Bug 897723 - Allow faulty.lib's on-demand decompression to be reentrant. r=nfroyd

mozglue/linker/Mappable.cpp

@@ -344,7 +344,11 @@ MappableSeekableZStream::Create(const char *name, Zip *zip,
   mozilla::ScopedDeletePtr<MappableSeekableZStream> mappable;
   mappable = new MappableSeekableZStream(zip);
 
-  if (pthread_mutex_init(&mappable->mutex, NULL))
+  pthread_mutexattr_t recursiveAttr;
+  pthread_mutexattr_init(&recursiveAttr);
+  pthread_mutexattr_settype(&recursiveAttr, PTHREAD_MUTEX_RECURSIVE);
+
+  if (pthread_mutex_init(&mappable->mutex, &recursiveAttr))
     return NULL;
 
   if (!mappable->zStream.Init(stream->GetBuffer(), stream->GetSize()))
@@ -466,6 +470,19 @@ MappableSeekableZStream::ensure(const void *addr)
 
   length = PageAlignedSize(length);
 
+  /* The following lock can be re-acquired by the thread holding it.
+   * If this happens, it means the following code is interrupted somehow by
+   * some signal, and ends up retriggering a chunk decompression for the
+   * same MappableSeekableZStream.
+   * If the chunk to decompress is different the second time, then everything
+   * is safe as the only common data touched below is chunkAvailNum, and it is
+   * atomically updated (leaving out any chance of an interruption while it is
+   * updated affecting the result). If the chunk to decompress is the same, the
+   * worst thing that can happen is chunkAvailNum being incremented one too
+   * many times, which doesn't affect functionality. The chances of it
+   * happening being pretty slim, and the effect being harmless, we can just
+   * ignore the issue. Other than that, we'd just be wasting time decompressing
+   * the same chunk twice. */
   AutoLock lock(&mutex);
 
   /* The very first page is mapped and accessed separately of the rest, and
@@ -522,7 +539,7 @@ MappableSeekableZStream::stats(const char *when, const char *name) const
 {
   size_t nEntries = zStream.GetChunksNum();
   DEBUG_LOG("%s: %s; %" PRIdSize "/%" PRIdSize " chunks decompressed",
-            name, when, chunkAvailNum, nEntries);
+            name, when, static_cast<size_t>(chunkAvailNum), nEntries);
 
   size_t len = 64;
   mozilla::ScopedDeleteArray<char> map;

mozglue/linker/Mappable.h

@@ -265,7 +265,7 @@ private:
   mozilla::ScopedDeleteArray<unsigned char> chunkAvail;
 
   /* Number of chunks that have already been decompressed. */
-  size_t chunkAvailNum;
+  mozilla::Atomic<size_t> chunkAvailNum;
 
   /* Mutex protecting decompression */
   pthread_mutex_t mutex;