    relying on the environment variables.
  
  ### API additions
  
  * You can now get the user-agent used by libgit2 using the
    `GIT_OPT_GET_USER_AGENT` option with `git_libgit2_opts()`.
    It is the counterpart to `GIT_OPT_SET_USER_AGENT`.
  
  * The `GIT_OPT_SET_SSL_CIPHERS` option for `git_libgit2_opts()` lets you specify
    a custom list of ciphers to use for OpenSSL.
  
  * `git_commit_create_buffer()` creates a commit and writes it into a
    user-provided buffer instead of writing it into the object db. Combine it with
    `git_commit_create_with_signature()` in order to create a commit with a
    cryptographic signature.
  
  * `git_blob_create_fromstream()` and
    `git_blob_create_fromstream_commit()` allow you to create a blob by
    writing into a stream. Useful when you do not know the final size or
    want to copy the contents from another stream.
  
  * New flags for `git_repository_open_ext`:
  
      * `GIT_REPOSITORY_OPEN_NO_DOTGIT` - Do not check for a repository by
        appending `/.git` to the `start_path`; only open the repository if
        `start_path` itself points to the git directory.
      * `GIT_REPOSITORY_OPEN_FROM_ENV` - Find and open a git repository,
        respecting the environment variables used by the git command-line
        tools. If set, `git_repository_open_ext` will ignore the other
        flags and the `ceiling_dirs` argument, and will allow a NULL
        `path` to use `GIT_DIR` or search from the current directory. The
        search for a repository will respect `$GIT_CEILING_DIRECTORIES`
        and `$GIT_DISCOVERY_ACROSS_FILESYSTEM`.  The opened repository
        will respect `$GIT_INDEX_FILE`, `$GIT_NAMESPACE`,
        `$GIT_OBJECT_DIRECTORY`, and `$GIT_ALTERNATE_OBJECT_DIRECTORIES`.
        In the future, this flag will also cause `git_repository_open_ext`
        to respect `$GIT_WORK_TREE` and `$GIT_COMMON_DIR`; currently,
        `git_repository_open_ext` with this flag will error out if either
        `$GIT_WORK_TREE` or `$GIT_COMMON_DIR` is set.
  
  * `git_diff_from_buffer()` can create a `git_diff` object from the contents
    of a git-style patch file.
  
  * `git_index_version()` and `git_index_set_version()` to get and set
    the index version
  
  * `git_odb_expand_ids()` lets you check for the existence of multiple
    objects at once.
  
  * The new `git_blob_dup()`, `git_commit_dup()`, `git_tag_dup()` and
    `git_tree_dup()` functions provide type-specific wrappers for
    `git_object_dup()` to reduce noise and increase type safety for callers.
  
  * `git_reference_dup()` lets you duplicate a reference to aid in ownership
    management and cleanup.
  
  * `git_signature_from_buffer()` lets you create a signature from a string in the
    format that appear in objects.
  
  * `git_tree_create_updated()` lets you create a tree based on another one
    together with a list of updates. For the covered update cases, it's more
    efficient than the `git_index` route.
  
  * `git_apply_patch()` applies hunks from a `git_patch` to a buffer.
  
  * `git_diff_to_buf()` lets you print an entire diff directory to a buffer,
    similar to how `git_patch_to_buf()` works.
  
  * `git_proxy_init_options()` is added to initialize a `git_proxy_options`
    structure at run-time.
  
  * `git_merge_driver_register()`, `git_merge_driver_unregister()` let you
    register and unregister a custom merge driver to be used when `.gitattributes`
    specifies it.
  
  * `git_merge_driver_lookup()` can be used to look up a merge driver by name.
  
  * `git_merge_driver_source_repo()`, `git_merge_driver_source_ancestor()`,
    `git_merge_driver_source_ours()`, `git_merge_driver_source_theirs()`,
    `git_merge_driver_source_file_options()` added as accessors to
    `git_merge_driver_source`.
  
  ### API removals
  
  * `git_blob_create_fromchunks()` has been removed in favour of
    `git_blob_create_fromstream()`.
  
  ### Breaking API changes
  
  * `git_packbuilder_object_count` and `git_packbuilder_written` now
    return a `size_t` instead of a `uint32_t` for more thorough
    compatibility with the rest of the library.
  
