thodg/libgit2/src/global.c

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• Hash : 4853d94c
  Author :
  Date : 2020-05-14T10:36:35
  

  global: separate global state from thread-local state Our "global initialization" has accumulated some debris over the years. It was previously responsible for both running the various global initializers (that set up various subsystems) _and_ setting up the "global state", which is actually the thread-local state for things like error reporting. Separate the thread local state out into "threadstate". Use the normal subsystem initialization functions that we already have to set it up. This makes both the global initialization system and the threadstate system simpler to reason about.

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• src/global.c

• /*
   * 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.
   */
  
  #include "global.h"
  
  #include "alloc.h"
  #include "threadstate.h"
  #include "hash.h"
  #include "sysdir.h"
  #include "filter.h"
  #include "settings.h"
  #include "mwindow.h"
  #include "merge_driver.h"
  #include "pool.h"
  #include "streams/registry.h"
  #include "streams/mbedtls.h"
  #include "streams/openssl.h"
  #include "thread-utils.h"
  #include "git2/global.h"
  #include "transports/ssh.h"
  #include "win32/w32_stack.h"
  
  typedef int (*git_global_init_fn)(void);
  
  static git_global_init_fn git__init_callbacks[] = {
  	git_allocator_global_init,
  	git_threadstate_global_init,
  	git_threads_global_init,
  	git_hash_global_init,
  	git_sysdir_global_init,
  	git_filter_global_init,
  	git_merge_driver_global_init,
  	git_transport_ssh_global_init,
  	git_stream_registry_global_init,
  	git_openssl_stream_global_init,
  	git_mbedtls_stream_global_init,
  	git_mwindow_global_init,
  	git_pool_global_init,
  	git_settings_global_init
  };
  
  static git_global_shutdown_fn git__shutdown_callbacks[ARRAY_SIZE(git__init_callbacks)];
  
  static git_atomic git__n_shutdown_callbacks;
  static git_atomic git__n_inits;
  
  void git__on_shutdown(git_global_shutdown_fn callback)
  {
  	int count = git_atomic_inc(&git__n_shutdown_callbacks);
  	assert(count <= (int) ARRAY_SIZE(git__shutdown_callbacks) && count > 0);
  	git__shutdown_callbacks[count - 1] = callback;
  }
  
  static int init_common(void)
  {
  	size_t i;
  	int ret;
  
  	/* Initialize subsystems that have global state */
  	for (i = 0; i < ARRAY_SIZE(git__init_callbacks); i++)
  		if ((ret = git__init_callbacks[i]()) != 0)
  			break;
  
  	GIT_MEMORY_BARRIER;
  
  	return ret;
  }
  
  static void shutdown_common(void)
  {
  	int pos;
  
  	/* Shutdown subsystems that have registered */
  	for (pos = git_atomic_get(&git__n_shutdown_callbacks);
  		pos > 0;
  		pos = git_atomic_dec(&git__n_shutdown_callbacks)) {
  
  		git_global_shutdown_fn cb = git__swap(
  			git__shutdown_callbacks[pos - 1], NULL);
  
  		if (cb != NULL)
  			cb();
  	}
  }
  
  /*
   * `git_libgit2_init()` allows subsystems to perform global setup,
   * which may take place in the global scope.  An explicit memory
   * fence exists at the exit of `git_libgit2_init()`.  Without this,
   * CPU cores are free to reorder cache invalidation of `_tls_init`
   * before cache invalidation of the subsystems' newly written global
   * state.
   */
  #if defined(GIT_THREADS) && defined(GIT_WIN32)
  
  /*
   * On Win32, we use a spinlock to provide locking semantics.  This is
   * lighter-weight than a proper critical section.
   */
  static volatile LONG init_spinlock = 0;
  
  GIT_INLINE(int) init_lock(void)
  {
  	while (InterlockedCompareExchange(&init_spinlock, 1, 0)) { Sleep(0); }
  	return 0;
  }
  
  GIT_INLINE(int) init_unlock(void)
  {
  	InterlockedExchange(&init_spinlock, 0);
  	return 0;
  }
  
  #elif defined(GIT_THREADS) && defined(_POSIX_THREADS)
  
  /*
   * On POSIX, we need to use a proper mutex for locking.  We might prefer
   * a spinlock here, too, but there's no static initializer for a
   * pthread_spinlock_t.
   */
  static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
  
  GIT_INLINE(int) init_lock(void)
  {
  	return pthread_mutex_lock(&mutex) == 0 ? 0 : -1;
  }
  
  GIT_INLINE(int) init_unlock(void)
  {
  	return pthread_mutex_unlock(&mutex) == 0 ? 0 : -1;
  }
  
  #elif defined(GIT_THREADS)
  # error unknown threading model
  #else
  
  # define init_lock() 0
  # define init_unlock() 0
  
  #endif
  
  int git_libgit2_init(void)
  {
  	int ret;
  
  	if (init_lock() < 0)
  		return -1;
  
  	/* Only do work on a 0 -> 1 transition of the refcount */
  	if ((ret = git_atomic_inc(&git__n_inits)) == 1) {
  		if (init_common() < 0)
  			ret = -1;
  	}
  
  	if (init_unlock() < 0)
  		return -1;
  
  	return ret;
  }
  
  int git_libgit2_shutdown(void)
  {
  	int ret;
  
  	/* Enter the lock */
  	if (init_lock() < 0)
  		return -1;
  
  	/* Only do work on a 1 -> 0 transition of the refcount */
  	if ((ret = git_atomic_dec(&git__n_inits)) == 0)
  		shutdown_common();
  
  	/* Exit the lock */
  	if (init_unlock() < 0)
  		return -1;
  
  	return ret;
  }
  

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