thodg/libgit2

diff --git a/include/git2/common.h b/include/git2/common.h
index f13dfd5..c909f86 100644
--- a/include/git2/common.h
+++ b/include/git2/common.h
@@ -179,6 +179,7 @@ typedef enum {
 	GIT_OPT_SET_SSL_CIPHERS,
 	GIT_OPT_GET_USER_AGENT,
 	GIT_OPT_ENABLE_OFS_DELTA,
+	GIT_OPT_ENABLE_SYNCHRONOUS_OBJECT_CREATION,
 } git_libgit2_opt_t;
 
 /**
@@ -316,6 +317,13 @@ typedef enum {
  *		> Packfiles containing offset deltas can still be read.
  *		> This defaults to enabled.
  *
+ *	* opts(GIT_OPT_ENABLE_SYNCHRONOUS_OBJECT_CREATION, int enabled)
+ *
+ *		> Enable synchronized writes of new objects using `fsync`
+ *		> (or the platform equivalent) to ensure that new object data
+ *		> is written to permanent storage, not simply cached.  This
+ *		> defaults to disabled.
+ *
  * @param option Option key
  * @param ... value to set the option
  * @return 0 on success, <0 on failure
diff --git a/include/git2/odb_backend.h b/include/git2/odb_backend.h
index b17cfd8..9199538 100644
--- a/include/git2/odb_backend.h
+++ b/include/git2/odb_backend.h
@@ -39,7 +39,7 @@ GIT_EXTERN(int) git_odb_backend_pack(git_odb_backend **out, const char *objects_
  * @param out location to store the odb backend pointer
  * @param objects_dir the Git repository's objects directory
  * @param compression_level zlib compression level to use
- * @param do_fsync whether to do an fsync() after writing (currently ignored)
+ * @param do_fsync whether to do an fsync() after writing
  * @param dir_mode permissions to use creating a directory or 0 for defaults
  * @param file_mode permissions to use creating a file or 0 for defaults
  *
diff --git a/src/config_cache.c b/src/config_cache.c
index dbea871..8407222 100644
--- a/src/config_cache.c
+++ b/src/config_cache.c
@@ -78,6 +78,7 @@ static struct map_data _cvar_maps[] = {
 	{"core.logallrefupdates", NULL, 0, GIT_LOGALLREFUPDATES_DEFAULT },
 	{"core.protecthfs", NULL, 0, GIT_PROTECTHFS_DEFAULT },
 	{"core.protectntfs", NULL, 0, GIT_PROTECTNTFS_DEFAULT },
+	{"core.fsyncobjectfiles", NULL, 0, GIT_FSYNCOBJECTFILES_DEFAULT },
 };
 
 int git_config__cvar(int *out, git_config *config, git_cvar_cached cvar)
diff --git a/src/filebuf.c b/src/filebuf.c
index 825b9c0..80250cc 100644
--- a/src/filebuf.c
+++ b/src/filebuf.c
@@ -291,6 +291,9 @@ int git_filebuf_open_withsize(git_filebuf *file, const char *path, int flags, mo
 	if (flags & GIT_FILEBUF_DO_NOT_BUFFER)
 		file->do_not_buffer = true;
 
+	if (flags & GIT_FILEBUF_FSYNC)
+		file->do_fsync = true;
+
 	file->buf_size = size;
 	file->buf_pos = 0;
 	file->fd = -1;
@@ -425,6 +428,11 @@ int git_filebuf_commit(git_filebuf *file)
 
 	file->fd_is_open = false;
 
+	if (file->do_fsync && p_fsync(file->fd) < 0) {
+		giterr_set(GITERR_OS, "failed to fsync '%s'", file->path_lock);
+		goto on_error;
+	}
+
 	if (p_close(file->fd) < 0) {
 		giterr_set(GITERR_OS, "failed to close file at '%s'", file->path_lock);
 		goto on_error;
@@ -437,6 +445,9 @@ int git_filebuf_commit(git_filebuf *file)
 		goto on_error;
 	}
 
+	if (file->do_fsync && git_futils_fsync_parent(file->path_original) < 0)
+		goto on_error;
+
 	file->did_rename = true;
 
 	git_filebuf_cleanup(file);
diff --git a/src/filebuf.h b/src/filebuf.h
index 467708d..c65aea7 100644
--- a/src/filebuf.h
+++ b/src/filebuf.h
@@ -20,7 +20,8 @@
 #define GIT_FILEBUF_FORCE				(1 << 3)
 #define GIT_FILEBUF_TEMPORARY			(1 << 4)
 #define GIT_FILEBUF_DO_NOT_BUFFER		(1 << 5)
-#define GIT_FILEBUF_DEFLATE_SHIFT		(6)
+#define GIT_FILEBUF_FSYNC				(1 << 6)
+#define GIT_FILEBUF_DEFLATE_SHIFT		(7)
 
 #define GIT_FILELOCK_EXTENSION ".lock\0"
 #define GIT_FILELOCK_EXTLENGTH 6
@@ -47,6 +48,7 @@ struct git_filebuf {
 	bool created_lock;
 	bool did_rename;
 	bool do_not_buffer;
+	bool do_fsync;
 	int last_error;
 };
 
diff --git a/src/fileops.c b/src/fileops.c
index acb3b87..28f414f 100644
--- a/src/fileops.c
+++ b/src/fileops.c
@@ -236,10 +236,16 @@ int git_futils_readbuffer(git_buf *buf, const char *path)
 int git_futils_writebuffer(
 	const git_buf *buf,	const char *path, int flags, mode_t mode)
 {
-	int fd, error = 0;
+	int fd, do_fsync = 0, error = 0;
 
