Make base:ReplaceFile return an informative error.

base/platform_file.h

Index: base/platform_file.h
diff --git a/base/platform_file.h b/base/platform_file.h
index 43758c51cee035771fe267ff8b7b57cd773cd828..7d6408f5efe93e47e18e676415c406b5cd9727f6 100644
--- a/base/platform_file.h
+++ b/base/platform_file.h
@@ -204,6 +204,12 @@ BASE_EXPORT bool TouchPlatformFile(PlatformFile file,
 // Returns some information for the given file.
 BASE_EXPORT bool GetPlatformFileInfo(PlatformFile file, PlatformFileInfo* info);
 
+#if defined(OS_WIN)
+PlatformFileError LastErrorToPlatformFileError(DWORD saved_errno);
+#elif defined(OS_POSIX)
+PlatformFileError ErrnoToPlatformFileError(int saved_errno);
+#endif

[jar (doing other things), 2013/05/07 01:12:36]

Who else uses these? Why do they need to be visible in the common header?  Why
wouldn't a caller that needed such include a platform specific header?

[dgrogan, 2013/05/07 19:02:05]

platform_file_{win,posix}.cc and file_util_{win,posix}.cc use them.

Why do they need to be visible in the common header?  Why
I think a platform-specific header would be fine. Is there an appropriate
existing header? Or should I create platform_file_win.h and
platform_file_posix.h?

[jar (doing other things), 2013/05/07 19:44:50]

If there are shared usage of these platform specific methods, then it is
reasonable to put them in platform specific header (perhaps as you suggested
mirroring the platform specific source).  I guess if this is the only thing in
that header, the above format would be OK.... but...

I didn't see anyone else calling these though.  Who else is calling them? Why
are they needed in a header?

[dgrogan, 2013/05/07 21:24:06]

As above (I suspect you just didn't see that comment last time around) file_util
and platform_file use them:
https://codereview.chromium.org/14886003/diff/1002/base/file_util_posix.cc#ne...
https://codereview.chromium.org/14886003/diff/1002/base/platform_file_posix.c...

If you DID see that comment, and know that both of those places use those
methods, then I am confused. They are in platform_file.h so that
file_util_{win,posix} can call them. platform_file_{win,posix} also calls it. Am
I missing something fundamental?

[jar (doing other things), 2013/05/07 21:39:02]

OK.  I didn't spot the uses.  Sorry to have missed your comment.

Is it worth moving these into the ifdef on line 22?  I don't think there is a
style rule, and seeing the related typdefs nearby probably makes this a tad
easier to read.

[dgrogan, 2013/05/07 21:44:43]

Heh, that's where I tried to put them originally. The problem is the functions
return PlatformFileError, which isn't defined until line 65.

[jar (doing other things), 2013/05/07 22:31:32]

How about splitting the difference... and uniting the ifdefs up before line 121,
which may be the first users of the typdefs seen on line 22?

(personal nit): The fewer conditional ifdef contexts my mind has to go
in-and-out of in a read, the better.

[dgrogan, 2013/05/07 22:45:16]

Done.

+
 // Use this class to pass ownership of a PlatformFile to a receiver that may or
 // may not want to accept it.  This class does not own the storage for the
 // PlatformFile.