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+==============
+Crash Reporter
+==============
+
+Overview
+========
+
+The **crash reporter** is a subsystem to record and manage application
+crash data.
+
+While the subsystem is known as *crash reporter*, it helps to think of
+it more as a *process dump manager*. This is because the heart of this
+subsystem is really managing process dump files and these files are
+created not only from process crashes but also from hangs and other
+exceptional events.
+
+The crash reporter subsystem is composed of a number of pieces working
+together.
+
+Breakpad
+   Breakpad is a library and set of tools to make collecting process
+   information (notably dumps from crashes) easy. Breakpad is a 3rd
+   party project (originaly developed by Google) that is imported into
+   the tree.
+
+Dump files
+   Breakpad produces files called *dump files* that hold process data
+   (stacks, heap data, etc).
+
+Crash Reporter Client
+   The crash reporter client is a standalone executable that is launched
+   to handle dump files. This application optionally submits crashes to
+   Mozilla (or the configured server).
+
+Minidump Analyzer
+   The minidump analyzer is a standalone executable that is launched by the
+   crash reporter client or by the browser itself to extract stack traces from
+   the dump files generated during a crash. It appends the stack traces to the
+   .extra file associated with the crash dump.
+
+How Main-Process Crash Handling Works
+=====================================
+
+The crash handler is hooked up very early in the Gecko process lifetime.
+It all starts in ``XREMain::XRE_mainInit()`` from ``nsAppRunner.cpp``.
+Assuming crash reporting is enabled, this startup function registers an
+exception handler for the process and tells the crash reporter subsystem
+about basic metadata such as the application name and version.
+
+The registration of the crash reporter exception handler doubles as
+initialization of the crash reporter itself. This happens in
+``CrashReporter::SetExceptionHandler()`` from ``nsExceptionHandler.cpp``.
+The crash reporter figures out what application to use for reporting
+dumped crashes and where to store these dump files on disk. The Breakpad
+exception handler (really just a mechanism for dumping process state) is
+initialized as part of this function. The Breakpad exception handler is
+a ``google_breakpad::ExceptionHandler`` instance and it's stored as
+``gExceptionHandler``.
+
+As the application runs, various other systems may write *annotations*
+or *notes* to the crash reporter to indicate state of the application,
+help with possible reasons for a current or future crash, etc. These are
+performed via ``CrashReporter::AnnotateCrashReport()`` and
+``CrashReporter::AppendAppNotesToCrashReport()`` from
+``nsExceptionHandler.h``.
+
+For well running applications, this is all that happens. However, if a
+crash or similar exceptional event occurs (such as a hang), we need to
+write a crash report.
+
+When an event worthy of writing a dump occurs, the Breakpad exception
+handler is invoked and Breakpad does its thing. When Breakpad has
+finished, it calls back into ``CrashReporter::MinidumpCallback()`` from
+``nsExceptionHandler.cpp`` to tell the crash reporter about what was
+written.
+
+``MinidumpCallback()`` performs a number of actions once a dump has been
+written. It writes a file with the time of the crash so other systems can
+easily determine the time of the last crash. It supplements the dump
+file with an *extra* file containing Mozilla-specific metadata. This data
+includes the annotations set via ``CrashReporter::AnnotateCrashReport()``
+as well as time since last crash, whether garbage collection was active at
+the time of the crash, memory statistics, etc.
+
+If the *crash reporter client* is enabled, ``MinidumpCallback()`` invokes
+it. It simply tries to create a new *crash reporter client* process (e.g.
+*crashreporter.exe*) with the path to the written minidump file as an
+argument.
+
+The *crash reporter client* performs a number of roles. There's a lot going
+on, so you may want to look at ``main()`` in ``crashreporter.cpp``. First,
+stack traces are extracted from the dump via the *minidump analyzer* tool.
+The resulting traces are appended to the .extra file of the crash. Then, the
+*crash reporter client* verifies that the dump data is sane. If it isn't
+(e.g. required metadata is missing), the dump data is ignored. If dump data
+looks sane, the dump data
+is moved into the *pending* directory for the configured data directory
+(defined via the ``MOZ_CRASHREPORTER_DATA_DIRECTORY`` environment variable
+or from the UI). Once this is done, the main crash reporter UI is displayed
+via ``UIShowCrashUI()``. The crash reporter UI is platform specific: there
+are separate versions for Windows, OS X, and various \*NIX presentation
+flavors (such as GTK). The basic gist is a dialog is displayed to the user
+and the user has the opportunity to submit this dump data to a remote
+server.