-	if (flags <= 0)
+	if (!flags)
 		flags = O_CREAT | O_TRUNC | O_WRONLY;
+
+	if ((flags & O_FSYNC) != 0)
+		do_fsync = 1;
+
+	flags &= ~O_FSYNC;
+
 	if (!mode)
 		mode = GIT_FILEMODE_BLOB;
 
@@ -254,8 +260,19 @@ int git_futils_writebuffer(
 		return error;
 	}
 
-	if ((error = p_close(fd)) < 0)
+	if (do_fsync && (error = p_fsync(fd)) < 0) {
+		giterr_set(GITERR_OS, "could not fsync '%s'", path);
+		p_close(fd);
+		return error;
+	}
+
+	if ((error = p_close(fd)) < 0) {
 		giterr_set(GITERR_OS, "error while closing '%s'", path);
+		return error;
+	}
+
+	if (do_fsync && (flags & O_CREAT))
+		error = git_futils_fsync_parent(path);
 
 	return error;
 }
@@ -1108,3 +1125,33 @@ void git_futils_filestamp_set_from_stat(
 		memset(stamp, 0, sizeof(*stamp));
 	}
 }
+
+int git_futils_fsync_dir(const char *path)
+{
+#ifdef GIT_WIN32
+	GIT_UNUSED(path);
+	return 0;
+#else
+	int fd, error = -1;
+
+	if ((fd = p_open(path, O_RDONLY)) < 0) {
+		giterr_set(GITERR_OS, "failed to open directory '%s' for fsync", path);
+		return -1;
+	}
+
+	if ((error = p_fsync(fd)) < 0)
+		giterr_set(GITERR_OS, "failed to fsync directory '%s'", path);
+
+	p_close(fd);
+	return error;
+#endif
+}
+
+int git_futils_fsync_parent(const char *path)
+{
+	char *parent = git_path_dirname(path);
+	int error = git_futils_fsync_dir(parent);
+
+	git__free(parent);
+	return error;
+}
diff --git a/src/fileops.h b/src/fileops.h
index 65c96a6..46886b0 100644
--- a/src/fileops.h
+++ b/src/fileops.h
@@ -25,6 +25,13 @@ extern int git_futils_readbuffer_updated(
 	git_buf *obj, const char *path, git_oid *checksum, int *updated);
 extern int git_futils_readbuffer_fd(git_buf *obj, git_file fd, size_t len);
 
+/* Additional constants for `git_futils_writebuffer`'s `open_flags`.  We
+ * support these internally and they will be removed before the `open` call.
+ */
+#ifndef O_FSYNC
+# define O_FSYNC (1 << 31)
+#endif
+
 extern int git_futils_writebuffer(
 	const git_buf *buf, const char *path, int open_flags, mode_t mode);
 
@@ -356,4 +363,22 @@ extern void git_futils_filestamp_set(
 extern void git_futils_filestamp_set_from_stat(
 	git_futils_filestamp *stamp, struct stat *st);
 
+/**
+ * `fsync` the parent directory of the given path, if `fsync` is
+ * supported for directories on this platform.
+ *
+ * @param path Path of the directory to sync.
+ * @return 0 on success, -1 on error
+ */
+extern int git_futils_fsync_dir(const char *path);
+
+/**
+ * `fsync` the parent directory of the given path, if `fsync` is
+ * supported for directories on this platform.
+ *
+ * @param path Path of the file whose parent directory should be synced.
+ * @return 0 on success, -1 on error
+ */
+extern int git_futils_fsync_parent(const char *path);
+
 #endif /* INCLUDE_fileops_h__ */
diff --git a/src/indexer.c b/src/indexer.c
index 869e229..ce67240 100644
--- a/src/indexer.c
+++ b/src/indexer.c
@@ -17,6 +17,7 @@
 #include "oid.h"
 #include "oidmap.h"
 #include "zstream.h"
+#include "object.h"
 
 extern git_mutex git__mwindow_mutex;
 
@@ -33,7 +34,8 @@ struct git_indexer {
 	unsigned int parsed_header :1,
 		pack_committed :1,
 		have_stream :1,
-		have_delta :1;
+		have_delta :1,
+		do_fsync :1;
 	struct git_pack_header hdr;
 	struct git_pack_file *pack;
 	unsigned int mode;
@@ -123,6 +125,9 @@ int git_indexer_new(
 	git_hash_ctx_init(&idx->hash_ctx);
 	git_hash_ctx_init(&idx->trailer);
 
+	if (git_object__synchronous_writing)
+		idx->do_fsync = 1;
+
 	error = git_buf_joinpath(&path, prefix, suff);
 	if (error < 0)
 		goto cleanup;
@@ -161,6 +166,11 @@ cleanup:
 	return -1;
 }
 
+void git_indexer__set_fsync(git_indexer *idx, int do_fsync)
+{
+	idx->do_fsync = !!do_fsync;
+}
+
 /* Try to store the delta so we can try to resolve it later */
 static int store_delta(git_indexer *idx)
 {
@@ -989,7 +999,9 @@ int git_indexer_commit(git_indexer *idx, git_transfer_progress *stats)
 		return -1;
 
 	if (git_filebuf_open(&index_file, filename.ptr,
-		GIT_FILEBUF_HASH_CONTENTS, idx->mode) < 0)
+		GIT_FILEBUF_HASH_CONTENTS |
+		(idx->do_fsync ? GIT_FILEBUF_FSYNC : 0),
+		idx->mode) < 0)
 		goto on_error;
 
 	/* Write out the header */
@@ -1066,6 +1078,11 @@ int git_indexer_commit(git_indexer *idx, git_transfer_progress *stats)
 		return -1;
 	}
 