  * `git_packbuiler_progress` now provides explicitly sized `uint32_t`
    values instead of `unsigned int`.
  
  * `git_diff_file` now includes an `id_abbrev` field that reflects the
    number of nibbles set in the `id` field.
  
  * `git_odb_backend` now has a `freshen` function pointer.  This optional
    function pointer is similar to the `exists` function, but it will update
    a last-used marker.  For filesystem-based object databases, this updates
    the timestamp of the file containing the object, to indicate "freshness".
    If this is `NULL`, then it will not be called and the `exists` function
    will be used instead.
  
  * `git_remote_connect()` now accepts `git_proxy_options` argument, and
    `git_fetch_options` and `git_push_options` each have a `proxy_opts` field.
  
  * `git_merge_options` now provides a `default_driver` that can be used
    to provide the name of a merge driver to be used to handle files changed
    during a merge.
  
  v0.24
  -------
  
  ### Changes or improvements
  
  * Custom merge drivers can now be registered, which allows callers to
    configure callbacks to honor `merge=driver` configuration in
    `.gitattributes`.
  
  * Custom filters can now be registered with wildcard attributes, for
    example `filter=*`.  Consumers should examine the attributes parameter
    of the `check` function for details.
  
  * Symlinks are now followed when locking a file, which can be
    necessary when multiple worktrees share a base repository.
  
  * You can now set your own user-agent to be sent for HTTP requests by
    using the `GIT_OPT_SET_USER_AGENT` with `git_libgit2_opts()`.
  
  * You can set custom HTTP header fields to be sent along with requests
    by passing them in the fetch and push options.
  
  * Tree objects are now assumed to be sorted. If a tree is not
    correctly formed, it will give bad results. This is the git approach
    and cuts a significant amount of time when reading the trees.
  
  * Filter registration is now protected against concurrent
    registration.
  
  * Filenames which are not valid on Windows in an index no longer cause
    to fail to parse it on that OS.
  
  * Rebases can now be performed purely in-memory, without touching the
    repository's workdir.
  
  * When adding objects to the index, or when creating new tree or commit
    objects, the inputs are validated to ensure that the dependent objects
    exist and are of the correct type.  This object validation can be
    disabled with the GIT_OPT_ENABLE_STRICT_OBJECT_CREATION option.
  
  * The WinHTTP transport's handling of bad credentials now behaves like
    the others, asking for credentials again.
  
  ### API additions
  
  * `git_config_lock()` has been added, which allow for
    transactional/atomic complex updates to the configuration, removing
    the opportunity for concurrent operations and not committing any
    changes until the unlock.
  
  * `git_diff_options` added a new callback `progress_cb` to report on the
    progress of the diff as files are being compared. The documentation of
    the existing callback `notify_cb` was updated to reflect that it only
    gets called when new deltas are added to the diff.
  
  * `git_fetch_options` and `git_push_options` have gained a `custom_headers`
    field to set the extra HTTP header fields to send.
  
  * `git_stream_register_tls()` lets you register a callback to be used
    as the constructor for a TLS stream instead of the libgit2 built-in
    one.
  
  * `git_commit_header_field()` allows you to look up a specific header
    field in a commit.
  
  * `git_commit_extract_signature()` extracts the signature from a
    commit and gives you both the signature and the signed data so you
    can verify it.
  
  ### API removals
  
  * No APIs were removed in this version.
  
  ### Breaking API changes
  
  * The `git_merge_tree_flag_t` is now `git_merge_flag_t`.  Subsequently,
    its members are no longer prefixed with `GIT_MERGE_TREE_FLAG` but are
    now prefixed with `GIT_MERGE_FLAG`, and the `tree_flags` field of the
    `git_merge_options` structure is now named `flags`.
  
  * The `git_merge_file_flags_t` enum is now `git_merge_file_flag_t` for
    consistency with other enum type names.
  
  * `git_cert` descendent types now have a proper `parent` member
  
  * It is the responsibility of the refdb backend to decide what to do
    with the reflog on ref deletion. The file-based backend must delete
    it, a database-backed one may wish to archive it.
  
  * `git_config_backend` has gained two entries. `lock` and `unlock`
    with which to implement the transactional/atomic semantics for the
    configuration backend.
  
