Runtime: Make swift_getObjCClassMetadata resilient against weakly linked classes #5055
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Description: Whenever we would link against a swift overlay (like CallKit) that has weakly linked Objective-C classes we run the risk of crashing in runtime functions that access type metadata.
The type metadata is initiatialized with swift_getObjCClassMetadata. If the class is weakly linked and not present this function is called with a null (the weakly linked) class.
For example, we crash on iOS9 on a simple “print(“foo”)” call when this application links against CallKit.
Scope: At least three bug reports so far.
Risk: Very low, the fix is to return null if the class passed is null.
Tested: Verified that the reported crashers work after this change
Radar: rdar://problem/28203571 (also rdar://problem/28340074)
Reviewed by: Joe Groff
rdar://28203571