The debugger shouldn't try to connect to the tracer actor if it is not present or if tracing is disabled (bug 970536). r=fitzgen

browser/devtools/debugger/debugger-controller.js

@@ -240,7 +240,11 @@ let DebuggerController = {
       } else {
         this._startDebuggingTab(startedDebugging.resolve);
         const startedTracing = promise.defer();
-        this._startTracingTab(traceActor, startedTracing.resolve);
+        if (Prefs.tracerEnabled && traceActor) {
+          this._startTracingTab(traceActor, startedTracing.resolve);
+        } else {
+          startedTracing.resolve();
+        }
 
         return promise.all([startedDebugging.promise, startedTracing.promise]);
       }

browser/devtools/debugger/test/browser.ini

@@ -214,6 +214,7 @@ support-files =
 [browser_dbg_tracing-03.js]
 [browser_dbg_tracing-04.js]
 [browser_dbg_tracing-05.js]
+[browser_dbg_tracing-06.js]
 [browser_dbg_variables-view-01.js]
 [browser_dbg_variables-view-02.js]
 [browser_dbg_variables-view-03.js]

browser/devtools/debugger/test/browser_dbg_tracing-06.js

@@ -0,0 +1,39 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/**
+ * Test that the tracer doesn't connect to the backend when tracing is disabled.
+ */
+
+const TAB_URL = EXAMPLE_URL + "doc_tracing-01.html";
+const TRACER_PREF = "devtools.debugger.tracer";
+
+let gTab, gDebuggee, gPanel, gDebugger;
+let gOriginalPref = Services.prefs.getBoolPref(TRACER_PREF);
+Services.prefs.setBoolPref(TRACER_PREF, false);
+
+function test() {
+  initDebugger(TAB_URL).then(([aTab, aDebuggee, aPanel]) => {
+    gTab = aTab;
+    gDebuggee = aDebuggee;
+    gPanel = aPanel;
+    gDebugger = gPanel.panelWin;
+
+    waitForSourceShown(gPanel, "code_tracing-01.js")
+      .then(() => {
+        ok(!gDebugger.DebuggerController.traceClient, "Should not have a trace client");
+        closeDebuggerAndFinish(gPanel);
+      })
+      .then(null, aError => {
+        ok(false, "Got an error: " + aError.message + "\n" + aError.stack);
+      });
+  });
+}
+
+registerCleanupFunction(function() {
+  gTab = null;
+  gDebuggee = null;
+  gPanel = null;
+  gDebugger = null;
+  Services.prefs.setBoolPref(TRACER_PREF, gOriginalPref);
+});