  * `git_index_add` and `git_index_conflict_add()` will now use the case
    as provided by the caller on case insensitive systems.  Previous
    versions would keep the case as it existed in the index.  This does
    not affect the higher-level `git_index_add_bypath` or
    `git_index_add_frombuffer` functions.
  
  * The `notify_payload` field of `git_diff_options` was renamed to `payload`
    to reflect that it's also the payload for the new progress callback.
  
  * The `git_config_level_t` enum has gained a higher-priority value
    `GIT_CONFIG_LEVEL_PROGRAMDATA` which represent a rough Windows equivalent
    to the system level configuration.
  
  * `git_rebase_options` now has a `merge_options` field.
  
  * The index no longer performs locking itself. This is not something
    users of the library should have been relying on as it's not part of
    the concurrency guarantees.
  
  * `git_remote_connect()` now takes a `custom_headers` argument to set
    the extra HTTP header fields to send.
  
  v0.23
  ------
  
  ### Changes or improvements
  
  * Patience and minimal diff drivers can now be used for merges.
  
  * Merges can now ignore whitespace changes.
  
  * Updated binary identification in CRLF filtering to avoid false positives in
    UTF-8 files.
  
  * Rename and copy detection is enabled for small files.
  
  * Checkout can now handle an initial checkout of a repository, making
    `GIT_CHECKOUT_SAFE_CREATE` unnecessary for users of clone.
  
  * The signature parameter in the ref-modifying functions has been
    removed. Use `git_repository_set_ident()` and
    `git_repository_ident()` to override the signature to be used.
  
  * The local transport now auto-scales the number of threads to use
    when creating the packfile instead of sticking to one.
  
  * Reference renaming now uses the right id for the old value.
  
  * The annotated version of branch creation, HEAD detaching and reset
    allow for specifying the expression from the user to be put into the
    reflog.
  
  * `git_rebase_commit` now returns `GIT_EUNMERGED` when you attempt to
    commit with unstaged changes.
  
  * On Mac OS X, we now use SecureTransport to provide the cryptographic
    support for HTTPS connections insead of OpenSSL.
  
  * Checkout can now accept an index for the baseline computations via the
    `baseline_index` member.
  
  * The configuration for fetching is no longer stored inside the
    `git_remote` struct but has been moved to a `git_fetch_options`. The
    remote functions now take these options or the callbacks instead of
    setting them beforehand.
  
  * `git_submodule` instances are no longer cached or shared across
    lookup. Each submodule represents the configuration at the time of
    loading.
  
  * The index now uses diffs for `add_all()` and `update_all()` which
    gives it a speed boost and closer semantics to git.
  
  * The ssh transport now reports the stderr output from the server as
    the error message, which allows you to get the "repository not
    found" messages.
  
  * `git_index_conflict_add()` will remove staged entries that exist for
    conflicted paths.
  
  * The flags for a `git_diff_file` will now have the `GIT_DIFF_FLAG_EXISTS`
    bit set when a file exists on that side of the diff.  This is useful
    for understanding whether a side of the diff exists in the presence of
    a conflict.
  
  * The constructor for a write-stream into the odb now takes
    `git_off_t` instead of `size_t` for the size of the blob, which
    allows putting large files into the odb on 32-bit systems.
  
  * The remote's push and pull URLs now honor the url.$URL.insteadOf
    configuration. This allows modifying URL prefixes to a custom
    value via gitconfig.
  
  * `git_diff_foreach`, `git_diff_blobs`, `git_diff_blob_to_buffer`,
    and `git_diff_buffers` now accept a new binary callback of type
    `git_diff_binary_cb` that includes the binary diff information.
  
  * The race condition mitigations described in `racy-git.txt` have been
    implemented.
  
  * If libcurl is installed, we will use it to connect to HTTP(S)
    servers.
  
  ### API additions
  
  * The `git_merge_options` gained a `file_flags` member.
  
  * Parsing and retrieving a configuration value as a path is exposed
    via `git_config_parse_path()` and `git_config_get_path()`
    respectively.
  
  * `git_repository_set_ident()` and `git_repository_ident()` serve to
    set and query which identity will be used when writing to the
    reflog.
  
  * `git_config_entry_free()` frees a config entry.
  
  * `git_config_get_string_buf()` provides a way to safely retrieve a
    string from a non-snapshot configuration.
  
  * `git_annotated_commit_from_revspec()` allows to get an annotated
    commit from an extended sha synatx string.
  
