Fix threading bug in the brlapi braille controller

chrome/browser/extensions/api/braille_display_private/braille_controller_brlapi.cc

Index: chrome/browser/extensions/api/braille_display_private/braille_controller_brlapi.cc
diff --git a/chrome/browser/extensions/api/braille_display_private/braille_controller_brlapi.cc b/chrome/browser/extensions/api/braille_display_private/braille_controller_brlapi.cc
index 4ab1194194a5e59fc6eb6abf19ae435f4b03e226..0fe8566b496a3f10c53632a83ef697023ad710af 100644
--- a/chrome/browser/extensions/api/braille_display_private/braille_controller_brlapi.cc
+++ b/chrome/browser/extensions/api/braille_display_private/braille_controller_brlapi.cc
@@ -134,7 +134,7 @@ void BrailleControllerImpl::SetCreateBrlapiConnectionForTesting(
 }
 
 void BrailleControllerImpl::PokeSocketDirForTesting() {
-  OnSocketDirChanged(base::FilePath(), false /*error*/);
+  OnSocketDirChangedOnIOThread();
 }
 
 void BrailleControllerImpl::StartConnecting() {
@@ -146,26 +146,44 @@ void BrailleControllerImpl::StartConnecting() {
   if (!libbrlapi_loader_.loaded()) {
     return;
   }
+  // One could argue that there is a race condition between starting to
+  // watch the socket asynchonously and trying to connect.  While this is true,
+  // we are actually retrying to connect for a while, so this doesn't matter
+  // in practice.
+  BrowserThread::PostTask(
+      BrowserThread::FILE, FROM_HERE, base::Bind(
+          &BrailleControllerImpl::StartWatchingSocketDirOnFileThread,
+          base::Unretained(this)));
+  ResetRetryConnectHorizon();
+  TryToConnect();
+}
+
+void BrailleControllerImpl::StartWatchingSocketDirOnFileThread() {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::FILE));
   base::FilePath brlapi_dir(BRLAPI_SOCKETPATH);
   if (!file_path_watcher_.Watch(
           brlapi_dir, false, base::Bind(
-              &BrailleControllerImpl::OnSocketDirChanged,
+              &BrailleControllerImpl::OnSocketDirChangedOnFileThread,
               base::Unretained(this)))) {
     LOG(WARNING) << "Couldn't watch brlapi directory " << BRLAPI_SOCKETPATH;
-    return;
   }
-  ResetRetryConnectHorizon();
-  TryToConnect();
 }
 
-void BrailleControllerImpl::OnSocketDirChanged(const base::FilePath& path,
-                                               bool error) {
-  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::IO));
-  DCHECK(libbrlapi_loader_.loaded());
+void BrailleControllerImpl::OnSocketDirChangedOnFileThread(
+    const base::FilePath& path, bool error) {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::FILE));
   if (error) {
     LOG(ERROR) << "Error watching brlapi directory: " << path.value();
     return;
   }
+  BrowserThread::PostTask(
+      BrowserThread::IO, FROM_HERE, base::Bind(
+          &BrailleControllerImpl::OnSocketDirChangedOnIOThread,
+          base::Unretained(this)));
+}
+
+void BrailleControllerImpl::OnSocketDirChangedOnIOThread() {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::IO));
   LOG(INFO) << "BrlAPI directory changed";
   // Every directory change resets the max retry time to the appropriate delay
   // into the future.