+
+If a dump is submitted via the crash reporter, the raw dump files are
+removed from the *pending* directory and a file containing the
+crash ID from the remote server for the submitted dump is created in the
+*submitted* directory.
+
+If the user chooses not to submit a dump in the crash reporter UI, the dump
+files are deleted.
+
+And that's pretty much what happens when a crash/dump is written!
+
+Plugin and Child Process Crashes
+================================
+
+Crashes in plugin and child processes are also managed by the crash
+reporting subsystem.
+
+Child process crashes are handled by the ``mozilla::dom::CrashReporterParent``
+class defined in ``dom/ipc``. When a child process crashes, the toplevel IPDL
+actor should check for it by calling TakeMinidump in its ``ActorDestroy``
+Method: see ``mozilla::plugins::PluginModuleParent::ActorDestroy`` and
+``mozilla::plugins::PluginModuleParent::ProcessFirstMinidump``. That method
+is responsible for calling
+``mozilla::dom::CrashReporterParent::GenerateCrashReportForMinidump`` with
+appropriate crash annotations specific to the crash. All child-process
+crashes are annotated with a ``ProcessType`` annotation, such as "content" or
+"plugin".
+
+Submission of child process crashes is handled by application code. This
+code prompts the user to submit crashes in context-appropriate UI and then
+submits the crashes using ``CrashSubmit.jsm``.
+
+Memory Reports
+==============
+
+When a process detects that it is running low on memory, a memory report is
+saved. If the process crashes, the memory report will be included with the crash
+report. ``nsThread::SaveMemoryReportNearOOM()`` checks to see if the process is
+low on memory every 30 seconds at most and saves a report every 3 minutes at
+most. Since a child process cannot actually save to the hard drive, it instead
+notifies its parent process, which saves the report for it. If a crash does
+occur, the memory report is moved to the *pending* directory with the other dump
+data and an annotation is added to indicate the presence of the report. This
+happens in ``nsExceptionHandler.cpp``, but occurs in different functions
+depending on what process crashed. When the main process crashes, this happens
+in ``MinidumpCallback()``. When a child process crashes, it happens in
+``OnChildProcessDumpRequested()``, with the annotation being added in
+``WriteExtraData()``.
+
+Flash Process Crashes
+=====================
+
+On Windows Vista+, the Adobe Flash plugin creates two extra processes in its
+Firefox plugin to implement OS-level sandboxing. In order to catch crashes in
+these processes, Firefox injects a crash report handler into the process using the code at ``InjectCrashReporter.cpp``. When these crashes occur, the
+ProcessType=plugin annotation is present, and an additional annotation
+FlashProcessDump has the value "Sandbox" or "Broker".
+
+Plugin Hangs
+============
+
+Plugin hangs are handled as crash reports. If a plugin doesn't respond to an
+IPC message after 60 seconds, the plugin IPC code will take minidumps of all
+of the processes involved and then kill the plugin.
+
+In this case, there will be only one .ini file with the crash report metadata,
+but there will be multiple dump files: at least one for the browser process and
+one for the plugin process, and perhaps also additional dumps for the Flash
+sandbox and broker processes. All of these files are submitted together as a
+unit. Before submission, the filenames of the files are linked:
+
+- **uuid.ini** - *annotations, includes an additional_minidumps field*
+- **uuid.dmp** - *plugin process dump file*
+- **uuid-<other>.dmp** - *other process dump file as listed in additional_minidumps*
+
+Browser Hangs
+=============
+
+There is a feature of Firefox that will crash Firefox if it stops processing
+messages after a certain period of time. This feature doesn't work well and is
+disabled by default. See ``xpcom/threads/HangMonitor.cpp``. Hang crashes
+are annotated with ``Hang=1``.
+
+about:crashes
+=============
+
+If the crash reporter subsystem is enabled, the *about:crashes*
+page will be registered with the application. This page provides
+information about previous and submitted crashes.
+
+It is also possible to submit crashes from *about:crashes*.