  * `git_repository_set_head_detached_from_annotated()`,
    `git_branch_create_from_annotated()` and
    `git_reset_from_annotated()` allow for the caller to provide an
    annotated commit through which they can control what expression is
    put into the reflog as the source/target.
  
  * `git_index_add_frombuffer()` can now create a blob from memory
    buffer and add it to the index which is attached to a repository.
  
  * The structure `git_fetch_options` has been added to determine the
    runtime configuration for fetching, such as callbacks, pruning and
    autotag behaviour. It has the runtime initializer
    `git_fetch_init_options()`.
  
  * The enum `git_fetch_prune_t` has been added, letting you specify the
    pruning behaviour for a fetch.
  
  * A push operation will notify the caller of what updates it indends
    to perform on the remote, which provides similar information to
    git's pre-push hook.
  
  * `git_stash_apply()` can now apply a stashed state from the stash list,
    placing the data into the working directory and index.
  
  * `git_stash_pop()` will apply a stashed state (like `git_stash_apply()`)
    but will remove the stashed state after a successful application.
  
  * A new error code `GIT_EEOF` indicates an early EOF from the
    server. This typically indicates an error with the URL or
    configuration of the server, and tools can use this to show messages
    about failing to communicate with the server.
  
  * A new error code `GIT_EINVALID` indicates that an argument to a
    function is invalid, or an invalid operation was requested.
  
  * `git_diff_index_to_workdir()` and `git_diff_tree_to_index()` will now
    produce deltas of type `GIT_DELTA_CONFLICTED` to indicate that the index
    side of the delta is a conflict.
  
  * The `git_status` family of functions will now produce status of type
    `GIT_STATUS_CONFLICTED` to indicate that a conflict exists for that file
    in the index.
  
  * `git_index_entry_is_conflict()` is a utility function to determine if
    a given index entry has a non-zero stage entry, indicating that it is
    one side of a conflict.
  
  * It is now possible to pass a keypair via a buffer instead of a
    path. For this, `GIT_CREDTYPE_SSH_MEMORY` and
    `git_cred_ssh_key_memory_new()` have been added.
  
  * `git_filter_list_contains` will indicate whether a particular
    filter will be run in the given filter list.
  
  * `git_commit_header_field()` has been added, which allows retrieving
    the contents of an arbitrary header field.
  
  * `git_submodule_set_branch()` allows to set the configured branch for
    a submodule.
  
  ### API removals
  
  * `git_remote_save()` and `git_remote_clear_refspecs()` have been
    removed. Remote's configuration is changed via the configuration
    directly or through a convenience function which performs changes to
    the configuration directly.
  
  * `git_remote_set_callbacks()`, `git_remote_get_callbacks()` and
    `git_remote_set_transport()` have been removed and the remote no
    longer stores this configuration.
  
  * `git_remote_set_fetch_refpecs()` and
    `git_remote_set_push_refspecs()` have been removed. There is no
    longer a way to set the base refspecs at run-time.
  
  * `git_submodule_save()` has been removed. The submodules are no
    longer configured via the objects.
  
  * `git_submodule_reload_all()` has been removed as we no longer cache
    submodules.
  
  ### Breaking API changes
  
  * `git_smart_subtransport_cb` now has a `param` parameter.
  
  * The `git_merge_options` structure member `flags` has been renamed
    to `tree_flags`.
  
  * The `git_merge_file_options` structure member `flags` is now
    an unsigned int. It was previously a `git_merge_file_flags_t`.
  
  * `GIT_CHECKOUT_SAFE_CREATE` has been removed.  Most users will generally
    be able to switch to `GIT_CHECKOUT_SAFE`, but if you require missing
    file handling during checkout, you may now use `GIT_CHECKOUT_SAFE |
    GIT_CHECKOUT_RECREATE_MISSING`.
  
  * The `git_clone_options` and `git_submodule_update_options`
    structures no longer have a `signature` field.
  
  * The following functions have removed the signature and/or log message
    parameters in favour of git-emulating ones.
  
