Bugfix: libeio race condition

Process at most 10 pending responses from the thread pool in one go.
10 was chosen arbitrarily.
Test and report by Felix Geisendörfer <felix@debuggable.com>

src/node.cc

@@ -949,6 +949,9 @@ int main(int argc, char *argv[]) {
   ev_async_init(&node::eio_watcher, node::EIOCallback);
   // 2. Actaully start the thread pool.
   eio_init(node::EIOWantPoll, NULL);
+  // Don't handle more than 10 reqs on each eio_poll(). This is to avoid
+  // race conditions. See test/mjsunit/test-eio-race.js
+  eio_set_max_poll_reqs(10);
   // 3. Start watcher.
   ev_async_start(EV_DEFAULT_UC_ &node::eio_watcher);
   // 4. Remove a reference to the async watcher. This means we'll drop out

test/mjsunit/test-eio-race.js

@@ -0,0 +1,63 @@
+process.mixin(require("./common"));
+
+var
+  count = 100,
+  posix = require('posix');
+
+function tryToKillEventLoop() {
+  puts('trying to kill event loop ...');
+
+  posix.stat(__filename)
+    .addCallback(function() {
+      puts('first posix.stat succeeded ...');
+
+      posix.stat(__filename)
+        .addCallback(function() {
+          puts('second posix.stat succeeded ...');
+          puts('could not kill event loop, retrying...');
+
+          setTimeout(function () {
+            if (--count) {
+              tryToKillEventLoop();
+            } else {
+              process.exit(0);
+            }
+          }, 1);
+        })
+        .addErrback(function() {
+          throw new Exception('second posix.stat failed')
+        })
+
+    })
+    .addErrback(function() {
+      throw new Exception('first posix.stat failed')
+    });
+}
+
+// Generate a lot of thread pool events
+var pos = 0;
+posix.open('/dev/zero', process.O_RDONLY, 0666).addCallback(function (rd) {
+  function readChunk () {
+    posix.read(rd, 1024, pos, 'binary').addCallback(function (chunk, bytesRead) {
+      if (chunk) {
+        pos += bytesRead;
+        //puts(pos);
+        readChunk();
+      } else {
+        posix.close(rd);
+        throw new Exception(BIG_FILE+' should not end before the issue shows up');
+      }
+    }).addErrback(function () {
+      throw new Exception('could not read from '+BIG_FILE);
+    });
+  }
+  readChunk();
+}).addErrback(function () {
+  throw new Exception('could not open '+BIG_FILE);
+});
+
+tryToKillEventLoop();
+
+process.addListener("exit", function () {
+  assert.ok(pos > 10000);
+});