Add Locking calls to file_io.h plus implementations and test

util/file/file_io_posix.cc

Index: util/file/file_io_posix.cc
diff --git a/util/file/file_io_posix.cc b/util/file/file_io_posix.cc
index 60bc9a8daaac1473a1f49c01f76433c77cc7b7e7..76454b235ad6b93773188ad17bdec9a29b3bf7d3 100644
--- a/util/file/file_io_posix.cc
+++ b/util/file/file_io_posix.cc
@@ -15,6 +15,7 @@
 #include "util/file/file_io.h"
 
 #include <fcntl.h>
+#include <sys/file.h>
 #include <unistd.h>
 
 #include "base/files/file_path.h"
@@ -100,6 +101,19 @@ FileHandle LoggingOpenFileForWrite(const base::FilePath& path,
   return fd;
 }
 
+bool LoggingLockFile(FileHandle file, FileLocking locking) {
+  int operation = locking == FileLocking::kShared ? LOCK_SH : LOCK_EX;

[Robert Sesek, 2015/03/12 15:28:03]

nit: () around the condition

[scottmg, 2015/03/19 22:06:19]

Done.

+  int rv = HANDLE_EINTR(flock(file, operation));

[Mark Mentovai, 2015/03/12 04:47:42]

Since we’re moving away from the BSD-specific O_SHLOCK/O_EXLOCK, we should also
move away from the BSD-derived flock() interface altogether, and use the more
reliable, well-specified, and portable POSIX fcntl() F_SETLKW/F_UNLCK lock
interface.

The differences are minor on Mac OS X and BSDs, but on other systems including
Linux, fcntl() locks are better-supported.

One major difference is that flock() support over NFS varies from nonexistent to
spotty. The interface with the best shot at working over NFS is the fcntl()
lock.

[scottmg, 2015/03/19 22:06:19]

I tried to do this, but I haven't been able to get exclusive fcntl locks to
work. The #if 0'd code doesn't work, the #else code does, but I've stared at
`man fcntl` for a while now and I'm not sure why. Can either of you enlighten
me?

[Mark Mentovai, 2015/03/20 15:03:54]

I think you got it right, but it seems that fcntl() locks belong to the process,
not the fd.

Oh well.

[scottmg, 2015/03/20 21:07:34]

Ah, yes. It does repeatedly say process in the docs, darn. I'm going to go back
to flock for now then.

+  PLOG_IF(ERROR, rv < 0) << "flock";
+  return rv == 0;
+}
+
+bool LoggingUnlockFile(FileHandle file) {
+  int rv = HANDLE_EINTR(flock(file, LOCK_UN));
+  PLOG_IF(ERROR, rv < 0) << "flock";
+  return rv == 0;
+}
+
 FileOffset LoggingSeekFile(FileHandle file, FileOffset offset, int whence) {
   off_t rv = lseek(file, offset, whence);
   PLOG_IF(ERROR, rv < 0) << "lseek";