[base/files] Respect MAC_CHROMIUM_TMPDIR instead of TMPDIR on macOS.

base/files/file_util_mac.mm

 // 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/files/file_util.h"
 
 #import <Foundation/Foundation.h>
 #include <copyfile.h>
 #include <stdlib.h>
 
 #include "base/files/file_path.h"
 #include "base/mac/foundation_util.h"
 #include "base/strings/string_util.h"
 #include "base/threading/thread_restrictions.h"
 
 namespace base {
 
 bool CopyFile(const FilePath& from_path, const FilePath& to_path) {
   ThreadRestrictions::AssertIOAllowed();
   if (from_path.ReferencesParent() || to_path.ReferencesParent())
     return false;
   return (copyfile(from_path.value().c_str(),
                    to_path.value().c_str(), NULL, COPYFILE_DATA) == 0);
 }
 
 bool GetTempDir(base::FilePath* path) {
-  // In order to facilitate hermetic runs on macOS, first check $TMPDIR.
-  // NOTE: $TMPDIR is ALMOST ALWAYS set on macOS (unless the user un-set it).
-  const char* env_tmpdir = getenv("TMPDIR");
+  // In order to facilitate hermetic runs on macOS, first check
+  // $MAC_CHROMIUM_TMPDIR.

[Mark Mentovai, 2017/03/27 20:36:31]

Explain why not TMPDIR here, since it’s a natural question to ask, and our kids
shouldn’t have to spelunk through file history to get the answer.

+  const char* env_tmpdir = getenv("MAC_CHROMIUM_TMPDIR");

[iannucci, 2017/03/25 00:45:04]

I don't have strong feelings about this name.. if someone else does, I'm happy
to change it.

[Mark Mentovai, 2017/03/27 20:36:31]

Maybe [D]CHECK that the length is within a reasonable range here, to avoid
hard-to-diagnose problems like bug 698759.

   if (env_tmpdir) {
     *path = base::FilePath(env_tmpdir);
     return true;
   }
 
   // If we didn't find it, fall back to the native function.
   NSString* tmp = NSTemporaryDirectory();
   if (tmp == nil)
     return false;
   *path = base::mac::NSStringToFilePath(tmp);
@@ skipping 11 matching lines @@
   // Fall back on temp dir if no home directory is defined.
   FilePath rv;
   if (GetTempDir(&rv))
     return rv;
 
   // Last resort.
   return FilePath("/tmp");
 }
 
 }  // namespace base