
"backgroundhangmonitor" ping
============================

Whenever a thread monitored by the Background Hang Monitor hangs, a stack and
some non-identifying information about the hang is captured. When 50 of these
hangs are collected across all processes, or the browser exits, this ping is
transmitted with the collected hang information.

This ping is only collected in nightly builds, to avoid the high volume of pings
which would be produced in Beta.

Structure:

.. code-block:: js

    {
      "type": "backgroundhangmonitor",
      ... // common ping data
      "environment": { ... },
      "payload": {
        "timeSinceLastPing": <number>, // uptime since last backgroundhangmonitor ping (ms).
        "modules": [
          [
            <string>, // Name of the file holding the debug information.
            <string> // Breakpad ID of this module.
          ],
          ...
        ],
        "hangs": [
          {
            "duration": <number>, // duration of the hang in milliseconds.
            "thread": <string>, // name of the hanging thread.
            "runnableName": <string>, // name of the runnable executing during the hang.
                                      // Runnable names are only collected for the XPCOM main thread.
            "process": <string>, // Type of process that hung, see below for a list of types.
            "remoteType": <string>, // Remote type of process which hung, see below.
            "annotations": [ ... ], // A list of annotations on the hang, see below.
            "pseudoStack": [ ... ], // List of label stack frames and js frames.
            "stack": [ ... ], // interleaved hang stack, see below.
          },
          ...
        ]
      }
    }

.. note::

  Hangs are collected whenever the current runnable takes over 128ms.

Process Types
-------------

The ``process`` field is a string denoting the kind of process that hung. Hangs
are currently sent only for the processes below:

+---------------+---------------------------------------------------+
| Kind          | Description                                       |
+===============+===================================================+
| default       | Main process, also known as the browser process   |
+---------------+---------------------------------------------------+
| tab           | Content process                                   |
+---------------+---------------------------------------------------+
| gpu           | GPU process                                       |
+---------------+---------------------------------------------------+

Remote Type
-----------

The ``remoteType`` field is a string denoting the type of content process that
hung. As such it is only non-null if ``processType`` contains the ``tab`` value.

The supported ``remoteType`` values are documented in the crash ping
documentation.

Stack Traces
------------

Each hang object contains a ``stack`` field which has been populated with an
interleaved stack trace of the hung thread. An interleaved stack consists of a
native backtrace with additional frames interleaved, representing chrome JS and
label stack entries.

The structure that manages the label stack and the JS stack was called
"PseudoStack" in the past and is now called "ProfilingStack".

Note that this field only contains native stack frames, label stack and chrome
JS script frames. If native stacks can not be collected on the target platform,
or stackwalking was not initialized, there will be no native frames present, and
the stack will consist only of label stack and chrome JS script frames.

A single frame in the stack is either a raw string, representing a label stack
or chrome JS script frame, or a native stack frame.

Label stack frames contain the static string from all instances of the
AUTO_PROFILER_LABEL* macros. Additionally, dynamic strings are collected from
all usages of the AUTO_PROFILER_LABEL_DYNAMIC*_NONSENSITIVE macros. The dynamic
strings are simply appended to the static strings after a space character.

Current dynamic string collections are as follows:

+--------------------------------------------------+-----------------------------------------+
| Static string                                    | Dynamic                                 |
+==================================================+=========================================+
| ChromeUtils::Import                              | Associated chrome:// or resource:// URI |
+--------------------------------------------------+-----------------------------------------+
| nsJSContext::GarbageCollectNow                   | GC reason string                        |
+--------------------------------------------------+-----------------------------------------+
| mozJSSubScriptLoader::DoLoadSubScriptWithOptions | Associated chrome:// or resource:// URI |
+--------------------------------------------------+-----------------------------------------+
| PresShell::DoFlushPendingNotifications           | Flush type                              |
+--------------------------------------------------+-----------------------------------------+
| nsObserverService::NotifyObservers               | Associated observer topic               |
+--------------------------------------------------+-----------------------------------------+

Native stack frames are as such:

.. code-block:: js

    [
      <number>, // Index in the payload.modules list of the module description.
                // -1 if this frame was not in a valid module.
      <string> // Hex string (e.g. "FF0F") of the frame offset in the module.
    ]

Annotations
-----------

The annotations field is an array of key-value pairs, for example if the user
was interacting during a hang the annotations field would look something like
this:

.. code-block:: js

    [
        [
            "UserInteracting",
            "true"
        ]
    ]

The following annotations are currently present in tree:

+-----------------+-------------------------------------------------+
| Name            | Description                                     |
+=================+=================================================+
| UserInteracting | "true" if the user was actively interacting     |
+-----------------+-------------------------------------------------+
| pluginName      | Name of the currently running plugin            |
+-----------------+-------------------------------------------------+
| pluginVersion   | Version of the currently running plugin         |
+-----------------+-------------------------------------------------+
| HangUIShown     | "true" if the hang UI was shown                 |
+-----------------+-------------------------------------------------+
| HangUIContinued | "true" if continue was selected in the hang UI  |
+-----------------+-------------------------------------------------+
| HangUIDontShow  | "true" if the hang UI was not shown             |
+-----------------+-------------------------------------------------+
| Unrecovered     | "true" if the hang persisted until process exit |
+-----------------+-------------------------------------------------+
