Merge pull request #24216 from mikeash/remote-mirror-weak-link-swift_reflection_classIsSwiftMask

[RemoteMirror] Mark swift_reflection_classIsSwiftMask as a weak import.

Author: mikeash

include/swift/SwiftRemoteMirror/SwiftRemoteMirror.h

@@ -37,7 +37,7 @@
 extern "C" {
 #endif
 
-SWIFT_REMOTE_MIRROR_LINKAGE
+SWIFT_REMOTE_MIRROR_LINKAGE __attribute__((__weak_import__))
 extern unsigned long long swift_reflection_classIsSwiftMask;
 
 /// Get the metadata version supported by the Remote Mirror library.

stdlib/public/SwiftRemoteMirror/SwiftRemoteMirror.cpp

@@ -14,7 +14,7 @@
 
 #define SWIFT_CLASS_IS_SWIFT_MASK swift_reflection_classIsSwiftMask
 extern "C" {
-SWIFT_REMOTE_MIRROR_LINKAGE
+SWIFT_REMOTE_MIRROR_LINKAGE __attribute__((__weak_import__))
 unsigned long long swift_reflection_classIsSwiftMask = 2;
 }
 

stdlib/tools/swift-reflection-test/swift-reflection-test.c

@@ -576,7 +576,10 @@ int main(int argc, char *argv[]) {
 
   const char *BinaryFilename = argv[1];
 
-  swift_reflection_classIsSwiftMask = computeClassIsSwiftMask();
+  // swift_reflection_classIsSwiftMask is weak linked so we can work
+  // with older Remote Mirror dylibs.
+  if (&swift_reflection_classIsSwiftMask != NULL)
+    swift_reflection_classIsSwiftMask = computeClassIsSwiftMask();
 
   uint16_t Version = swift_reflection_getSupportedMetadataVersion();
   printf("Metadata version: %u\n", Version);