The Tor Browser / file revision

 ============

 Crash Events

 ============

 **Crash Events** refers to a special subsystem of Gecko that aims to capture

 events of interest related to process crashing and hanging.

 When an event worthy of recording occurs, a file containing that event's

 information is written to a well-defined location on the filesystem. The Gecko

 process periodically scans for produced files and consolidates information

 into a more unified and efficient backend store.

 Crash Event Files

 =================

 When a crash-related event occurs, a file describing that event is written

 to a well-defined directory. That directory is likely in the directory of

 the currently-active profile. However, if a profile is not yet active in

 the Gecko process, that directory likely resides in the user's *app data*

 directory (*UAppData* from the directory service).

 The filename of the event file is not relevant. However, producers need

 to choose a filename intelligently to avoid name collisions and race

 conditions. Since file locking is potentially dangerous at crash time,

 the convention of generating a UUID and using it as a filename has been

 adopted.

 File Format

 -----------

 All crash event files share the same high-level file format. The format

 consists of the following fields delimited by a UNIX newline (*\n*)

 character:

 * String event name (valid UTF-8, but likely ASCII)

 * String representation of integer seconds since UNIX epoch

 * Payload

 The payload is event specific and may contain UNIX newline characters.

 The recommended method for parsing is to split at most 3 times on UNIX

 newline and then dispatch to an event-specific parsed based on the

 event name.

 If an unknown event type is encountered, the event can safely be ignored

 until later. This helps ensure that application downgrades (potentially

 due to elevated crash rate) don't result in data loss.

 The format and semantics of each event type are meant to be constant once

 that event type is committed to the main Firefox repository. If new metadata

 needs to be captured or the meaning of data captured in an event changes,

 that change should be expressed through the invention of a new event type.

 For this reason, event names are highly recommended to contain a version.

 e.g. instead of a *Gecko process crashed* event, we prefer a *Gecko process

 crashed v1* event.

 Event Types

 -----------

 Each subsection documents the different types of crash events that may be

 produced. Each section name corresponds to the first line of the crash

 event file.

 crash.main.1

 ^^^^^^^^^^^^

 This event is produced when the main process crashes.

 The payload of this event is the string crash ID, very likely a UUID.

 There should be ``UUID.dmp`` and ``UUID.extra`` files on disk, saved by

 Breakpad.

 crash.plugin.1

 ^^^^^^^^^^^^^^

 This event is produced when a plugin process crashes.

 The payload is identical to ``crash.main.1``'s.

 hang.plugin.1

 ^^^^^^^^^^^^^

 This event is produced when a plugin process hangs.

 The payload is identical to ``crash.main.1``'s.

 Aggregated Event Log

 ====================

 Crash events are aggregated together into a unified event *log*. Currently,

 this *log* is really a JSON file. However, this is an implementation detail

 and it could change at any time. The interface to crash data provided by

 the JavaScript API is the only supported interface.

 Design Considerations

 =====================

 There are many considerations influencing the design of this subsystem.

 We attempt to document them in this section.

 Decoupling of Event Files from Final Data Structure

 ---------------------------------------------------

 While it is certainly possible for the Gecko process to write directly to

 the final data structure on disk, there is an intentional decoupling between

 the production of events and their transition into final storage. Along the

 same vein, the choice to have events written to multiple files by producers

 is deliberate.

 Some recorded events are written immediately after a process crash. This is

 a very uncertain time for the host system. There is a high liklihood the

 system is in an exceptional state, such as memory exhaustion. Therefore, any

 action taken after crashing needs to be very deliberate about what it does.

 Excessive memory allocation and certain system calls may cause the system

 to crash again or the machine's condition to worsen. This means that the act

 of recording a crash event must be very light weight. Writing a new file from

 nothing is very light weight. This is one reason we write separate files.

 Another reason we write separate files is because if the main Gecko process

 itself crashes (as opposed to say a plugin process), the crash reporter (not

 Gecko) is running and the crash reporter needs to handle the writing of the

 event info. If this writing is involved (say loading, parsing, updating, and

 reserializing back to disk), this logic would need to be implemented in both

 Gecko and the crash reporter or would need to be implemented in such a way

 that both could use. Neither of these is very practical from a software

 lifecycle management perspective. It's much easier to have separate processes

 write a simple file and to let a single implementation do all the complex

 work.

 Idempotent Event Processing

 ===========================

 Processing of event files has been designed such that the result is

 idempotent regardless of what order those files are processed in. This is

 not only a good design decision, but it is arguably necessary. While event

 files are processed in order by file mtime, filesystem times may not have

 the resolution required for proper sorting. Therefore, processing order is

 merely an optimistic assumption.

 Aggregated Storage Format

 =========================

 Crash events are aggregated into a unified data structure on disk. That data

 structure is currently LZ4-compressed JSON and is represented by a single file.

 The choice of a single JSON file was initially driven by time and complexity

 concerns. Before changing the format or adding significant amounts of new

 data, some considerations must be taken into account.

 First, in well-behaving installs, crash data should be minimal. Crashes and

 hangs will be rare and thus the size of the crash data should remain small

 over time.

 The choice of a single JSON file has larger implications as the amount of

 crash data grows. As new data is accumulated, we need to read and write

 an entire file to make small updates. LZ4 compression helps reduce I/O.

 But, there is a potential for unbounded file growth. We establish a

 limit for the max age of records. Anything older than that limit is

 pruned. We also establish a daily limit on the number of crashes we will

 store. All crashes beyond the first N in a day have no payload and are

 only recorded by the presence of a count. This count ensures we can

 distinguish between ``N`` and ``100 * N``, which are very different

 values!