fs: make unwatchFile() remove a specific listener

Before this commit, `fs.unwatchFile(path)` removed *all* listeners for `path`.

The function is overloaded now: `fs.unwatchFile(path)` still removes all
listeners, but `fs.unwatchFile(path, cb)` lets you remove a specific listener.

Fixes #3660.

doc/api/fs.markdown

@@ -461,12 +461,16 @@ These stat objects are instances of `fs.Stat`.
 If you want to be notified when the file was modified, not just accessed
 you need to compare `curr.mtime` and `prev.mtime`.
 
-
-## fs.unwatchFile(filename)
+## fs.unwatchFile(filename, [listener])
 
     Stability: 2 - Unstable.  Use fs.watch instead, if available.
 
-Stop watching for changes on `filename`.
+Stop watching for changes on `filename`. If `listener` is specified, only that
+particular listener is removed. Otherwise, *all* listeners are removed and you
+have effectively stopped watching `filename`.
+
+Calling `fs.unwatchFile()` with a filename that is not being watched is a
+no-op, not an error.
 
 ## fs.watch(filename, [options], [listener])
 

lib/fs.js

@@ -943,10 +943,18 @@ fs.watchFile = function(filename) {
   return stat;
 };
 
-fs.unwatchFile = function(filename) {
-  var stat;
-  if (inStatWatchers(filename)) {
-    stat = statWatchers[filename];
+fs.unwatchFile = function(filename, listener) {
+  if (!inStatWatchers(filename)) return;
+
+  var stat = statWatchers[filename];
+
+  if (typeof listener === 'function') {
+    stat.removeListener('change', listener);
+  } else {
+    stat.removeAllListeners('change');
+  }
+
+  if (stat.listeners('change').length === 0) {
     stat.stop();
     statWatchers[filename] = undefined;
   }

test/pummel/test-fs-watch-file.js

@@ -26,6 +26,7 @@ var fs = require('fs');
 
 var watchSeenOne = 0;
 var watchSeenTwo = 0;
+var watchSeenThree = 0;
 
 var startDir = process.cwd();
 var testDir = common.fixturesDir;
@@ -37,12 +38,16 @@ var filenameTwo = 'hasOwnProperty';
 var filepathTwo = filenameTwo;
 var filepathTwoAbs = path.join(testDir, filenameTwo);
 
+var filenameThree = 'charm'; // because the third time is
+
 
 process.on('exit', function() {
   fs.unlinkSync(filepathOne);
   fs.unlinkSync(filepathTwoAbs);
+  fs.unlinkSync(filenameThree);
   assert.equal(1, watchSeenOne);
-  assert.equal(1, watchSeenTwo);
+  assert.equal(2, watchSeenTwo);
+  assert.equal(1, watchSeenThree);
 });
 
 
@@ -86,13 +91,38 @@ assert.throws(
 
 assert.doesNotThrow(
     function() {
-      fs.watchFile(filepathTwo, function(curr, prev) {
-        fs.unwatchFile(filepathTwo);
+      function a(curr, prev) {
+        fs.unwatchFile(filepathTwo, a);
         ++watchSeenTwo;
-      });
+      }
+      function b(curr, prev) {
+        fs.unwatchFile(filepathTwo, b);
+        ++watchSeenTwo;
+      }
+      fs.watchFile(filepathTwo, a);
+      fs.watchFile(filepathTwo, b);
     }
 );
 
 setTimeout(function() {
   fs.writeFileSync(filepathTwoAbs, 'pardner');
 }, 1000);
+
+assert.doesNotThrow(
+    function() {
+      function a(curr, prev) {
+        assert.ok(0); // should not run
+      }
+      function b(curr, prev) {
+        fs.unwatchFile(filenameThree, b);
+        ++watchSeenThree;
+      }
+      fs.watchFile(filenameThree, a);
+      fs.watchFile(filenameThree, b);
+      fs.unwatchFile(filenameThree, a);
+    }
+);
+
+setTimeout(function() {
+  fs.writeFileSync(filenameThree, 'pardner');
+}, 1000);