This document is old and some of the information is out-of-date.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) 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
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?
|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
||All tier 1 platforms||Debug build (or build opt with
|Leaksoup (part of TraceMalloc)||All objects? (or allocations?)||All tier 1 platforms||Build with
|Leak tools for simple objects and summary statistics|
|TraceMalloc||All allocations||All tier 1 platforms||Build with
|Valgrind||All allocations||Mac, Linux||Build with
|LSAN||All allocations||Mac, Linux||Any build|
|Apple tools||?||Mac||Any build|
|Leak tools for debugging memory growth that is cleaned up on shutdown|
|diffbloatdump (part of TraceMalloc)||All allocations||Linux only?||Build with
Common leak patterns
When trying to find a leak of reference-counted objects, there are a number of patterns that could cause the leak:
- 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 and bug 84136 for examples. (Is this advice still accurate now that we have a cycle collector? --Jesse)
- Dropping a reference on the floor by:
- Forgetting to release (because you weren't using
nsCOMPtrwhen you should have been): See bug 99180 or bug 93087 for an example or bug 28555 for a slightly more interesting one. This is also a frequent problem around early returns when not using
- Double-AddRef: This happens most often when assigning the result of a function that returns an AddRefed pointer (bad!) into an
dont_AddRef(). See bug 76091 or bug 49648 for an example.
- [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 and bug 287847 for examples.
- Forgetting to release (because you weren't using
- Dropping a non-refcounted object on the floor (especially one that owns references to reference counted objects). See bug 109671 for an example.
- Destructors that should have been virtual: If you expect to override an object's destructor (which includes giving a derived class of it an
nsCOMPtrmember 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
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 and bug 378255. (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 scripts
tools/rb/fix_macosx_stack.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) and
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. 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_linux_stack.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
DEBUG_TRACEMALLOC_FRAMEARENAwhere it is commented out in
Set the environment variable