      * `git_branch_create()`, `git_branch_move()`
      * `git_rebase_init()`, `git_rebase_abort()`
      * `git_reference_symbolic_create_matching()`,
        `git_reference_symbolic_create()`, `git_reference_create()`,
        `git_reference_create_matching()`,
        `git_reference_symbolic_set_target()`,
        `git_reference_set_target()`, `git_reference_rename()`
      * `git_remote_update_tips()`, `git_remote_fetch()`, `git_remote_push()`
      * `git_repository_set_head()`,
        `git_repository_set_head_detached()`,
        `git_repository_detach_head()`
      * `git_reset()`
  
  * `git_config_get_entry()` now gives back a ref-counted
    `git_config_entry`. You must free it when you no longer need it.
  
  * `git_config_get_string()` will return an error if used on a
    non-snapshot configuration, as there can be no guarantee that the
    returned pointer is valid.
  
  * `git_note_default_ref()` now uses a `git_buf` to return the string,
    as the string is otherwise not guaranteed to stay allocated.
  
  * `git_rebase_operation_current()` will return `GIT_REBASE_NO_OPERATION`
    if it is called immediately after creating a rebase session but before
    you have applied the first patch.
  
  * `git_rebase_options` now contains a `git_checkout_options` struct
    that will be used for functions that modify the working directory,
    namely `git_rebase_init`, `git_rebase_next` and
    `git_rebase_abort`.  As a result, `git_rebase_open` now also takes
    a `git_rebase_options` and only the `git_rebase_init` and
    `git_rebase_open` functions take a `git_rebase_options`, where they
    will persist the options to subsequent `git_rebase` calls.
  
  * The `git_clone_options` struct now has fetch options in a
    `fetch_opts` field instead of remote callbacks in
    `remote_callbacks`.
  
  * The remote callbacks has gained a new member `push_negotiation`
    which gets called before sending the update commands to the server.
  
  * The following functions no longer act on a remote instance but
    change the repository's configuration. Their signatures have changed
    accordingly:
  
      * `git_remote_set_url()`, `git_remote_seturl()`
      * `git_remote_add_fetch()`, `git_remote_add_push()` and
      * `git_remote_set_autotag()`
  
  * `git_remote_connect()` and `git_remote_prune()` now take a pointer
    to the callbacks.
  
  * `git_remote_fetch()` and `git_remote_download()` now take a pointer
    to fetch options which determine the runtime configuration.
  
  * The `git_remote_autotag_option_t` values have been changed. It has
    gained a `_UNSPECIFIED` default value to specify no override for the
    configured setting.
  
  * `git_remote_update_tips()` now takes a pointer to the callbacks as
    well as a boolean whether to write `FETCH_HEAD` and the autotag
    setting.
  
  * `git_remote_create_anonymous()` no longer takes a fetch refspec as
    url-only remotes cannot have configured refspecs.
  
  * The `git_submodule_update_options` struct now has fetch options in
    the `fetch_opts` field instead of callbacks in the
    `remote_callbacks` field.
  
  * The following functions no longer act on a submodule instance but
    change the repository's configuration. Their signatures have changed
    accordingly:
  
      * `git_submodule_set_url()`, `git_submodule_set_ignore()`,
        `git_submodule_set_update()`,
        `git_submodule_set_fetch_recurse_submodules()`.
  
  * `git_submodule_status()` no longer takes a submodule instance but a
    repsitory, a submodule name and an ignore setting.
  
  * The `push` function in the `git_transport` interface now takes a
    pointer to the remote callbacks.
  
  * The `git_index_entry` struct's fields' types have been changed to
    more accurately reflect what is in fact stored in the
    index. Specifically, time and file size are 32 bits intead of 64, as
    these values are truncated.
  
  * `GIT_EMERGECONFLICT` is now `GIT_ECONFLICT`, which more accurately
    describes the nature of the error.
  
  * It is no longer allowed to call `git_buf_grow()` on buffers
    borrowing the memory they point to.
  
  v0.22
  ------
  
  ### Changes or improvements
  
  * `git_signature_new()` now requires a non-empty email address.
  
  * Use CommonCrypto libraries for SHA-1 calculation on Mac OS X.
  
  * Disable SSL compression and SSLv2 and SSLv3 ciphers in favor of TLSv1
    in OpenSSL.
  
  * The fetch behavior of remotes with autotag set to `GIT_REMOTE_DOWNLOAD_TAGS_ALL`
    has been changed to match git 1.9.0 and later. In this mode, libgit2 now
    fetches all tags in addition to whatever else needs to be fetched.
  
