From 3c88c233b446cc6eb10ceae6fabc7d7f278d4a27 Mon Sep 17 00:00:00 2001
From: "John \"Elwin\" Edwards" <elwin@rlgallery.org>
Date: Wed, 7 Jan 2015 13:18:35 -0500
Subject: [PATCH] rlgwebd.js: prevent races when reading ttyrecs.

DglSession objects read a 12-byte TTYREC header, extract therefrom the
length of the data chunk, and then read the data.  In between these two
reads, the file watcher could trigger another readchunk() invocation,
which might attempt to read a header from the beginning of the data
chunk.  This usually results in expecting a data chunk of several GB
and failing to create a Buffer for it.

The race is remedied by setting a flag on the DglSession object
whenever readchunk() is called, clearing it when both reads complete,
and refusing to read if it is already set.
---
 rlgwebd.js | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/rlgwebd.js b/rlgwebd.js
index 8bfe779..3f24d74 100755
--- a/rlgwebd.js
+++ b/rlgwebd.js
@@ -205,6 +205,9 @@ function DglSession(filename) {
   this.pname = basename.slice(0, firstsep);
   var fname = basename.slice(firstsep + 1);
   this.ttyrec = path.join("/dgldir/ttyrec", this.pname, this.gname, fname);
+  /* Flag to prevent multiple handlers from reading simultaneously and
+   * getting into a race. */
+  this.reading = false;
   this.framebuf = new Buffer(1024);
   this.frameoff = 0;
   this.framepush = function(chunk) {
@@ -229,17 +232,24 @@ function DglSession(filename) {
     this.frameoff += chunk.length;
   };
   this.readchunk = function () {
+    if (this.reading)
+      return;
+    this.reading = true;
     var header = new Buffer(12);
     fs.read(ss.fd, header, 0, 12, null, function (err, n, buf) {
       /* Stop recursion if end of file has been reached. */
-      if (err || n < 12)
+      if (err || n < 12) {
+        ss.reading = false;
         return;
+      }
       var datalen = buf.readUInt32LE(8);
       //tslog("Allocating %d bytes", datalen);
       var databuf = new Buffer(datalen);
       fs.read(ss.fd, databuf, 0, datalen, null, function (err, n, buf) {
-        if (err || n < datalen)
+        ss.reading = false;
+        if (err || n < datalen) {
           return;
+        }
         /* Process the data */
         ss.framepush(buf);
         ss.emit("data", buf);
@@ -278,6 +288,8 @@ function DglSession(filename) {
   };
   this.close = function () {
     this.watcher.close()
+    /* Ensure all data is handled before quitting. */
+    this.readchunk();
     fs.close(this.fd);
     this.emit("close");
     tslog("DGL %s: closed", ss.tag());