Make source map cache in StackTraceDeobfuscator thread-safe

[Samin061]

StackTraceDeobfuscator caches per-permutation source maps in a plain HashMap, and a single shared instance reads and writes it from every request thread (RemoteLoggingServiceImpl and RequestFactoryServlet each hold one). Concurrent deobfuscation of sourcemap-capable client stack traces then races on loadSourceMap's unsynchronized get/put, which can drop cache entries, hand back a mapping stored under a different key, or spin on a corrupted table. The sibling symbol cache in this class is already synchronized; sourceMaps was overlooked. Switch it to ConcurrentHashMap.

[niloc132]

Unless I'm mistaken, this is still slightly racy when it is accessed - the get/put calls should be replaced with computeIfAbsent so that we only write once? Probably not a dangerous race, but in the same cases where the old code could lead to a race/deadlock, we could end up doing the code twice. If you're cleaning this up, let's clean it all?

Related, but not necessarily blocking: I think SymbolCache could be more efficient too, at least not requiring exclusive readers, only writing should block reading (and writes are only possible for the first read, likewise a computeIfAbsent).

[niloc132]

Generic args need not be explicitly specified

Suggested change

	new ConcurrentHashMap<String, SourceMapping>();
	private final Map<String, SourceMapping> sourceMaps = new ConcurrentHashMap<>();

[Samin061]

Done, applied.

[Samin061]

Good point. Switched loadSourceMap to computeIfAbsent so the parse only runs once per key, and dropped the explicit generic args per your inline note.

I left SymbolCache alone to keep this focused on the sourceMaps race. The read/write split there is a bit more involved since the inner maps currently rely on the synchronized block, so I'd rather do it as a separate change if that's alright.