Merge pull request #12978 from benlangmuir/sk-edit-race

[sourcekitd] Fix a race in EditableTextBuffer::getSnapshot()

Author: benlangmuir

Diff for: tools/SourceKit/include/SourceKit/Support/ImmutableTextBuffer.h

@@ -153,7 +153,7 @@ friend class EditableTextBuffer;
 };
 
 class EditableTextBuffer : public ThreadSafeRefCountedBase<EditableTextBuffer> {
-  llvm::sys::Mutex EditMtx;
+  mutable llvm::sys::Mutex EditMtx;
   ImmutableTextBufferRef Root;
   ImmutableTextUpdateRef CurrUpd;
   std::string Filename;

Diff for: tools/SourceKit/lib/Support/ImmutableTextBuffer.cpp

@@ -126,6 +126,7 @@ EditableTextBuffer::EditableTextBuffer(StringRef Filename, StringRef Text) {
 }
 
 ImmutableTextSnapshotRef EditableTextBuffer::getSnapshot() const {
+  llvm::sys::ScopedLock L(EditMtx);
   return new ImmutableTextSnapshot(const_cast<EditableTextBuffer*>(this), Root,
                                    CurrUpd);
 }