Remove call to IsRunningSequenceOnCurrentThread() in media_file_system_backend.cc

chrome/browser/media_galleries/fileapi/media_file_system_backend.cc

Index: chrome/browser/media_galleries/fileapi/media_file_system_backend.cc
diff --git a/chrome/browser/media_galleries/fileapi/media_file_system_backend.cc b/chrome/browser/media_galleries/fileapi/media_file_system_backend.cc
index 69bd2adfb18b6808aa295b0f6e5c3c0ce1b55c6c..54179adbe89775b57e478465e8588540d0d5dffc 100644
--- a/chrome/browser/media_galleries/fileapi/media_file_system_backend.cc
+++ b/chrome/browser/media_galleries/fileapi/media_file_system_backend.cc
@@ -10,6 +10,7 @@
 
 #include "base/bind.h"
 #include "base/files/file_path.h"
+#include "base/lazy_instance.h"
 #include "base/logging.h"
 #include "base/sequenced_task_runner.h"
 #include "base/strings/string_number_conversions.h"
@@ -57,6 +58,11 @@ namespace {
 
 const char kMediaGalleryMountPrefix[] = "media_galleries-";
 
+#if DCHECK_IS_ON()
+base::LazyInstance<base::SequenceChecker>::Leaky g_media_sequence_checker =
+    LAZY_INSTANCE_INITIALIZER;
+#endif
+
 void OnPreferencesInit(
     const content::ResourceRequestInfo::WebContentsGetter& web_contents_getter,
     const extensions::Extension* extension,
@@ -150,11 +156,10 @@ MediaFileSystemBackend::~MediaFileSystemBackend() {
 }
 
 // static
-bool MediaFileSystemBackend::CurrentlyOnMediaTaskRunnerThread() {
-  base::SequencedWorkerPool* pool = content::BrowserThread::GetBlockingPool();
-  base::SequencedWorkerPool::SequenceToken media_sequence_token =
-      pool->GetNamedSequenceToken(kMediaTaskRunnerName);
-  return pool->IsRunningSequenceOnCurrentThread(media_sequence_token);
+void MediaFileSystemBackend::AssertCurrentlyOnMediaSequence() {
+#if DCHECK_IS_ON()
+  DCHECK(g_media_sequence_checker.Get().CalledOnValidSequence());

[Reilly Grant (use Gerrit), 2016/09/07 18:07:05]

I don't like consolidating the DCHECK into a single place because it means the
line number reported is not as useful.

[fdoray, 2016/09/07 19:21:51]

I don't want a public CurrentlyOnMediaTaskRunnerThread() method that relies on
SequenceChecker because people could be tempted to use it outside of DCHECKs.
e.g.:
  if (MediaFileSystemBackend::CurrentlyOnMediaTaskRunnerThread())
    ThisIsAlwaysCalledInANonDCHECKBuild();

Since DCHECK prints a stack trace, does this CL really makes it harder to
understand reported errors?

Potential solutions:
1. Keep this CL as-is.
2. Have a CurrentlyOnMediaSequence() method based on SequenceChecker - with a
comment that says that it shouldn't be used outside of DCHECKs since it always
returns true in non-DCHECK builds.
3. Have a GetMediaSequenceChecker() method that returns a SequenceChecker. I
believe that people are familiar with the fact that SequenceChecker is a no-op
in non-DCHECK builds and won't try to do if
(MediaFileSystemBackend::GetMediaSequenceChecker().CalledOnValidSequence()) {
... }

[Reilly Grant (use Gerrit), 2016/09/07 20:58:55]

I prefer option 2. Stack traces often do not include line numbers.

+#endif
 }
 
 // static