Bug 1524864 - nsMIMEInputStream should expose nsISeekableStream only if the underlying stream is seekable, r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D18528

--HG--
extra : moz-landing-system : lando

netwerk/base/nsMIMEInputStream.cpp

@@ -72,6 +72,7 @@ class nsMIMEInputStream : public nsIMIMEInputStream,
                             const char* aFromRawSegment, uint32_t aToOffset,
                             uint32_t aCount, uint32_t* aWriteCount);
 
+  bool IsSeekableInputStream() const;
   bool IsAsyncInputStream() const;
   bool IsIPCSerializable() const;
   bool IsInputStreamLength() const;
@@ -101,8 +102,8 @@ NS_IMPL_CLASSINFO(nsMIMEInputStream, nullptr, nsIClassInfo::THREADSAFE,
 NS_INTERFACE_MAP_BEGIN(nsMIMEInputStream)
   NS_INTERFACE_MAP_ENTRY(nsIMIMEInputStream)
   NS_INTERFACE_MAP_ENTRY_AMBIGUOUS(nsIInputStream, nsIMIMEInputStream)
-  NS_INTERFACE_MAP_ENTRY(nsISeekableStream)
   NS_INTERFACE_MAP_ENTRY(nsITellableStream)
+  NS_INTERFACE_MAP_ENTRY_CONDITIONAL(nsISeekableStream, IsSeekableInputStream())
   NS_INTERFACE_MAP_ENTRY_CONDITIONAL(nsIIPCSerializableInputStream,
                                      IsIPCSerializable())
   NS_INTERFACE_MAP_ENTRY_CONDITIONAL(nsIAsyncInputStream, IsAsyncInputStream())
@@ -464,6 +465,11 @@ nsMIMEInputStream::OnInputStreamLengthReady(nsIAsyncInputStreamLength* aStream,
   return callback->OnInputStreamLengthReady(this, aLength);
 }
 
+bool nsMIMEInputStream::IsSeekableInputStream() const {
+  nsCOMPtr<nsISeekableStream> seekable = do_QueryInterface(mStream);
+  return !!seekable;
+}
+
 bool nsMIMEInputStream::IsAsyncInputStream() const {
   nsCOMPtr<nsIAsyncInputStream> asyncStream = do_QueryInterface(mStream);
   return !!asyncStream;