Add try/catch block and logger call in post-parsing action to make future debugging easier

Author: sungshik

rascal-lsp/src/main/java/org/rascalmpl/vscode/lsp/TextDocumentState.java

@@ -205,26 +205,33 @@ private void parse() {
             try {
                 parser.apply(location, content)
                     .whenCompleteAsync((ITree t, Throwable e) -> {
-                        if (e instanceof CompletionException && e.getCause() != null) {
-                            e = e.getCause();
-                        }
-                        var diagnosticsList = toDiagnosticsList(t, e); // `t` and `e` are nullable
-
-                        // Complete future to get the tree
-                        if (t == null) {
-                            treeAsync.completeExceptionally(e);
-                        } else {
-                            var tree = new Versioned<>(version, t, timestamp);
-                            Versioned.replaceIfNewer(last, tree);
-                            if (diagnosticsList.isEmpty()) {
-                                Versioned.replaceIfNewer(lastWithoutErrors, tree);
+                        try {
+                            if (e instanceof CompletionException && e.getCause() != null) {
+                                e = e.getCause();
+                            }
+                            var diagnosticsList = toDiagnosticsList(t, e); // `t` and `e` are nullable
+
+                            // Complete future to get the tree
+                            if (t == null) {
+                                treeAsync.completeExceptionally(e);
+                            } else {
+                                var tree = new Versioned<>(version, t, timestamp);
+                                Versioned.replaceIfNewer(last, tree);
+                                if (diagnosticsList.isEmpty()) {
+                                    Versioned.replaceIfNewer(lastWithoutErrors, tree);
+                                }
+                                treeAsync.complete(tree);
                             }
-                            treeAsync.complete(tree);
-                        }
 
-                        // Complete future to get diagnostics
-                        var diagnostics = new Versioned<>(version, diagnosticsList);
-                        diagnosticsAsync.complete(diagnostics);
+                            // Complete future to get diagnostics
+                            var diagnostics = new Versioned<>(version, diagnosticsList);
+                            diagnosticsAsync.complete(diagnostics);
+                        } catch (Exception exc) {
+                            // The action of `whenCompleteAsync` shouldn't throw an exception (see JavaDoc): if it
+                            // unexpectedly does, then it is almost surely a bug, but the exception is swallowed, so it
+                            // is very hard to debug. The try/catch block and logger call aim to make it easier.
+                            logger.error("Unexpected exception after parsing: {}", exc);
+                        }
                     }, exec);
             } catch (NoContributionException e) {
                 logger.debug("Ignoring missing parser for {}", location, e);