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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