+	if (idx->do_fsync && p_fsync(idx->pack->mwf.fd) < 0) {
+		giterr_set(GITERR_OS, "failed to fsync packfile");
+		goto on_error;
+	}
+
 	/* We need to close the descriptor here so Windows doesn't choke on commit_at */
 	if (p_close(idx->pack->mwf.fd) < 0) {
 		giterr_set(GITERR_OS, "failed to close packfile");
@@ -1078,7 +1095,14 @@ int git_indexer_commit(git_indexer *idx, git_transfer_progress *stats)
 		goto on_error;
 
 	/* And don't forget to rename the packfile to its new place. */
-	p_rename(idx->pack->pack_name, git_buf_cstr(&filename));
+	if (p_rename(idx->pack->pack_name, git_buf_cstr(&filename)) < 0)
+		goto on_error;
+
+	/* And fsync the parent directory if we're asked to. */
+	if (idx->do_fsync &&
+		git_futils_fsync_parent(git_buf_cstr(&filename)) < 0)
+		goto on_error;
+
 	idx->pack_committed = 1;
 
 	git_buf_free(&filename);
diff --git a/src/indexer.h b/src/indexer.h
new file mode 100644
index 0000000..702694b
--- /dev/null
+++ b/src/indexer.h
@@ -0,0 +1,12 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+#ifndef INCLUDE_indexer_h__
+#define INCLUDE_indexer_h__
+
+extern int git_indexer__set_fsync(git_indexer *idx, int do_fsync);
+
+#endif
diff --git a/src/object.c b/src/object.c
index 2da36a2..bd87c93 100644
--- a/src/object.c
+++ b/src/object.c
@@ -16,6 +16,7 @@
 #include "tag.h"
 
 bool git_object__strict_input_validation = true;
+bool git_object__synchronous_writing = false;
 
 typedef struct {
 	const char	*str;	/* type name string */
diff --git a/src/object.h b/src/object.h
index dd227d1..13117e4 100644
--- a/src/object.h
+++ b/src/object.h
@@ -10,6 +10,7 @@
 #include "repository.h"
 
 extern bool git_object__strict_input_validation;
+extern bool git_object__synchronous_writing;
 
 /** Base git object for inheritance */
 struct git_object {
diff --git a/src/odb.c b/src/odb.c
index 3f2ac02..cf321f5 100644
--- a/src/odb.c
+++ b/src/odb.c
@@ -496,7 +496,7 @@ int git_odb_get_backend(git_odb_backend **out, git_odb *odb, size_t pos)
 	return GIT_ENOTFOUND;
 }
 
-static int add_default_backends(
+int git_odb__add_default_backends(
 	git_odb *db, const char *objects_dir,
 	bool as_alternates, int alternate_depth)
 {
@@ -531,7 +531,7 @@ static int add_default_backends(
 #endif
 
 	/* add the loose object backend */
-	if (git_odb_backend_loose(&loose, objects_dir, -1, 0, 0, 0) < 0 ||
+	if (git_odb_backend_loose(&loose, objects_dir, -1, db->do_fsync, 0, 0) < 0 ||
 		add_backend_internal(db, loose, GIT_LOOSE_PRIORITY, as_alternates, inode) < 0)
 		return -1;
 
@@ -586,7 +586,7 @@ static int load_alternates(git_odb *odb, const char *objects_dir, int alternate_
 			alternate = git_buf_cstr(&alternates_path);
 		}
 
-		if ((result = add_default_backends(odb, alternate, true, alternate_depth + 1)) < 0)
+		if ((result = git_odb__add_default_backends(odb, alternate, true, alternate_depth + 1)) < 0)
 			break;
 	}
 
@@ -598,7 +598,7 @@ static int load_alternates(git_odb *odb, const char *objects_dir, int alternate_
 
 int git_odb_add_disk_alternate(git_odb *odb, const char *path)
 {
-	return add_default_backends(odb, path, true, 0);
+	return git_odb__add_default_backends(odb, path, true, 0);
 }
 
 int git_odb_open(git_odb **out, const char *objects_dir)
@@ -612,7 +612,7 @@ int git_odb_open(git_odb **out, const char *objects_dir)
 	if (git_odb_new(&db) < 0)
 		return -1;
 
-	if (add_default_backends(db, objects_dir, 0, 0) < 0) {
+	if (git_odb__add_default_backends(db, objects_dir, 0, 0) < 0) {
 		git_odb_free(db);
 		return -1;
 	}
@@ -621,6 +621,24 @@ int git_odb_open(git_odb **out, const char *objects_dir)
 	return 0;
 }
 
+int git_odb__set_caps(git_odb *odb, int caps)
+{
+	if (caps == GIT_ODB_CAP_FROM_OWNER) {
+		git_repository *repo = odb->rc.owner;
+		int val;
+
+		if (!repo) {
+			giterr_set(GITERR_ODB, "cannot access repository to set odb caps");
+			return -1;
+		}
+
+		if (!git_repository__cvar(&val, repo, GIT_CVAR_FSYNCOBJECTFILES))
+			odb->do_fsync = !!val;
+	}
+
+	return 0;
+}
+
 static void odb_free(git_odb *db)
 {
 	size_t i;
diff --git a/src/odb.h b/src/odb.h
index 76e82f1..4f548bb 100644
--- a/src/odb.h
+++ b/src/odb.h
@@ -38,8 +38,25 @@ struct git_odb {
 	git_refcount rc;
 	git_vector backends;
 	git_cache own_cache;
+	unsigned int do_fsync :1;
 };
 
