Leak hunting strategies and tips

This document is old and some of the information is out-of-date. Use with caution.

Strategy for finding leaks

When trying to make a particular testcase not leak, I recommend focusing first on the largest object graphs (since these entrain many smaller objects), then on smaller reference-counted object graphs, and then on any remaining individual objects or small object graphs that don’t entrain other objects.

Because (1) large graphs of leaked objects tend to include some objects pointed to by global variables that confuse GC-based leak detectors, which can make leaks look smaller (as in bug 99180{.external .text}) or hide them completely and (2) large graphs of leaked objects tend to hide smaller ones, it’s much better to go after the large graphs of leaks first.

A good general pattern for finding and fixing leaks is to start with a task that you want not to leak (for example, reading email). Start finding and fixing leaks by running part of the task under nsTraceRefcnt logging, gradually building up from as little as possible to the complete task, and fixing most of the leaks in the first steps before adding additional steps. (By most of the leaks, I mean the leaks of large numbers of different types of objects or leaks of objects that are known to entrain many non-logged objects such as JS objects. Seeing a leaked GlobalWindowImpl, nsXULPDGlobalObject, nsXBLDocGlobalObject, or nsXPCWrappedJS is a sign that there could be significant numbers of JS objects leaked.)

For example, start with bringing up the mail window and closing the window without doing anything. Then go on to selecting a folder, then selecting a message, and then other activities one does while reading mail.

Once you’ve done this, and it doesn’t leak much, then try the action under trace-malloc or LSAN or Valgrind to find the leaks of smaller graphs of objects. (When I refer to the size of a graph of objects, I’m referring to the number of objects, not the size in bytes. Leaking many copies of a string could be a very large leak, but the object graphs are small and easy to identify using GC-based leak detection.)

What leak tools do we have?

Tool Finds Platforms Requires
Leak tools for large object graphs
Leak Gauge Windows, documents, and docshells only All platforms Any build
GC and CC logs JS objects, DOM objects, many other kinds of objects All platforms Any build
Leak tools for medium-size object graphs
BloatView, refcount tracing and balancing Objects that implement nsISupports or use MOZ_COUNT_{CTOR,DTOR} All tier 1 platforms Debug build (or build opt with --enable-logrefcnt)
Leak tools for debugging memory growth that is cleaned up on shutdown

Common leak patterns

When trying to find a leak of reference-counted objects, there are a number of patterns that could cause the leak:

  1. Ownership cycles. The most common source of hard-to-fix leaks is ownership cycles. If you can avoid creating cycles in the first place, please do, since it’s often hard to be sure to break the cycle in every last case. Sometimes these cycles extend through JS objects (discussed further below), and since JS is garbage-collected, every pointer acts like an owning pointer and the potential for fan-out is larger. See bug 106860{.external .text} and bug 84136{.external .text} for examples. (Is this advice still accurate now that we have a cycle collector? –Jesse)

  2. Dropping a reference on the floor by:

    1. Forgetting to release (because you weren’t using nsCOMPtr when you should have been): See bug 99180{.external .text} or bug 93087{.external .text} for an example or bug 28555{.external .text} for a slightly more interesting one. This is also a frequent problem around early returns when not using nsCOMPtr.

    2. Double-AddRef: This happens most often when assigning the result of a function that returns an AddRefed pointer (bad!) into an nsCOMPtr without using dont_AddRef(). See bug 76091{.external .text} or bug 49648{.external .text} for an example.

    3. [Obscure] Double-assignment into the same variable: If you release a member variable and then assign into it by calling another function that does the same thing, you can leak the object assigned into the variable by the inner function. (This can happen equally with or without nsCOMPtr.) See bug 38586{.external .text} and bug 287847{.external .text} for examples.

  3. Dropping a non-refcounted object on the floor (especially one that owns references to reference counted objects). See bug 109671{.external .text} for an example.

  4. Destructors that should have been virtual: If you expect to override an object’s destructor (which includes giving a derived class of it an nsCOMPtr member variable) and delete that object through a pointer to the base class using delete, its destructor better be virtual. (But we have many virtual destructors in the codebase that don’t need to be – don’t do that.)

Debugging leaks that go through XPConnect

Many large object graphs that leak go through XPConnect{.external .text}. This can mean there will be XPConnect wrapper objects showing up as owning the leaked objects, but it doesn’t mean it’s XPConnect’s fault (although that has been known to happen{.external .text}, it’s rare). Debugging leaks that go through XPConnect requires a basic understanding of what XPConnect does. XPConnect allows an XPCOM object to be exposed to JavaScript, and it allows certain JavaScript objects to be exposed to C++ code as normal XPCOM objects.

When a C++ object is exposed to JavaScript (the more common of the two), an XPCWrappedNative object is created. This wrapper owns a reference to the native object until the corresponding JavaScript object is garbage-collected. This means that if there are leaked GC roots from which the wrapper is reachable, the wrapper will never release its reference on the native object. While this can be debugged in detail, the quickest way to solve these problems is often to simply debug the leaked JS roots. These roots are printed on shutdown in DEBUG builds, and the name of the root should give the type of object it is associated with.

One of the most common ways one could leak a JS root is by leaking an nsXPCWrappedJS object. This is the wrapper object in the reverse direction – when a JS object is used to implement an XPCOM interface and be used transparently by native code. The nsXPCWrappedJS object creates a GC root that exists as long as the wrapper does. The wrapper itself is just a normal reference-counted object, so a leaked nsXPCWrappedJS can be debugged using the normal refcount-balancer tools.

If you really need to debug leaks that involve JS objects closely, you can get detailed printouts of the paths JS uses to mark objects when it is determining the set of live objects by using the functions added in bug 378261{.external .text} and bug 378255{.external .text}. (More documentation of this replacement for GC_MARK_DEBUG, the old way of doing it, would be useful. It may just involve setting the XPC_SHUTDOWN_HEAP_DUMP environment variable to a file name, but I haven’t tested that.)

Post-processing of stack traces

On Mac and Linux, the stack traces generated by our internal debugging tools don’t have very good symbol information (since they just show the results of dladdr). The stacks can be significantly improved (better symbols, and file name / line number information) by post-processing. Stacks can be piped through the script tools/rb/fix_stacks.py to do this. These scripts are designed to be run on balance trees in addition to raw stacks; since they are rather slow, it is often much faster to generate balance trees (e.g., using make-tree.pl for the refcount balancer or diffbloatdump.pl --use-address for trace-malloc) andthen run the balance trees (which are much smaller) through the post-processing.

Getting symbol information for system libraries


Setting the environment variable _NT_SYMBOL_PATH to something like symsrv*symsrv.dll*f:\localsymbols*http://msdl.microsoft.com/download/symbols as described in Microsoft’s article{.external .text}. This needs to be done when running, since we do the address to symbol mapping at runtime.


Many Linux distros provide packages containing external debugging symbols for system libraries. fix_stacks.py uses this debugging information (although it does not verify that they match the library versions on the system).

For example, on Fedora, these are in *-debuginfo RPMs (which are available in yum repositories that are disabled by default, but easily enabled by editing the system configuration).


Disabling Arena Allocation

With many lower-level leak tools (particularly trace-malloc based ones, like leaksoup) it can be helpful to disable arena allocation of objects that you’re interested in, when possible, so that each object is allocated with a separate call to malloc. Some places you can do this are:

layout engine : Define DEBUG_TRACEMALLOC_FRAMEARENA where it is commented out in layout/base/nsPresShell.cpp

glib : Set the environment variable G_SLICE=always-malloc

Other References