Bug 1433982 - Make nsAtomicFileOutputStream::DoOpen() fail if the file is read-only. r=glandium

This means we don't leave behind prefs-<n>.js files when prefs.js is read-only.

MozReview-Commit-ID: H6KKnoYGdhH

--HG--
extra : rebase_source : 263dd6fb75204a4565c8af89e7b21fc37a10d52e

netwerk/base/nsFileStreams.cpp

@@ -805,11 +805,18 @@ nsAtomicFileOutputStream::DoOpen()
     }
 
     if (NS_SUCCEEDED(rv) && mTargetFileExists) {
+        // Abort if |file| is not writable; it won't work as an output stream.
+        bool isWritable;
+        if (NS_SUCCEEDED(file->IsWritable(&isWritable)) && !isWritable) {
+            return NS_ERROR_FILE_ACCESS_DENIED;
+        }
+
         uint32_t origPerm;
         if (NS_FAILED(file->GetPermissions(&origPerm))) {
             NS_ERROR("Can't get permissions of target file");
             origPerm = mOpenParams.perm;
         }
+
         // XXX What if |perm| is more restrictive then |origPerm|?
         // This leaves the user supplied permissions as they were.
         rv = tempResult->CreateUnique(nsIFile::NORMAL_FILE_TYPE, origPerm);