Fix PP_FileOpenFlags_Dev handling:

base/platform_file_posix.cc

Index: base/platform_file_posix.cc
diff --git a/base/platform_file_posix.cc b/base/platform_file_posix.cc
index 2ffac06456aac130f20622b0547319fec99bf3f1..64070f896d071df6150e540e4c23d95757acc133 100644
--- a/base/platform_file_posix.cc
+++ b/base/platform_file_posix.cc
@@ -44,6 +44,12 @@ PlatformFile CreatePlatformFile(const FilePath& name, int flags,
     open_flags = O_CREAT | O_TRUNC;
   }
 
+  if (flags & PLATFORM_FILE_TRUNCATE) {
+    DCHECK(!open_flags);

[darin (slow to review), 2011/05/20 21:02:13]

so this means that it is not possible to specify PLATFORM_FILE_CREATE and
PLATFORM_FILE_TRUNCATE at the same time.  but, POSIX allows you to specify
O_CREAT and O_TRUNC together.  while it may seem redundant to indicate that a
file should be truncated when created (since the file is 0 length anyways), i
don't see the harm in doing so.

[yzshen1, 2011/05/20 21:29:24]

I think PlatformFileFlags are probably designed to look similar to Win flags.
As a result, the followings have an 1-1 mapping to Win flags:
PLATFORM_FILE_OPEN <-> OPEN_EXISTING
PLATFORM_FILE_CREATE <-> CREATE_NEW
PLATFORM_FILE_OPEN_ALWAYS <-> OPEN_ALWAYS
PLATFORM_FILE_CREATE_ALWAYS <-> CREATE_ALWAYS
PLATFORM_FILE_TRUNCATE <-> TRUNCATE_EXISTING
And platform_file_win enforces that these five PlatformFileFlags cannot be used
together with each other.
It would be good to be consistent.

The way to specify O_CREAT | O_TRUNC is PLATFORM_FILE_CREATE_ALWAYS.

+    DCHECK(flags & PLATFORM_FILE_WRITE);
+    open_flags = O_TRUNC;
+  }
+
   if (!open_flags && !(flags & PLATFORM_FILE_OPEN) &&

[darin (slow to review), 2011/05/20 21:02:13]

it looks like you might be breaking this check, which was trying to assert that
at least one of PLATFORM_FILE_CREATE, PLATFORM_FILE_CREATE_ALWAYS,
PLATFORM_FILE_OPEN, or PLATFORM_FILE_OPEN_ALWAYS was specified.  with your
change above, it is possible for PLATFORM_FILE_TRUNCATE to be specified without
having specified any of those other 4 flags.  is that really what you intended?

i always thought that truncate should be a modifier for opening an existing
file.

[yzshen1, 2011/05/20 21:29:24]

This is what I have described in the CL description:
CreatePlatformFile: resolve the conflict between the win and posix
implementation. Before this change, the win implementation didn't allow
PLATFORM_FILE_TRUNCATE to be used with any of the (OPEN|CREATE)(_ALWAYS)? flags;
while the posix implementation required it to be used with them.

I don't have a strong opinion about which way is better. But we have to keep the
win and posix implementation consistent.

       !(flags & PLATFORM_FILE_OPEN_ALWAYS)) {
     NOTREACHED();
@@ -62,11 +68,6 @@ PlatformFile CreatePlatformFile(const FilePath& name, int flags,
     NOTREACHED();
   }
 
-  if (flags & PLATFORM_FILE_TRUNCATE) {
-    DCHECK(flags & PLATFORM_FILE_WRITE);
-    open_flags |= O_TRUNC;
-  }
-
   COMPILE_ASSERT(O_RDONLY == 0, O_RDONLY_must_equal_zero);
 
   int descriptor =