fs: add sync open flags 'rs' and 'rs+'

doc/api/fs.markdown

@@ -270,6 +270,13 @@ An exception occurs if the file does not exist.
 * `'r+'` - Open file for reading and writing.
 An exception occurs if the file does not exist.
 
+* `'rs'` - Open file for reading, telling the OS to open it synchronously
+(ie using the O_SYNC flag).  Whilst rarely useful, when used with caution by
+those who know what they're doing it can be sometimes necessary.  Note that
+this doesn't turn `fs.open()` into a synchronous blocking call, if that's what
+you want then you should be using `fs.openSync()`
+An exception occurs if the file does not exist.
+
 * `'w'` - Open file for writing.
 The file is created (if it does not exist) or truncated (if it exists).
 

lib/fs.js

@@ -210,7 +210,9 @@ function stringToFlags(flag) {
 
   switch (flag) {
     case 'r' : return O_RDONLY;
+    case 'rs' : return O_RDONLY | O_SYNC;
     case 'r+' : return O_RDWR;
+    case 'rs+' : return O_RDWR | O_SYNC;
 
     case 'w' : return O_TRUNC | O_CREAT | O_WRONLY;
     case 'wx' : // fall through

test/simple/test-fs-open.js

@@ -41,11 +41,19 @@ fs.open(__filename, 'r', function(err, fd) {
   if (err) {
     throw err;
   }
-
   openFd = fd;
 });
 
+var openFd2;
+fs.open(__filename, 'rs', function(err, fd) {
+  if (err) {
+    throw err;
+  }
+  openFd2 = fd;
+});
+
 process.on('exit', function() {
   assert.ok(openFd);
+  assert.ok(openFd2);
 });