+typedef enum {
+	GIT_ODB_CAP_FROM_OWNER = -1,
+} git_odb_cap_t;
+
+/*
+ * Set the capabilities for the object database.
+ */
+int git_odb__set_caps(git_odb *odb, int caps);
+
+/*
+ * Add the default loose and packed backends for a database.
+ */
+int git_odb__add_default_backends(
+	git_odb *db, const char *objects_dir,
+	bool as_alternates, int alternate_depth);
+
 /*
  * Hash a git_rawobj internally.
  * The `git_rawobj` is supposed to be previously initialized
diff --git a/src/odb_loose.c b/src/odb_loose.c
index b2e2f1f..e14af4f 100644
--- a/src/odb_loose.c
+++ b/src/odb_loose.c
@@ -14,6 +14,7 @@
 #include "odb.h"
 #include "delta.h"
 #include "filebuf.h"
+#include "object.h"
 
 #include "git2/odb_backend.h"
 #include "git2/types.h"
@@ -838,6 +839,17 @@ static void loose_backend__stream_free(git_odb_stream *_stream)
 	git__free(stream);
 }
 
+static int filebuf_flags(loose_backend *backend)
+{
+	int flags = GIT_FILEBUF_TEMPORARY |
+		(backend->object_zlib_level << GIT_FILEBUF_DEFLATE_SHIFT);
+
+	if (backend->fsync_object_files || git_object__synchronous_writing)
+		flags |= GIT_FILEBUF_FSYNC;
+
+	return flags;
+}
+
 static int loose_backend__stream(git_odb_stream **stream_out, git_odb_backend *_backend, git_off_t length, git_otype type)
 {
 	loose_backend *backend;
@@ -864,9 +876,7 @@ static int loose_backend__stream(git_odb_stream **stream_out, git_odb_backend *_
 	stream->stream.mode = GIT_STREAM_WRONLY;
 
 	if (git_buf_joinpath(&tmp_path, backend->objects_dir, "tmp_object") < 0 ||
-		git_filebuf_open(&stream->fbuf, tmp_path.ptr,
-			GIT_FILEBUF_TEMPORARY |
-			(backend->object_zlib_level << GIT_FILEBUF_DEFLATE_SHIFT),
+		git_filebuf_open(&stream->fbuf, tmp_path.ptr, filebuf_flags(backend),
 			backend->object_file_mode) < 0 ||
 		stream->stream.write((git_odb_stream *)stream, hdr, hdrlen) < 0)
 	{
@@ -894,9 +904,7 @@ static int loose_backend__write(git_odb_backend *_backend, const git_oid *oid, c
 	header_len = git_odb__format_object_header(header, sizeof(header), len, type);
 
 	if (git_buf_joinpath(&final_path, backend->objects_dir, "tmp_object") < 0 ||
-		git_filebuf_open(&fbuf, final_path.ptr,
-			GIT_FILEBUF_TEMPORARY |
-			(backend->object_zlib_level << GIT_FILEBUF_DEFLATE_SHIFT),
+		git_filebuf_open(&fbuf, final_path.ptr, filebuf_flags(backend),
 			backend->object_file_mode) < 0)
 	{
 		error = -1;
diff --git a/src/pack-objects.c b/src/pack-objects.c
index 58b7b94..ef272e8 100644
--- a/src/pack-objects.c
+++ b/src/pack-objects.c
@@ -1385,6 +1385,7 @@ int git_packbuilder_write(
 	git_indexer *indexer;
 	git_transfer_progress stats;
 	struct pack_write_context ctx;
+	int t;
 
 	PREPARE_PACK;
 
@@ -1392,6 +1393,9 @@ int git_packbuilder_write(
 		&indexer, path, mode, pb->odb, progress_cb, progress_cb_payload) < 0)
 		return -1;
 
+	if (!git_repository__cvar(&t, pb->repo, GIT_CVAR_FSYNCOBJECTFILES) && t)
+		git_indexer__set_fsync(indexer, 1);
+
 	ctx.indexer = indexer;
 	ctx.stats = &stats;
 
diff --git a/src/pack-objects.h b/src/pack-objects.h
index 5a84f41..e1e0ee3 100644
--- a/src/pack-objects.h
+++ b/src/pack-objects.h
@@ -16,6 +16,7 @@
 #include "netops.h"
 #include "zstream.h"
 #include "pool.h"
+#include "indexer.h"
 
 #include "git2/oid.h"
 #include "git2/pack.h"
diff --git a/src/posix.c b/src/posix.c
index e68f324..94deb6a 100644
--- a/src/posix.c
+++ b/src/posix.c
@@ -10,6 +10,8 @@
 #include <stdio.h>
 #include <ctype.h>
 
+size_t p_fsync__cnt = 0;
+
 #ifndef GIT_WIN32
 
 #ifdef NO_ADDRINFO
diff --git a/src/posix.h b/src/posix.h
index f204751..bd5a98e 100644
--- a/src/posix.h
+++ b/src/posix.h
@@ -111,6 +111,12 @@ extern int p_rename(const char *from, const char *to);
 extern int git__page_size(size_t *page_size);
 extern int git__mmap_alignment(size_t *page_size);
 
+/* The number of times `p_fsync` has been called.  Note that this is for
+ * test code only; it it not necessarily thread-safe and should not be
+ * relied upon in production.
+ */
+extern size_t p_fsync__cnt;
+
 /**
  * Platform-dependent methods
  */
diff --git a/src/rebase.c b/src/rebase.c
index bf1d086..f528031 100644
--- a/src/rebase.c
+++ b/src/rebase.c
@@ -447,8 +447,8 @@ static int rebase_setupfiles_merge(git_rebase *rebase)
 	size_t i;
 	int error = 0;
 
