[SourceKit] Add defensive guard for invalid offset

[rintaro]

Invalid (too high) offset used to cause a inifinite loop in Lexer. This could happens if:

1. Client send a request to SourceKit
2. User somehow truncate the file
3. SourceKit handle the request asynchronously

This is a quick fix until we fix desynchronization problem in SourceKit.
Making a test case is not easy at this point because we currently don't have a tool to test this scenario.
rdar://problem/30346106

[rintaro]

@swift-ci Please smoke test

[akyrtzi]

Is it not possible to create a unit-test for this ? See unittests/SourceKit/SwiftLang/CursorInfoTest.cpp

[rintaro]

Ah, I forgot about unittests, I will.

[rintaro]

@akyrtzi Sorry, it was not so easy. This private getSourceToken() function is only called from
https://github.com/rintaro/swift/blob/4fe5ead3/tools/SourceKit/lib/SwiftLang/SwiftSourceDocInfo.cpp#L1167
And I haven't found the code path to reach here. Specifically, do you know how to pass this Snap->getStamp() == InputSnap->getStamp() condition?
My scenario in the description seems wrong.

[akyrtzi]

I think you'll go pass that condition if you make a cursor info call, make an edit, then make another cursor call. But this may depend on timing, not sure.

If you can't get it to trigger via a unit test as is, I'd be fine with exposing the getSourceToken() function somewhere in a header and testing it directly.

[rintaro]

I managed to reproduce the crash (assertion failure) with sourcekitd-test.
Added test case. Thanks!

[rintaro]

@swift-ci Please smoke test