Bug 1759569 - Retain nsIInputStreamLength through NS_CloneInputStream, r=asuth

This will help in bug 1754004 as in some cases we will want to clone a
DataPipe, which is non-cloneable, and without this change length information
would be lost.

In the future it might be worth making a more generic mechanism for efficiently
cloning wrapper input streams with a fallback rather than hard-coding this
interface.

Differential Revision: https://phabricator.services.mozilla.com/D141035

xpcom/io/nsStreamUtils.cpp

@@ -6,6 +6,8 @@
 
 #include "mozilla/Mutex.h"
 #include "mozilla/Attributes.h"
+#include "mozilla/InputStreamLengthWrapper.h"
+#include "nsIInputStreamLength.h"
 #include "nsStreamUtils.h"
 #include "nsCOMPtr.h"
 #include "nsICloneableInputStream.h"
@@ -870,6 +872,16 @@ nsresult NS_CloneInputStream(nsIInputStream* aSource,
     return rv;
   }
 
+  // Propagate length information provided by nsIInputStreamLength. We don't use
+  // InputStreamLengthHelper::GetSyncLength to avoid the risk of blocking when
+  // called off-main-thread.
+  int64_t length = -1;
+  if (nsCOMPtr<nsIInputStreamLength> streamLength = do_QueryInterface(aSource);
+      streamLength && NS_SUCCEEDED(streamLength->Length(&length)) &&
+      length != -1) {
+    reader = new mozilla::InputStreamLengthWrapper(reader.forget(), length);
+  }
+
   cloneable = do_QueryInterface(reader);
   MOZ_ASSERT(cloneable && cloneable->GetCloneable());