-	if ((error = rebase_setupfile(rebase, END_FILE, -1, "%" PRIuZ "\n", git_array_size(rebase->operations))) < 0 ||
-		(error = rebase_setupfile(rebase, ONTO_NAME_FILE, -1, "%s\n", rebase->onto_name)) < 0)
+	if ((error = rebase_setupfile(rebase, END_FILE, 0, "%" PRIuZ "\n", git_array_size(rebase->operations))) < 0 ||
+		(error = rebase_setupfile(rebase, ONTO_NAME_FILE, 0, "%s\n", rebase->onto_name)) < 0)
 		goto done;
 
 	for (i = 0; i < git_array_size(rebase->operations); i++) {
@@ -459,7 +459,7 @@ static int rebase_setupfiles_merge(git_rebase *rebase)
 
 		git_oid_fmt(id_str, &operation->id);
 
-		if ((error = rebase_setupfile(rebase, commit_filename.ptr, -1,
+		if ((error = rebase_setupfile(rebase, commit_filename.ptr, 0,
 				"%.*s\n", GIT_OID_HEXSZ, id_str)) < 0)
 			goto done;
 	}
@@ -486,10 +486,10 @@ static int rebase_setupfiles(git_rebase *rebase)
 		rebase->orig_head_name;
 
 	if (git_repository__set_orig_head(rebase->repo, &rebase->orig_head_id) < 0 ||
-		rebase_setupfile(rebase, HEAD_NAME_FILE, -1, "%s\n", orig_head_name) < 0 ||
-		rebase_setupfile(rebase, ONTO_FILE, -1, "%.*s\n", GIT_OID_HEXSZ, onto) < 0 ||
-		rebase_setupfile(rebase, ORIG_HEAD_FILE, -1, "%.*s\n", GIT_OID_HEXSZ, orig_head) < 0 ||
-		rebase_setupfile(rebase, QUIET_FILE, -1, rebase->quiet ? "t\n" : "\n") < 0)
+		rebase_setupfile(rebase, HEAD_NAME_FILE, 0, "%s\n", orig_head_name) < 0 ||
+		rebase_setupfile(rebase, ONTO_FILE, 0, "%.*s\n", GIT_OID_HEXSZ, onto) < 0 ||
+		rebase_setupfile(rebase, ORIG_HEAD_FILE, 0, "%.*s\n", GIT_OID_HEXSZ, orig_head) < 0 ||
+		rebase_setupfile(rebase, QUIET_FILE, 0, rebase->quiet ? "t\n" : "\n") < 0)
 		return -1;
 
 	return rebase_setupfiles_merge(rebase);
@@ -823,8 +823,8 @@ static int rebase_next_merge(
 	normalize_checkout_options_for_apply(&checkout_opts, rebase, current_commit);
 
 	if ((error = git_indexwriter_init_for_operation(&indexwriter, rebase->repo, &checkout_opts.checkout_strategy)) < 0 ||
-		(error = rebase_setupfile(rebase, MSGNUM_FILE, -1, "%" PRIuZ "\n", rebase->current+1)) < 0 ||
-		(error = rebase_setupfile(rebase, CURRENT_FILE, -1, "%.*s\n", GIT_OID_HEXSZ, current_idstr)) < 0 ||
+		(error = rebase_setupfile(rebase, MSGNUM_FILE, 0, "%" PRIuZ "\n", rebase->current+1)) < 0 ||
+		(error = rebase_setupfile(rebase, CURRENT_FILE, 0, "%.*s\n", GIT_OID_HEXSZ, current_idstr)) < 0 ||
 		(error = git_merge_trees(&index, rebase->repo, parent_tree, head_tree, current_tree, &rebase->options.merge_options)) < 0 ||
 		(error = git_merge__check_result(rebase->repo, index)) < 0 ||
 		(error = git_checkout_index(rebase->repo, index, &checkout_opts)) < 0 ||
diff --git a/src/refdb_fs.c b/src/refdb_fs.c
index ac5a6a6..450b3f3 100644
--- a/src/refdb_fs.c
+++ b/src/refdb_fs.c
@@ -62,6 +62,7 @@ typedef struct refdb_fs_backend {
 	int peeling_mode;
 	git_iterator_flag_t iterator_flags;
 	uint32_t direach_flags;
+	int fsync;
 } refdb_fs_backend;
 
 static int refdb_reflog_fs__delete(git_refdb_backend *_backend, const char *name);
@@ -736,7 +737,7 @@ static int reference_path_available(
 
 static int loose_lock(git_filebuf *file, refdb_fs_backend *backend, const char *name)
 {
-	int error;
+	int error, filebuf_flags;
 	git_buf ref_path = GIT_BUF_INIT;
 
 	assert(file && backend && name);
@@ -755,7 +756,11 @@ static int loose_lock(git_filebuf *file, refdb_fs_backend *backend, const char *
 	if (git_buf_joinpath(&ref_path, backend->gitpath, name) < 0)
 		return -1;
 
-	error = git_filebuf_open(file, ref_path.ptr, GIT_FILEBUF_FORCE, GIT_REFS_FILE_MODE);
+	filebuf_flags = GIT_FILEBUF_FORCE;
+	if (backend->fsync)
+		filebuf_flags |= GIT_FILEBUF_FSYNC;
+
+	error = git_filebuf_open(file, ref_path.ptr, filebuf_flags, GIT_REFS_FILE_MODE);
 
 	if (error == GIT_EDIRECTORY)
 		giterr_set(GITERR_REFERENCE, "cannot lock ref '%s', there are refs beneath that folder", name);
@@ -990,15 +995,18 @@ static int packed_write(refdb_fs_backend *backend)
 {
 	git_sortedcache *refcache = backend->refcache;
 	git_filebuf pack_file = GIT_FILEBUF_INIT;
-	int error;
+	int error, open_flags = 0;
 	size_t i;
 
