Tcl 8.4.7 introduced memory leak in Tcl threaded memory allocator

Today, Zoran Vasiljevic confirmed that there is a leak in the threaded Tcl memory allocator [1] (affectionately known as “Zippy” by the AOLserver crowd). I unfortunately already knew about this (see comments) because Qiong Wei told me about it back in December 2004, but didn’t really act on it because I originally thought that it was something in his application code that may have been causing the leak, not something in the Tcl core itself. Today, I verified that it was indeed a leak in the Tcl core, introduced in Tcl 8.4.7 [2].

Here’s my test run against Tcl 8.4.6 (no leak):

$ LD_LIBRARY_PATH=`pwd` ./tclsh
% info patchlevel
8.4.6
% lappend auto_path /usr/lib/tcl8.4; package require Thread
2.6.1
% exec ps -p [pid] -o pid,vsz,rss,args
  PID   VSZ  RSS COMMAND
 8317 12704 1996 ./tclsh
% for {set i 0} {$i < 1000} {incr i} {
    thread::join [thread::create -joinable {}]
}
% exec ps -p [pid] -o pid,vsz,rss,args
  PID   VSZ  RSS COMMAND
 8317 21192 2272 ./tclsh

Here's my test run against Tcl 8.4.7 (has leak):

$ LD_LIBRARY_PATH=`pwd` ./tclsh
% info patchlevel
8.4.7
% lappend auto_path /usr/lib/tcl8.4; package require Thread
2.6.1
% exec ps -p [pid] -o pid,vsz,rss,args
  PID   VSZ  RSS COMMAND
 9358 12692 2000 ./tclsh
% for {set i 0} {$i < 1000} {incr i} {
    thread::join [thread::create -joinable {}]
}
% exec ps -p [pid] -o pid,vsz,rss,args
  PID   VSZ  RSS COMMAND
 9358 48272 29432 ./tclsh

As I said in email to the AOLSERVER mailing list, if you're using Tcl >= 8.4.7, I strongly suggest you roll back to Tcl 8.4.6, and hope that the yet-to-be-released Tcl 8.4.10 contains the necessary fix for this leak issue.