Fix ArgumentNullException under a race condition in analyzer driver

src/Compilers/Core/Portable/DiagnosticAnalyzer/AnalysisState.PerAnalyzerState.cs

@@ -319,6 +319,24 @@ private static void FreeState_NoLock<TAnalyzerStateData>(TAnalyzerStateData? sta
             {
                 if (state != null && !ReferenceEquals(state, AnalyzerStateData.FullyProcessedInstance))
                 {
+                    // Do not free state if it is still being processed for analysis, i.e. StateKind is 'InProcess'.
+                    // This can happen in rare cases when multiple threads are trying to analyze the same
+                    // symbol/syntax/operation, such that one thread has finished all the analyzer callbacks,
+                    // but before this thread marks the state as complete and invokes this FreeState call,
+                    // second thread starts processing the same symbol/syntax/operation and sets the StateKind to
+                    // 'InProcess' for the same state object. If we free the state here in the first thread,
+                    // then this can lead to data corruption/exceptions in the second thread.
+                    // For example, see https://github.com/dotnet/roslyn/issues/59988.
+                    //
+                    // CONSIDER: Note that our current approach of not freeing the state for StateKind.InProcess
+                    // leads to a leak, as we will never return this state object to the pool. However, this should
+                    // happen in extremely rare cases as described above, so it should be fine. In future, if we do
+                    // see negative performance impact from this leak, we can do a more complex state tracking and
+                    // ensure that we free this state object and return it to the pool once the StateKind is reset
+                    // to StateKind.ReadyToProcess state.
+                    if (state.StateKind == StateKind.InProcess)
+                        return;
+
                     state.Free();
                     pool.Free(state);
                 }