 	/* lock the cache to updates while we do this */
 	if ((error = git_sortedcache_wlock(refcache)) < 0)
 		return error;
 
+	if (backend->fsync)
+		open_flags = GIT_FILEBUF_FSYNC;
+
 	/* Open the file! */
-	if ((error = git_filebuf_open(&pack_file, git_sortedcache_path(refcache), 0, GIT_PACKEDREFS_FILE_MODE)) < 0)
+	if ((error = git_filebuf_open(&pack_file, git_sortedcache_path(refcache), open_flags, GIT_PACKEDREFS_FILE_MODE)) < 0)
 		goto fail;
 
 	/* Packfiles have a header... apparently
@@ -1786,7 +1794,7 @@ success:
 /* Append to the reflog, must be called under reference lock */
 static int reflog_append(refdb_fs_backend *backend, const git_reference *ref, const git_oid *old, const git_oid *new, const git_signature *who, const char *message)
 {
-	int error, is_symbolic;
+	int error, is_symbolic, open_flags;
 	git_oid old_id = {{0}}, new_id = {{0}};
 	git_buf buf = GIT_BUF_INIT, path = GIT_BUF_INIT;
 	git_repository *repo = backend->repo;
@@ -1854,7 +1862,12 @@ static int reflog_append(refdb_fs_backend *backend, const git_reference *ref, co
 			goto cleanup;
 	}
 
-	error = git_futils_writebuffer(&buf, git_buf_cstr(&path), O_WRONLY|O_CREAT|O_APPEND, GIT_REFLOG_FILE_MODE);
+	open_flags = O_WRONLY | O_CREAT | O_APPEND;
+
+	if (backend->fsync)
+		open_flags |= O_FSYNC;
+
+	error = git_futils_writebuffer(&buf, git_buf_cstr(&path), open_flags, GIT_REFLOG_FILE_MODE);
 
 cleanup:
 	git_buf_free(&buf);
@@ -2011,6 +2024,9 @@ int git_refdb_backend_fs(
 		backend->iterator_flags |= GIT_ITERATOR_PRECOMPOSE_UNICODE;
 		backend->direach_flags  |= GIT_PATH_DIR_PRECOMPOSE_UNICODE;
 	}
+	if ((!git_repository__cvar(&t, backend->repo, GIT_CVAR_FSYNCOBJECTFILES) && t) ||
+		git_object__synchronous_writing)
+		backend->fsync = 1;
 
 	backend->parent.exists = &refdb_fs_backend__exists;
 	backend->parent.lookup = &refdb_fs_backend__lookup;
diff --git a/src/repository.c b/src/repository.c
index 0db4816..425ef79 100644
--- a/src/repository.c
+++ b/src/repository.c
@@ -1055,18 +1055,22 @@ int git_repository_odb__weakptr(git_odb **out, git_repository *repo)
 		git_odb *odb;
 
 		if ((error = git_repository_item_path(&odb_path, repo,
-				GIT_REPOSITORY_ITEM_OBJECTS)) < 0)
+				GIT_REPOSITORY_ITEM_OBJECTS)) < 0 ||
+			(error = git_odb_new(&odb)) < 0)
 			return error;
 
-		error = git_odb_open(&odb, odb_path.ptr);
-		if (!error) {
-			GIT_REFCOUNT_OWN(odb, repo);
+		GIT_REFCOUNT_OWN(odb, repo);
 
-			odb = git__compare_and_swap(&repo->_odb, NULL, odb);
-			if (odb != NULL) {
-				GIT_REFCOUNT_OWN(odb, NULL);
-				git_odb_free(odb);
-			}
+		if ((error = git_odb__set_caps(odb, GIT_ODB_CAP_FROM_OWNER)) < 0 ||
+			(error = git_odb__add_default_backends(odb, odb_path.ptr, 0, 0)) < 0) {
+			git_odb_free(odb);
+			return error;
+		}
+
+		odb = git__compare_and_swap(&repo->_odb, NULL, odb);
+		if (odb != NULL) {
+			GIT_REFCOUNT_OWN(odb, NULL);
+			git_odb_free(odb);
 		}
 
 		git_buf_free(&odb_path);
diff --git a/src/repository.h b/src/repository.h
index c328ecd..33adfa6 100644
--- a/src/repository.h
+++ b/src/repository.h
@@ -46,6 +46,7 @@ typedef enum {
 	GIT_CVAR_LOGALLREFUPDATES, /* core.logallrefupdates */
 	GIT_CVAR_PROTECTHFS,    /* core.protectHFS */
 	GIT_CVAR_PROTECTNTFS,   /* core.protectNTFS */
+	GIT_CVAR_FSYNCOBJECTFILES, /* core.fsyncObjectFiles */
 	GIT_CVAR_CACHE_MAX
 } git_cvar_cached;
 
@@ -106,6 +107,8 @@ typedef enum {
 	GIT_PROTECTHFS_DEFAULT = GIT_CVAR_FALSE,
 	/* core.protectNTFS */
 	GIT_PROTECTNTFS_DEFAULT = GIT_CVAR_FALSE,
+	/* core.fsyncObjectFiles */
+	GIT_FSYNCOBJECTFILES_DEFAULT = GIT_CVAR_FALSE,
 } git_cvar_value;
 
 /* internal repository init flags */
diff --git a/src/settings.c b/src/settings.c
index 2158567..24e549e 100644
--- a/src/settings.c
+++ b/src/settings.c
@@ -227,6 +227,10 @@ int git_libgit2_opts(int key, ...)
 		git_smart__ofs_delta_enabled = (va_arg(ap, int) != 0);
 		break;
 
