[DisplayLink] Semi-speculative fix for crasher in CVCGDL::getDisplayT…

…imes

This patch applies a workaround discovered by mozilla to avoid crashes
in CVCGDisplayLink::getDisplayTimes(). The call to
CVDisplayLinkCreateWithCGDisplay() can sometimes tickle a bug that
will lead to a crash. By checking another condition we can detect the
error state and avoid a crash.

Bug: 1218720
Change-Id: Ic7e54d8a03d05143d6e9f54f26ebd79277cca934
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3217658
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Jayson Adams <shrike@chromium.org>
Cr-Commit-Position: refs/heads/main@{#930712}

ui/display/mac/display_link_mac.cc

@@ -82,7 +82,25 @@ scoped_refptr<DisplayLinkMac> DisplayLinkMac::GetForDisplay(
   ret = CVDisplayLinkCreateWithCGDisplay(display_id,
                                          display_link.InitializeInto());
   if (ret != kCVReturnSuccess) {
-    LOG(ERROR) << "CVDisplayLinkCreateWithActiveCGDisplays failed: " << ret;
+    LOG(ERROR) << "CVDisplayLinkCreateWithCGDisplay failed: " << ret;
+    return nullptr;
+  }
+
+  // Workaround for bug https://crbug.com/1218720. According to
+  // https://hg.mozilla.org/releases/mozilla-esr68/rev/db0628eadb86,
+  // CVDisplayLinkCreateWithCGDisplays()
+  // (called by CVDisplayLinkCreateWithCGDisplay()) sometimes
+  // creates a CVDisplayLinkRef with an uninitialized (nulled) internal
+  // pointer. If we continue to use this CVDisplayLinkRef, we will
+  // eventually crash in CVCGDisplayLink::getDisplayTimes(), where the
+  // internal pointer is dereferenced. Fortunately, when this happens
+  // another internal variable is also left uninitialized (zeroed),
+  // which is accessible via CVDisplayLinkGetCurrentCGDisplay(). In
+  // normal conditions the current display is never zero.
+  if ((ret == kCVReturnSuccess) &&
+      (CVDisplayLinkGetCurrentCGDisplay(display_link) == 0)) {
+    LOG(ERROR)
+        << "CVDisplayLinkCreateWithCGDisplay failed (no current display)";
     return nullptr;
   }