thodg/libgit2/src/global.h

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• Hash : 5c6180b5
  Author :
  Date : 2019-11-29T11:06:11
  

  global: convert to fiber-local storage to fix exit races
  
  On Windows platforms, we automatically clean up the thread-local storage
  upon detaching a thread via `DllMain()`. The thing is that this happens
  for every thread of applications that link against the libgit2 DLL, even
  those that don't have anything to do with libgit2 itself. As a result,
  we cannot assume that these unsuspecting threads make use of our
  `git_libgit2_init()` and `git_libgit2_shutdow()` reference counting,
  which may lead to racy situations:
  
      Thread 1                    Thread 2
  
      git_libgit2_shutdown()
                                  DllMain(DETACH_THREAD)
                                  git__free_tls_data()
      git_atomic_dec() == 0
      git__free_tls_data()
      TlsFree(_tls_index)
                                  TlsGetValue(_tls_index)
  
  Due to the second thread never having executed `git_libgit2_init()`, the
  first thread will clean up TLS data and as a result also free the
  `_tls_index` variable. When detaching the second thread, we
  unconditionally access the now-free'd `_tls_index` variable, which is
  obviously not going to work out well.
  
  Fix the issue by converting the code to use fiber-local storage instead
  of thread-local storage. While FLS will behave the exact same as TLS if
  no fibers are in use, it does allow us to specify a destructor similar
  to the one that is accepted by pthread_key_create(3P). Like this, we do
  not have to manually free indices anymore, but will let the FLS handle
  calling the destructor. This allows us to get rid of `DllMain()`
  completely, as we only used it to keep track of when threads were
  exiting and results in an overall simplification of TLS cleanup.
  

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• src/global.h

• /*
   * 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_global_h__
  #define INCLUDE_global_h__
  
  #include "common.h"
  
  #include "mwindow.h"
  #include "hash.h"
  
  typedef struct {
  	git_error *last_error;
  	git_error error_t;
  	git_buf error_buf;
  	char oid_fmt[GIT_OID_HEXSZ+1];
  
  	/* On Windows, this is the current child thread that was started by
  	 * `git_thread_create`.  This is used to set the thread's exit code
  	 * when terminated by `git_thread_exit`.  It is unused on POSIX.
  	 */
  	git_thread *current_thread;
  } git_global_st;
  
  git_global_st *git__global_state(void);
  
  extern git_mutex git__mwindow_mutex;
  
  #define GIT_GLOBAL (git__global_state())
  
  typedef void (*git_global_shutdown_fn)(void);
  
  extern void git__on_shutdown(git_global_shutdown_fn callback);
  
  extern const char *git_libgit2__user_agent(void);
  extern const char *git_libgit2__ssl_ciphers(void);
  
  #endif
  

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