+	case GIT_OPT_ENABLE_SYNCHRONOUS_OBJECT_CREATION:
+		git_object__synchronous_writing = (va_arg(ap, int) != 0);
+		break;
+
 	default:
 		giterr_set(GITERR_INVALID, "invalid option key");
 		error = -1;
diff --git a/src/unix/posix.h b/src/unix/posix.h
index b478640..52985fd 100644
--- a/src/unix/posix.h
+++ b/src/unix/posix.h
@@ -40,9 +40,14 @@ typedef int GIT_SOCKET;
 #define p_link(o,n) link(o, n)
 #define p_unlink(p) unlink(p)
 #define p_mkdir(p,m) mkdir(p, m)
-#define p_fsync(fd) fsync(fd)
 extern char *p_realpath(const char *, char *);
 
+GIT_INLINE(int) p_fsync(int fd)
+{
+	p_fsync__cnt++;
+	return fsync(fd);
+}
+
 #define p_recv(s,b,l,f) recv(s,b,l,f)
 #define p_send(s,b,l,f) send(s,b,l,f)
 #define p_inet_pton(a, b, c) inet_pton(a, b, c)
diff --git a/src/win32/posix_w32.c b/src/win32/posix_w32.c
index fea634b..5172627 100644
--- a/src/win32/posix_w32.c
+++ b/src/win32/posix_w32.c
@@ -113,6 +113,8 @@ int p_fsync(int fd)
 {
 	HANDLE fh = (HANDLE)_get_osfhandle(fd);
 
+	p_fsync__cnt++;
+
 	if (fh == INVALID_HANDLE_VALUE) {
 		errno = EBADF;
 		return -1;
diff --git a/tests/odb/loose.c b/tests/odb/loose.c
index c91927c..dd686aa 100644
--- a/tests/odb/loose.c
+++ b/tests/odb/loose.c
@@ -3,6 +3,7 @@
 #include "git2/odb_backend.h"
 #include "posix.h"
 #include "loose_data.h"
+#include "repository.h"
 
 #ifdef __ANDROID_API__
 # define S_IREAD        S_IRUSR
@@ -56,11 +57,13 @@ static void test_read_object(object_data *data)
 
 void test_odb_loose__initialize(void)
 {
+	p_fsync__cnt = 0;
 	cl_must_pass(p_mkdir("test-objects", GIT_OBJECT_DIR_MODE));
 }
 
 void test_odb_loose__cleanup(void)
 {
+	cl_git_pass(git_libgit2_opts(GIT_OPT_ENABLE_SYNCHRONOUS_OBJECT_CREATION, 0));
 	cl_fixture_cleanup("test-objects");
 }
 
@@ -150,3 +153,55 @@ void test_odb_loose__permissions_readwrite(void)
 {
 	test_write_object_permission(0777, 0666, 0777, 0666);
 }
+
+static void write_object_to_loose_odb(int fsync)
+{
+	git_odb *odb;
+	git_odb_backend *backend;
+	git_oid oid;
+
+	cl_git_pass(git_odb_new(&odb));
+	cl_git_pass(git_odb_backend_loose(&backend, "test-objects", -1, fsync, 0777, 0666));
+	cl_git_pass(git_odb_add_backend(odb, backend, 1));
+	cl_git_pass(git_odb_write(&oid, odb, "Test data\n", 10, GIT_OBJ_BLOB));
+	git_odb_free(odb);
+}
+
+void test_odb_loose__does_not_fsync_by_default(void)
+{
+	write_object_to_loose_odb(0);
+	cl_assert_equal_sz(0, p_fsync__cnt);
+}
+
+void test_odb_loose__fsync_obeys_odb_option(void)
+{
+	write_object_to_loose_odb(1);
+	cl_assert(p_fsync__cnt > 0);
+}
+
+void test_odb_loose__fsync_obeys_global_setting(void)
+{
+	cl_git_pass(git_libgit2_opts(GIT_OPT_ENABLE_SYNCHRONOUS_OBJECT_CREATION, 1));
+	write_object_to_loose_odb(0);
+	cl_assert(p_fsync__cnt > 0);
+}
+
+void test_odb_loose__fsync_obeys_repo_setting(void)
+{
+	git_repository *repo;
+	git_odb *odb;
+	git_oid oid;
+
+	cl_git_pass(git_repository_init(&repo, "test-objects", 1));
+	cl_git_pass(git_repository_odb__weakptr(&odb, repo));
+	cl_git_pass(git_odb_write(&oid, odb, "No fsync here\n", 14, GIT_OBJ_BLOB));
+	cl_assert(p_fsync__cnt == 0);
+	git_repository_free(repo);
+
+	cl_git_pass(git_repository_open(&repo, "test-objects"));
+	cl_repo_set_bool(repo, "core.fsyncObjectFiles", true);
+	cl_git_pass(git_repository_odb__weakptr(&odb, repo));
+	cl_git_pass(git_odb_write(&oid, odb, "Now fsync\n", 10, GIT_OBJ_BLOB));
+	cl_assert(p_fsync__cnt > 0);
+	git_repository_free(repo);
+}
diff --git a/tests/pack/packbuilder.c b/tests/pack/packbuilder.c
index 29f3e2d..1d7bece 100644
--- a/tests/pack/packbuilder.c
+++ b/tests/pack/packbuilder.c
@@ -23,6 +23,7 @@ void test_pack_packbuilder__initialize(void)
 	cl_git_pass(git_vector_init(&_commits, 0, NULL));
 	_commits_is_initialized = 1;
 	memset(&_stats, 0, sizeof(_stats));
+	p_fsync__cnt = 0;
 }
 
 void test_pack_packbuilder__cleanup(void)
@@ -30,6 +31,8 @@ void test_pack_packbuilder__cleanup(void)
 	git_oid *o;
 	unsigned int i;
 