  * `git_checkout()` now handles case-changing renames correctly on
    case-insensitive filesystems; for example renaming "readme" to "README".
  
  * The search for libssh2 is now done via pkg-config instead of a
    custom search of a few directories.
  
  * Add support for core.protectHFS and core.protectNTFS. Add more
    validation for filenames which we write such as references.
  
  * The local transport now generates textual progress output like
    git-upload-pack does ("counting objects").
  
  * `git_checkout_index()` can now check out an in-memory index that is not
    necessarily the repository's index, so you may check out an index
    that was produced by git_merge and friends while retaining the cached
    information.
  
  * Remove the default timeout for receiving / sending data over HTTP using
    the WinHTTP transport layer.
  
  * Add SPNEGO (Kerberos) authentication using GSSAPI on Unix systems.
  
  * Provide built-in objects for the empty blob (e69de29) and empty
    tree (4b825dc) objects.
  
  * The index' tree cache is now filled upon read-tree and write-tree
    and the cache is written to disk.
  
  * LF -> CRLF filter refuses to handle mixed-EOL files
  
  * LF -> CRLF filter now runs when * text = auto (with Git for Windows 1.9.4)
  
  * File unlocks are atomic again via rename. Read-only files on Windows are
    made read-write if necessary.
  
  * Share open packfiles across repositories to share descriptors and mmaps.
  
  * Use a map for the treebuilder, making insertion O(1)
  
  * The build system now accepts an option EMBED_SSH_PATH which when set
    tells it to include a copy of libssh2 at the given location. This is
    enabled for MSVC.
  
  * Add support for refspecs with the asterisk in the middle of a
    pattern.
  
  * Fetching now performs opportunistic updates. To achieve this, we
    introduce a difference between active and passive refspecs, which
    make `git_remote_download()` and `git_remote_fetch()` to take a list of
    resfpecs to be the active list, similarly to how git fetch accepts a
    list on the command-line.
  
  * The THREADSAFE option to build libgit2 with threading support has
    been flipped to be on by default.
  
  * The remote object has learnt to prune remote-tracking branches. If
    the remote is configured to do so, this will happen via
    `git_remote_fetch()`. You can also call `git_remote_prune()` after
    connecting or fetching to perform the prune.
  
  
  ### API additions
  
  * Introduce `git_buf_text_is_binary()` and `git_buf_text_contains_nul()` for
    consumers to perform binary detection on a git_buf.
  
  * `git_branch_upstream_remote()` has been introduced to provide the
    branch.<name>.remote configuration value.
  
  * Introduce `git_describe_commit()` and `git_describe_workdir()` to provide
    a description of the current commit (and working tree, respectively)
    based on the nearest tag or reference
  
  * Introduce `git_merge_bases()` and the `git_oidarray` type to expose all
    merge bases between two commits.
  
  * Introduce `git_merge_bases_many()` to expose all merge bases between
    multiple commits.
  
  * Introduce rebase functionality (using the merge algorithm only).
    Introduce `git_rebase_init()` to begin a new rebase session,
    `git_rebase_open()` to open an in-progress rebase session,
    `git_rebase_commit()` to commit the current rebase operation,
    `git_rebase_next()` to apply the next rebase operation,
    `git_rebase_abort()` to abort an in-progress rebase and `git_rebase_finish()`
    to complete a rebase operation.
  
  * Introduce `git_note_author()` and `git_note_committer()` to get the author
    and committer information on a `git_note`, respectively.
  
  * A factory function for ssh has been added which allows to change the
    path of the programs to execute for receive-pack and upload-pack on
    the server, `git_transport_ssh_with_paths()`.
  
  * The ssh transport supports asking the remote host for accepted
    credential types as well as multiple challeges using a single
    connection. This requires to know which username you want to connect
    as, so this introduces the USERNAME credential type which the ssh
    transport will use to ask for the username.
  
  * The `GIT_EPEEL` error code has been introduced when we cannot peel a tag
    to the requested object type; if the given object otherwise cannot be
    peeled, `GIT_EINVALIDSPEC` is returned.
  
  * Introduce `GIT_REPOSITORY_INIT_RELATIVE_GITLINK` to use relative paths
    when writing gitlinks, as is used by git core for submodules.
  
  * `git_remote_prune()` has been added. See above for description.
  
