Fixing a couple of minor bugs with the expression evaluator

 * need to keep a pointer to the `XDebuggerEvaluator` to prevent it from getting
   garbage-collected
 * right after evaluating an expression, the erlang debugger will trigger a new
   "breakpoint reached" event; unfortunately, that means this plugin will try to
   concurrently render the new state of the bindings as well as the result from
   the evaluation, which Intellij doesn't seem to handle too well, resulting in
   the debugging session losing track of what the current frame is... so to fix
   that we just ignore the next "breakpoint reached" event right after evaluating
   an expression... better ideas welcome!

Author: wk8

src/org/intellij/erlang/debugger/xdebug/ErlangStackFrame.java

@@ -41,6 +41,7 @@ public class ErlangStackFrame extends XStackFrame {
   private final ErlangXDebugProcess myDebugProcess;
   private final ErlangTraceElement myTraceElement;
   private final ErlangSourcePosition mySourcePosition;
+  private XDebuggerEvaluator myEvaluator = null;
 
   public ErlangStackFrame(@NotNull ErlangXDebugProcess debugProcess,
                           @NotNull ErlangTraceElement traceElement) {
@@ -58,14 +59,15 @@ public ErlangStackFrame(@NotNull ErlangXDebugProcess debugProcess,
   @Nullable
   @Override
   public XDebuggerEvaluator getEvaluator() {
-    return new XDebuggerEvaluator() {
+    myEvaluator = new XDebuggerEvaluator() {
       @Override
       public void evaluate(@NotNull String expression,
                            @NotNull XEvaluationCallback callback,
                            @Nullable XSourcePosition expressionPosition) {
         myDebugProcess.evaluateExpression(expression, callback, myTraceElement);
       }
     };
+    return myEvaluator;
   }
 
   @Nullable

src/org/intellij/erlang/debugger/xdebug/ErlangXDebugProcess.java

@@ -16,8 +16,10 @@
 
 package org.intellij.erlang.debugger.xdebug;
 
+import com.ericsson.otp.erlang.OtpErlangAtom;
 import com.ericsson.otp.erlang.OtpErlangObject;
 import com.ericsson.otp.erlang.OtpErlangPid;
+import com.ericsson.otp.erlang.OtpErlangTuple;
 import com.intellij.execution.ExecutionException;
 import com.intellij.execution.configurations.GeneralCommandLine;
 import com.intellij.execution.process.OSProcessHandler;
@@ -72,7 +74,6 @@
 import static org.intellij.erlang.debugger.ErlangDebuggerLog.LOG;
 
 public class ErlangXDebugProcess extends XDebugProcess implements ErlangDebuggerEventListener {
-  private final XDebugSession mySession;
   private final ExecutionEnvironment myExecutionEnvironment;
   private final ErlangRunningState myRunningState;
   private final ErlangDebuggerNode myDebuggerNode;
@@ -83,11 +84,16 @@ public class ErlangXDebugProcess extends XDebugProcess implements ErlangDebugger
   private ConcurrentHashMap<ErlangSourcePosition, XLineBreakpoint<ErlangLineBreakpointProperties>> myPositionToLineBreakpointMap =
     new ConcurrentHashMap<>();
   private XDebuggerEvaluator.XEvaluationCallback myEvalCallback = null;
+  // right after evaluating an expression, the erlang debugger will trigger a new "breakpoint reached" event
+  // unfortunately, that means this plugin will try to concurrently render the new state of the bindings as
+  // well as the result from the evaluation, which Intellij doesn't seem to handle too well, resulting in
+  // the debugging session losing track of what the current frame is... so we just ignore the next "breakpoint
+  // reached" event right after evaluating an expression - better ideas welcome!
+  private boolean ignoreNextBreakpointReachedEvent = false;
 
   public ErlangXDebugProcess(@NotNull XDebugSession session, ExecutionEnvironment env) throws ExecutionException {
     //TODO add debug build targets and make sure the project is built using them.
     super(session);
-    mySession = session;
 
     session.setPauseActionSupported(false);
 
@@ -124,17 +130,36 @@ public ErlangDebugLocationResolver getLocationResolver() {
   public synchronized void evaluateExpression(@NotNull String expression,
                                               @NotNull XDebuggerEvaluator.XEvaluationCallback callback,
                                               @NotNull ErlangTraceElement traceElement) {
-    // need to pause the debugging session otherwise the callback might get invalidated
-    mySession.pause();
     myEvalCallback = callback;
+    // see the comment about ignoreNextBreakpointReachedEvent
+    ignoreNextBreakpointReachedEvent = true;
     myDebuggerNode.evaluate(expression, traceElement);
   }
 
   @Override
   public synchronized void handleEvaluationResponse(OtpErlangObject response) {
     if (myEvalCallback != null) {
-      myEvalCallback.evaluated(ErlangXValueFactory.create(response));
-      mySession.resume();
+      String error = null;
+
+      if (response instanceof OtpErlangAtom && ((OtpErlangAtom) response).atomValue().equals("Parse error")) {
+        // it is a parsing error
+        error = "Parse error";
+      } else if (response instanceof OtpErlangTuple) {
+        OtpErlangObject[] elements = ((OtpErlangTuple) response).elements();
+
+        // is it an uncaught exception?
+        if (elements.length == 2
+            && elements[0] instanceof OtpErlangAtom
+            && ((OtpErlangAtom) elements[0]).atomValue().equals("EXIT")) {
+          error = "Uncaught exception: " + elements[1];
+        }
+      }
+
+      if (error == null) {
+        myEvalCallback.evaluated(ErlangXValueFactory.create(response));
+      } else {
+        myEvalCallback.errorOccurred(error);
+      }
     }
   }
 
@@ -178,6 +203,12 @@ public void breakpointIsSet(String module, int line) {
 
   @Override
   public void breakpointReached(final OtpErlangPid pid, List<ErlangProcessSnapshot> snapshots) {
+    if (ignoreNextBreakpointReachedEvent) {
+      // see the comment about ignoreNextBreakpointReachedEvent
+      ignoreNextBreakpointReachedEvent = false;
+      return;
+    }
+
     ErlangProcessSnapshot processInBreakpoint = ContainerUtil.find(snapshots, erlangProcessSnapshot -> erlangProcessSnapshot.getPid().equals(pid));
     assert processInBreakpoint != null;
     ErlangSourcePosition breakPosition = ErlangSourcePosition.create(myLocationResolver, processInBreakpoint);