+	cl_git_pass(git_libgit2_opts(GIT_OPT_ENABLE_SYNCHRONOUS_OBJECT_CREATION, 0));
+
 	if (_commits_is_initialized) {
 		_commits_is_initialized = 0;
 		git_vector_foreach(&_commits, i, o) {
@@ -188,6 +191,40 @@ void test_pack_packbuilder__permissions_readwrite(void)
 	test_write_pack_permission(0666, 0666);
 }
 
+void test_pack_packbuilder__does_not_fsync_by_default(void)
+{
+	seed_packbuilder();
+	git_packbuilder_write(_packbuilder, ".", 0666, NULL, NULL);
+	cl_assert_equal_sz(0, p_fsync__cnt);
+}
+
+/* We fsync the packfile and index.  On non-Windows, we also fsync
+ * the parent directories.
+ */
+#ifdef GIT_WIN32
+static int expected_fsyncs = 2;
+#else
+static int expected_fsyncs = 4;
+#endif
+
+void test_pack_packbuilder__fsync_global_setting(void)
+{
+	cl_git_pass(git_libgit2_opts(GIT_OPT_ENABLE_SYNCHRONOUS_OBJECT_CREATION, 1));
+	p_fsync__cnt = 0;
+	seed_packbuilder();
+	git_packbuilder_write(_packbuilder, ".", 0666, NULL, NULL);
+	cl_assert_equal_sz(expected_fsyncs, p_fsync__cnt);
+}
+
+void test_pack_packbuilder__fsync_repo_setting(void)
+{
+	cl_repo_set_bool(_repo, "core.fsyncObjectFiles", true);
+	p_fsync__cnt = 0;
+	seed_packbuilder();
+	git_packbuilder_write(_packbuilder, ".", 0666, NULL, NULL);
+	cl_assert_equal_sz(expected_fsyncs, p_fsync__cnt);
+}
+
 static int foreach_cb(void *buf, size_t len, void *payload)
 {
 	git_indexer *idx = (git_indexer *) payload;
diff --git a/tests/refs/create.c b/tests/refs/create.c
index aca808c..4ecc605 100644
--- a/tests/refs/create.c
+++ b/tests/refs/create.c
@@ -13,6 +13,7 @@ static git_repository *g_repo;
 void test_refs_create__initialize(void)
 {
 	g_repo = cl_git_sandbox_init("testrepo");
+	p_fsync__cnt = 0;
 }
 
 void test_refs_create__cleanup(void)
@@ -21,6 +22,7 @@ void test_refs_create__cleanup(void)
 
 	cl_git_pass(git_libgit2_opts(GIT_OPT_ENABLE_STRICT_OBJECT_CREATION, 1));
 	cl_git_pass(git_libgit2_opts(GIT_OPT_ENABLE_STRICT_SYMBOLIC_REF_CREATION, 1));
+	cl_git_pass(git_libgit2_opts(GIT_OPT_ENABLE_SYNCHRONOUS_OBJECT_CREATION, 0));
 }
 
 void test_refs_create__symbolic(void)
@@ -297,3 +299,69 @@ void test_refs_create__creating_a_loose_ref_with_invalid_windows_name(void)
 
 	test_win32_name("refs/heads/com1");
 }
+
+/* Creating a loose ref involves fsync'ing the reference, the
+ * reflog and (on non-Windows) the containing directories.
+ * Creating a packed ref involves fsync'ing the packed ref file
+ * and (on non-Windows) the containing directory.
+ */
+#ifdef GIT_WIN32
+static int expected_fsyncs_create = 2, expected_fsyncs_compress = 1;
+#else
+static int expected_fsyncs_create = 4, expected_fsyncs_compress = 2;
+#endif
+
+static void count_fsyncs(size_t *create_count, size_t *compress_count)
+{
+	git_reference *ref = NULL;
+	git_refdb *refdb;
+	git_oid id;
+
+	p_fsync__cnt = 0;
+
+	git_oid_fromstr(&id, current_master_tip);
+	cl_git_pass(git_reference_create(&ref, g_repo, "refs/heads/fsync_test", &id, 0, "log message"));
+	git_reference_free(ref);
+
+	*create_count = p_fsync__cnt;
+	p_fsync__cnt = 0;
+
+	cl_git_pass(git_repository_refdb(&refdb, g_repo));
+	cl_git_pass(git_refdb_compress(refdb));
+	git_refdb_free(refdb);
+
+	*compress_count = p_fsync__cnt;
+	p_fsync__cnt = 0;
+}
+
+void test_refs_create__does_not_fsync_by_default(void)
+{
+	size_t create_count, compress_count;
+	count_fsyncs(&create_count, &compress_count);
+
+	cl_assert_equal_i(0, create_count);
+	cl_assert_equal_i(0, compress_count);
+}
+
+void test_refs_create__fsyncs_when_global_opt_set(void)
+{
+	size_t create_count, compress_count;
+
+	cl_git_pass(git_libgit2_opts(GIT_OPT_ENABLE_SYNCHRONOUS_OBJECT_CREATION, 1));
+	count_fsyncs(&create_count, &compress_count);
+
+	cl_assert_equal_i(expected_fsyncs_create, create_count);
+	cl_assert_equal_i(expected_fsyncs_compress, compress_count);
+}
+
+void test_refs_create__fsyncs_when_repo_config_set(void)
+{
+	size_t create_count, compress_count;
+
+	cl_repo_set_bool(g_repo, "core.fsyncObjectFiles", true);
+
+	count_fsyncs(&create_count, &compress_count);
+
+	cl_assert_equal_i(expected_fsyncs_create, create_count);
+	cl_assert_equal_i(expected_fsyncs_compress, compress_count);
+}