nextTick: Handle tick callbacks after each tick

src/node.js

@@ -239,6 +239,8 @@
 
       if (domain) domain.exit();
 
+      // process the nextTicks after each time we get called.
+      process._tickCallback();
       return ret;
     };
   };
@@ -247,8 +249,19 @@
     var nextTickQueue = [];
     var nextTickIndex = 0;
     var inTick = false;
+    var tickDepth = 0;
+
+    // the maximum number of times it'll process something like
+    // nextTick(function f(){nextTick(f)})
+    // It's unlikely, but not illegal, to hit this limit.  When
+    // that happens, it yields to libuv's tick spinner.
+    // This is a loop counter, not a stack depth, so we aren't using
+    // up lots of memory here.  I/O can sneak in before nextTick if this
+    // limit is hit, which is not ideal, but not terrible.
+    process.maxTickDepth = 1000;
 
     function tickDone() {
+      tickDepth = 0;
       nextTickQueue.splice(0, nextTickIndex);
       nextTickIndex = 0;
       inTick = false;
@@ -259,28 +272,38 @@
 
     process._tickCallback = function() {
       if (inTick) return;
-      var nextTickLength = nextTickQueue.length;
-      if (nextTickLength === 0) return;
       inTick = true;
 
-      while (nextTickIndex < nextTickLength) {
-        var tock = nextTickQueue[nextTickIndex++];
-        var callback = tock.callback;
-        if (tock.domain) {
-          if (tock.domain._disposed) continue;
-          tock.domain.enter();
-        }
-        var threw = true;
-        try {
-          callback();
-          threw = false;
-        } finally {
-          if (threw) tickDone();
-        }
-        if (tock.domain) {
-          tock.domain.exit();
+      // always do this at least once.  otherwise if process.maxTickDepth
+      // is set to some negative value, we'd never process any of them.
+      do {
+        tickDepth++;
+        var nextTickLength = nextTickQueue.length;
+        if (nextTickLength === 0) return tickDone();
+        while (nextTickIndex < nextTickLength) {
+          var tock = nextTickQueue[nextTickIndex++];
+          var callback = tock.callback;
+          if (tock.domain) {
+            if (tock.domain._disposed) continue;
+            tock.domain.enter();
+          }
+          var threw = true;
+          try {
+            callback();
+            threw = false;
+          } finally {
+            if (threw) tickDone();
+          }
+          if (tock.domain) {
+            tock.domain.exit();
+          }
         }
-      }
+        nextTickQueue.splice(0, nextTickIndex);
+        nextTickIndex = 0;
+
+        // continue until the max depth or we run out of tocks.
+      } while (tickDepth < process.maxTickDepth &&
+               nextTickQueue.length > 0);
 
       tickDone();
     };

test/simple/test-next-tick-intentional-starvation.js

@@ -0,0 +1,64 @@
+// Copyright Joyent, Inc. and other Node contributors.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the
+// "Software"), to deal in the Software without restriction, including
+// without limitation the rights to use, copy, modify, merge, publish,
+// distribute, sublicense, and/or sell copies of the Software, and to permit
+// persons to whom the Software is furnished to do so, subject to the
+// following conditions:
+//
+// The above copyright notice and this permission notice shall be included
+// in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN
+// NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+// DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
+// USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+var common = require('../common');
+var assert = require('assert');
+
+// this is the inverse of test-next-tick-starvation.
+// it verifies that process.nextTick will *always* come before other
+// events, up to the limit of the process.maxTickDepth value.
+
+// WARNING: unsafe!
+process.maxTickDepth = Infinity;
+
+var ran = false;
+var starved = false;
+var start = +new Date();
+var timerRan = false;
+
+function spin() {
+  ran = true;
+  var now = +new Date();
+  if (now - start > 100) {
+    console.log('The timer is starving, just as we planned.');
+    starved = true;
+
+    // now let it out.
+    return;
+  }
+
+  process.nextTick(spin);
+}
+
+function onTimeout() {
+  if (!starved) throw new Error('The timer escaped!');
+  console.log('The timer ran once the ban was lifted');
+  timerRan = true;
+}
+
+spin();
+setTimeout(onTimeout, 50);
+
+process.on('exit', function() {
+  assert.ok(ran);
+  assert.ok(starved);
+  assert.ok(timerRan);
+});

test/simple/test-process-active-wraps.js

@@ -46,11 +46,16 @@ function expect(activeHandles, activeRequests) {
   expect(2, 1); // client handle doesn't shut down until next tick
 })();
 
+// Force the nextTicks to be deferred to a later time.
+process.maxTickDepth = 1;
+
 process.nextTick(function() {
   process.nextTick(function() {
     process.nextTick(function() {
-      // the handles should be gone but the connect req could still be alive
-      assert.equal(process._getActiveHandles().length, 0);
+      process.nextTick(function() {
+        // the handles should be gone but the connect req could still be alive
+        assert.equal(process._getActiveHandles().length, 0);
+      });
     });
   });
 });