Bug 1475612: Fix double file close on background thread. r=erahm

LSBUtils closes a file descriptor twice, once with fclose and then again with
close. It also does this on a background thread, during startup, which means
it tends to race with main thread code which opens files.

This patch fixes that, and also removes a work-around for the issue in the
MemMapSnapshot code.

MozReview-Commit-ID: JdDHt9ayFEl

--HG--
extra : rebase_source : 2000ede41108d6312734d8df7db98272b33528fa
extra : amend_source : 1443a596fab3e3126a22d44787adaf5e12e4587c

dom/ipc/MemMapSnapshot.cpp

@@ -118,19 +118,7 @@ MemMapSnapshot::Freeze(AutoMemMap& aMem)
   MOZ_TRY(NS_NewNativeLocalFile(mPath, /* followLinks = */ false,
                                 getter_AddRefs(file)));
 
-  auto result = aMem.init(file);
-#ifdef XP_LINUX
-  // On Linux automation runs, every few hundred thousand calls, our attempt to
-  // stat the file that we just successfully opened fails with EBADF (bug
-  // 1472889). Presumably this is a race with a background thread that double
-  // closes a file, but is difficult to diagnose, so work around it by making a
-  // second mapping attempt if the first one fails.
-  if (!result.isOk()) {
-    aMem.reset();
-    result = aMem.init(file);
-  }
-#endif
-  return result;
+  return aMem.init(file);
 }
 
 #else

widget/LSBUtils.cpp

@@ -8,6 +8,7 @@
 
 #include <unistd.h>
 #include "base/process_util.h"
+#include "mozilla/FileUtils.h"
 
 namespace mozilla {
 namespace widget {
@@ -47,7 +48,7 @@ GetLSBRelease(nsACString& aDistributor,
     return false;
   }
 
-  FILE* stream = fdopen(pipefd[0], "r");
+  ScopedCloseFile stream(fdopen(pipefd[0], "r"));
   if (!stream) {
     NS_WARNING("Could not wrap fd!");
     close(pipefd[0]);
@@ -62,12 +63,8 @@ GetLSBRelease(nsACString& aDistributor,
              dist, desc, release, codename) != 4)
   {
     NS_WARNING("Failed to parse lsb_release!");
-    fclose(stream);
-    close(pipefd[0]);
     return false;
   }
-  fclose(stream);
-  close(pipefd[0]);
 
   aDistributor.Assign(dist);
   aDescription.Assign(desc);