Add an "append flag" to base::PlatformFile.

base/platform_file_posix.cc

 // Copyright (c) 2012 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.
 
 #include "base/platform_file.h"
 
 #include <fcntl.h>
 #include <errno.h>
 #include <sys/stat.h>
 #include <unistd.h>
@@ skipping 63 matching lines @@
       *error = PLATFORM_FILE_ERROR_FAILED;
     return kInvalidPlatformFileValue;
   }
 
   if (flags & PLATFORM_FILE_WRITE && flags & PLATFORM_FILE_READ) {
     open_flags |= O_RDWR;
   } else if (flags & PLATFORM_FILE_WRITE) {
     open_flags |= O_WRONLY;
   } else if (!(flags & PLATFORM_FILE_READ) &&
              !(flags & PLATFORM_FILE_WRITE_ATTRIBUTES) &&
+             !(flags & PLATFORM_FILE_APPEND) &&
              !(flags & PLATFORM_FILE_OPEN_ALWAYS)) {
     NOTREACHED();
   }
 
   if (flags & PLATFORM_FILE_TERMINAL_DEVICE)
     open_flags |= O_NOCTTY | O_NDELAY;
 
+  if (flags & PLATFORM_FILE_APPEND && flags & PLATFORM_FILE_READ)
+    open_flags |= O_APPEND | O_RDWR;
+  else if (flags & PLATFORM_FILE_APPEND)
+    open_flags |= O_APPEND | O_WRONLY;
+
   COMPILE_ASSERT(O_RDONLY == 0, O_RDONLY_must_equal_zero);
 
   int mode = S_IRUSR | S_IWUSR;
 #if defined(OS_CHROMEOS)
   mode |= S_IRGRP | S_IROTH;
 #endif
 
   int descriptor =
       HANDLE_EINTR(open(name.value().c_str(), open_flags, mode));
 
@@ skipping 228 matching lines @@
     default:
 #if !defined(OS_NACL)  // NaCl build has no metrics code.
       UMA_HISTOGRAM_SPARSE_SLOWLY("PlatformFile.UnknownErrors.Posix",
                                   saved_errno);
 #endif
       return PLATFORM_FILE_ERROR_FAILED;
   }
 }
 
 }  // namespace base