Transition POSIX shmem to use lockf(), not semaphores.

base/shared_memory.h

 // Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_SHARED_MEMORY_H_
 #define BASE_SHARED_MEMORY_H_
 
 #include "build/build_config.h"
 
 #if defined(OS_POSIX)
 #include <semaphore.h>
 #endif
 #include <string>
 
 #include "base/basictypes.h"
 #include "base/process.h"
 
 namespace base {
 
 // SharedMemoryHandle is a platform specific type which represents
 // the underlying OS handle to a shared memory segment.
 #if defined(OS_WIN)
 typedef HANDLE SharedMemoryHandle;
 typedef HANDLE SharedMemoryLock;
 #elif defined(OS_POSIX)
 typedef int SharedMemoryHandle;
-// TODO(port): these semaphores can leak if we crash, causing
-// autobuilder problems.  Transition to something easier to clean up
-// (e.g. lockf/flock).
-// TODO(port): make sure what we transition to is fast enough.
-typedef sem_t* SharedMemoryLock;
+// On POSIX, the lock is implemented as a lockf() on the mapped file,
+// so no additional member (or definition of SharedMemoryLock) is
+// needed.
 #endif
 
 // Platform abstraction for shared memory.  Provides a C++ wrapper
 // around the OS primitive for a memory mapped file.
 class SharedMemory {
  public:
   // Create a new SharedMemory object.
   SharedMemory();
 
   // Create a new SharedMemory object from an existing, open
@@ skipping 74 matching lines @@
   //   Close();
   //   return ok;
   bool GiveToProcess(ProcessHandle process,
                      SharedMemoryHandle* new_handle) {
     return ShareToProcessCommon(process, new_handle, true);
   }
 
   // Lock the shared memory.
   // This is a cross-process lock which may be recursively
   // locked by the same thread.
+  // TODO(port):
+  // WARNING: on POSIX the lock only works across processes, not
+  // across threads.  2 threads in the same process can both grab the
+  // lock at the same time.  There are several solutions for this
+  // (futex, lockf+anon_semaphore) but none are both clean and common
+  // across Mac and Linux.
   void Lock();
 
   // Release the shared memory lock.
   void Unlock();
 
  private:
 #if defined(OS_POSIX)
   bool CreateOrOpen(const std::wstring &name, int posix_flags, size_t size);
   bool FilenameForMemoryName(const std::wstring &memname,
                              std::wstring *filename);
+  void LockOrUnlockCommon(int function);
+
 #endif
   bool ShareToProcessCommon(ProcessHandle process,
                             SharedMemoryHandle* new_handle,
                             bool close_self);
 
   std::wstring       name_;
   SharedMemoryHandle mapped_file_;
   void*              memory_;
   bool               read_only_;
   size_t             max_size_;
+#if !defined(OS_POSIX)
   SharedMemoryLock   lock_;
-#if defined(OS_POSIX)
-  std::string        sem_name_;
 #endif
 
   DISALLOW_EVIL_CONSTRUCTORS(SharedMemory);
 };
 
 // A helper class that acquires the shared memory lock while
 // the SharedMemoryAutoLock is in scope.
 class SharedMemoryAutoLock {
  public:
   explicit SharedMemoryAutoLock(SharedMemory* shared_memory)
       : shared_memory_(shared_memory) {
     shared_memory_->Lock();
   }
 
   ~SharedMemoryAutoLock() {
     shared_memory_->Unlock();
   }
 
  private:
   SharedMemory* shared_memory_;
   DISALLOW_EVIL_CONSTRUCTORS(SharedMemoryAutoLock);
 };
 
 }  // namespace base
 
 #endif  // BASE_SHARED_MEMORY_H_