Do not retain zombie CGL contexts

There are reports of failures when releasing the previous_context_ in
ScopedCGLSetCurrentContext's destructor. The exact reason is unknown,
but test applications have shown that it is possible for the current
context to have a reference count of zero (in which case, the retain
before setting the new context current does nothing, and the release
after the new context is not current anymore may crash).

If the previous context has a zero reference count, do not retain and
release it, rather, set the context to NULL after the call completes.

BUG=358473

Review URL: https://codereview.chromium.org/224723021

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@262510 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 8 insertions, 2 deletions

diff --git a/ui/gl/scoped_cgl.cc b/ui/gl/scoped_cgl.cc
index 9fef385..18eb980 100644
--- a/ui/gl/scoped_cgl.cc
+++ b/ui/gl/scoped_cgl.cc
@@ -7,8 +7,14 @@
 
 namespace gfx {
 
-ScopedCGLSetCurrentContext::ScopedCGLSetCurrentContext(CGLContextObj context)
-    : previous_context_(CGLGetCurrentContext(), base::scoped_policy::RETAIN) {
+ScopedCGLSetCurrentContext::ScopedCGLSetCurrentContext(CGLContextObj context) {
+  CGLContextObj previous_context = CGLGetCurrentContext();
+  // It is possible for the previous context to have a zero reference count,
+  // because making a context current does not increment the reference count.
+  // In that case, do not restore the previous context.
+  if (previous_context && CGLGetContextRetainCount(previous_context)) {
+    previous_context_.reset(previous_context, base::scoped_policy::RETAIN);
+  }
   CGLError error = CGLSetCurrentContext(context);
   DCHECK_EQ(error, kCGLNoError) << "CGLSetCurrentContext should never fail";
 }