# HG changeset patch
# User John "Elwin" Edwards
# Date 1420654715 18000
# Node ID 59e62710cbb5cb5dddab1950e86e68d2d2bee1a4
# Parent  3a97e4ee50f098987f86d35fe8967552cfc93538
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.

diff -r 3a97e4ee50f0 -r 59e62710cbb5 rlgwebd.js
--- a/rlgwebd.js	Tue Jan 06 16:59:12 2015 -0500
+++ b/rlgwebd.js	Wed Jan 07 13:18:35 2015 -0500
@@ -205,6 +205,9 @@
   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 @@
     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 @@
   };
   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());