  
  * Introduce reference transactions, which allow multiple references to
    be locked at the same time and updates be queued. This also allows
    us to safely update a reflog with arbitrary contents, as we need to
    do for stash.
  
  ### API removals
  
  * `git_remote_supported_url()` and `git_remote_is_valid_url()` have been
    removed as they have become essentially useless with rsync-style ssh paths.
  
  * `git_clone_into()` and `git_clone_local_into()` have been removed from the
    public API in favour of `git_clone callbacks`.
  
  * The option to ignore certificate errors via `git_remote_cert_check()`
    is no longer present. Instead, `git_remote_callbacks` has gained a new
    entry which lets the user perform their own certificate checks.
  
  ### Breaking API changes
  
  * `git_cherry_pick()` is now `git_cherrypick()`.
  
  * The `git_submodule_update()` function was renamed to
    `git_submodule_update_strategy()`. `git_submodule_update()` is now used to
    provide functionalty similar to "git submodule update".
  
  * `git_treebuilder_create()` was renamed to `git_treebuilder_new()` to better
    reflect it being a constructor rather than something which writes to
    disk.
  
  * `git_treebuilder_new()` (was `git_treebuilder_create()`) now takes a
    repository so that it can query repository configuration.
    Subsequently, `git_treebuilder_write()` no longer takes a repository.
  
  * `git_threads_init()` and `git_threads_shutdown()` have been renamed to
    `git_libgit2_init()` and `git_libgit2_shutdown()` to better explain what
    their purpose is, as it's grown to be more than just about threads.
  
  * `git_libgit2_init()` and `git_libgit2_shutdown()` now return the number of
    initializations of the library, so consumers may schedule work on the
    first initialization.
  
  * The `git_transport_register()` function no longer takes a priority and takes
    a URL scheme name (eg "http") instead of a prefix like "http://"
  
  * `git_index_name_entrycount()` and `git_index_reuc_entrycount()` now
    return size_t instead of unsigned int.
  
  * The `context_lines` and `interhunk_lines` fields in `git_diff`_options are
    now `uint32_t` instead of `uint16_t`. This allows to set them to `UINT_MAX`,
    in effect asking for "infinite" context e.g. to iterate over all the
    unmodified lines of a diff.
  
  * `git_status_file()` now takes an exact path. Use `git_status_list_new()` if
    pathspec searching is needed.
  
  * `git_note_create()` has changed the position of the notes reference
    name to match `git_note_remove()`.
  
  * Rename `git_remote_load()` to `git_remote_lookup()` to bring it in line
    with the rest of the lookup functions.
  
  * `git_remote_rename()` now takes the repository and the remote's
    current name. Accepting a remote indicates we want to change it,
    which we only did partially. It is much clearer if we accept a name
    and no loaded objects are changed.
  
  * `git_remote_delete()` now accepts the repository and the remote's name
    instead of a loaded remote.
  
  * `git_merge_head` is now `git_annotated_commit`, to better reflect its usage
    for multiple functions (including rebase)
  
  * The `git_clone_options` struct no longer provides the `ignore_cert_errors` or
    `remote_name` members for remote customization.
  
    Instead, the `git_clone_options` struct has two new members, `remote_cb` and
    `remote_cb_payload`, which allow the caller to completely override the remote
    creation process. If needed, the caller can use this callback to give their
    remote a name other than the default (origin) or disable cert checking.
  
    The `remote_callbacks` member has been preserved for convenience, although it
    is not used when a remote creation callback is supplied.
  
  * The `git_clone`_options struct now provides `repository_cb` and
    `repository_cb_payload` to allow the user to create a repository with
    custom options.
  
  * The `git_push` struct to perform a push has been replaced with
    `git_remote_upload()`. The refspecs and options are passed as a
    function argument. `git_push_update_tips()` is now also
    `git_remote_update_tips()` and the callbacks are in the same struct as
    the rest.
  
  * The `git_remote_set_transport()` function now sets a transport factory function,
    rather than a pre-existing transport instance.
  
  * The `git_transport` structure definition has moved into the sys/transport.h
    file.
  
  * libgit2 no longer automatically sets the OpenSSL locking
    functions. This is not something which we can know to do. A
    last-resort convenience function is provided in sys/openssl.h,
    `git_openssl_set_locking()` which can be used to set the locking.
  

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