fixes a crash when skipping array fields during decoding

[t089]

Fixes a crash in skipValue() when skipping array/dictionary values during decoding.

Motivation:

When implementing JSONDecodable conformance with decodeEachKeyAndValue and a default: break to skip unknown keys, decoding crashes with preconditionFailure("Wrong node type") if any of the unknown fields values is an array (or a nested object containing arrays). This is because skipValue() delegates to BlackHoleVisitor via ArrayDecoder/StructDecoder, but these decoders inherit the parent's coding path node (e.g. .dictionary) instead of pushing their own. When decodeEachElement calls incrementArrayIndex() on the inherited .dictionary node, it hits the precondition failure.

Modifications:

• In ParserState.skipValue(), push the appropriate CodingPathNode before creating the sub-decoder for both ._openbrace and ._openbracket cases.
• Point currentTopCodingPathNode at the new node before constructing the StructDecoder/ArrayDecoder
• After the sub-decoder finishes, unwind back to the parent node (using withExtendedLifetime to ensure the node stays alive).

Result:

Skipping keys in JSON whose values are objects or array no longer crashes.

Testing:

Validated with a reproducer that no longer crashes after the patch is applied.

[kperryua]

@t089 is there a reproducer test case you could add to the suite with this fix?

[t089]

Sure, before I check myself, does the Suite Support "exit tests" to observe a crash?

[kperryua]

Looks like it's supported on a number of platforms. We'd have to crib a conditional compiler setting from Foundation's testOnlySwiftSettings in the Package.swift to enable it safely.

But we're not testing an expected assertion here right? Just a crash fix? The test running to completion should be verification enough. Using the process exit infrastructure might even make it harder for us to get backtrace diagnostics if it were to start crashing again later.

[t089]

Ok got it. Yes then I can add a test case for this.

[kperryua]

Thanks a ton! You probably want to rebase the branch on top of the latest version that has the various test fixes.

[t089]

@kperryua added a couple of tests