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diff rlgwebd.js @ 165:59e62710cbb5

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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.
author John "Elwin" Edwards
date Wed, 07 Jan 2015 13:18:35 -0500
parents 3a97e4ee50f0
children fba1b34e7554
line wrap: on
line diff
--- 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());