From: Matthew Fernandez <matthew.fernandez@gmail.com>
Date: Sat, 27 Nov 2021 03:43:33 +0000 (-0800)
Subject: GDI+ plugin: rewrite 'FileStream' reference counting to be thread safe
X-Git-Tag: 2.50.0~3^2
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=0db8dc7480dd4a544c6f39222ddfc3d905eb7061;p=graphviz

GDI+ plugin: rewrite 'FileStream' reference counting to be thread safe

Based on Microsoft sample code,¹ it seems like the expectation is that consumers
of the `IStream` API may expect to concurrently manipulate reference counts.
This change guards against this by making the referencing counting code use
lock-free atomics, based on the same Microsoft sample.

Note that this is essentially proactively addressing a latent issue as it is, in
general, unsafe to use Graphviz libraries or plugins in a multithreaded
environment right now.

¹ https://docs.microsoft.com/en-us/previous-versions/windows/desktop/ms752876(v=vs.85)
---

diff --git a/plugin/gdiplus/FileStream.cpp b/plugin/gdiplus/FileStream.cpp
index 0fe700814..aa2d030d6 100644
--- a/plugin/gdiplus/FileStream.cpp
+++ b/plugin/gdiplus/FileStream.cpp
@@ -53,12 +53,12 @@ HRESULT FileStream::QueryInterface(
 
 ULONG FileStream::AddRef()
 {
-	return ++_ref;
+	return (ULONG)InterlockedIncrement((ULONG*)&_ref);
 }
 
 ULONG FileStream::Release()
 {
-	ULONG ref = --_ref;
+	ULONG ref = (ULONG)InterlockedDecrement((ULONG*)&_ref);
 	if (ref == 0)
 		delete this;
 	return ref;