[platform/upstream/git.git] / Documentation / technical / api-trace2.txt

= Trace2 API

The Trace2 API can be used to print debug, performance, and telemetry
information to stderr or a file.  The Trace2 feature is inactive unless
explicitly enabled by enabling one or more Trace2 Targets.

The Trace2 API is intended to replace the existing (Trace1)
printf-style tracing provided by the existing `GIT_TRACE` and
`GIT_TRACE_PERFORMANCE` facilities.  During initial implementation,
Trace2 and Trace1 may operate in parallel.

The Trace2 API defines a set of high-level messages with known fields,
such as (`start`: `argv`) and (`exit`: {`exit-code`, `elapsed-time`}).

Trace2 instrumentation throughout the Git code base sends Trace2
messages to the enabled Trace2 Targets.  Targets transform these
messages content into purpose-specific formats and write events to
their data streams.  In this manner, the Trace2 API can drive
many different types of analysis.

Targets are defined using a VTable allowing easy extension to other
formats in the future.  This might be used to define a binary format,
for example.

Trace2 is controlled using `trace2.*` config values in the system and
global config files and `GIT_TRACE2*` environment variables.  Trace2 does
not read from repo local or worktree config files or respect `-c`
command line config settings.

== Trace2 Targets

Trace2 defines the following set of Trace2 Targets.
Format details are given in a later section.

=== The Normal Format Target

The normal format target is a tradition printf format and similar
to GIT_TRACE format.  This format is enabled with the `GIT_TR`
environment variable or the `trace2.normalTarget` system or global
config setting.

For example

------------
$ export GIT_TRACE2=~/log.normal
$ git version
git version 2.20.1.155.g426c96fcdb
------------

or

------------
$ git config --global trace2.normalTarget ~/log.normal
$ git version
git version 2.20.1.155.g426c96fcdb
------------

yields

------------
$ cat ~/log.normal
12:28:42.620009 common-main.c:38                  version 2.20.1.155.g426c96fcdb
12:28:42.620989 common-main.c:39                  start git version
12:28:42.621101 git.c:432                         cmd_name version (version)
12:28:42.621215 git.c:662                         exit elapsed:0.001227 code:0
12:28:42.621250 trace2/tr2_tgt_normal.c:124       atexit elapsed:0.001265 code:0
------------

=== The Performance Format Target

The performance format target (PERF) is a column-based format to
replace GIT_TRACE_PERFORMANCE and is suitable for development and
testing, possibly to complement tools like gprof.  This format is
enabled with the `GIT_TRACE2_PERF` environment variable or the
`trace2.perfTarget` system or global config setting.

For example

------------
$ export GIT_TRACE2_PERF=~/log.perf
$ git version
git version 2.20.1.155.g426c96fcdb
------------

or

------------
$ git config --global trace2.perfTarget ~/log.perf
$ git version
git version 2.20.1.155.g426c96fcdb
------------

yields

------------
$ cat ~/log.perf
12:28:42.620675 common-main.c:38                  | d0 | main                     | version      |     |           |           |            | 2.20.1.155.g426c96fcdb
12:28:42.621001 common-main.c:39                  | d0 | main                     | start        |     |  0.001173 |           |            | git version
12:28:42.621111 git.c:432                         | d0 | main                     | cmd_name     |     |           |           |            | version (version)
12:28:42.621225 git.c:662                         | d0 | main                     | exit         |     |  0.001227 |           |            | code:0
12:28:42.621259 trace2/tr2_tgt_perf.c:211         | d0 | main                     | atexit       |     |  0.001265 |           |            | code:0
------------

=== The Event Format Target

The event format target is a JSON-based format of event data suitable
for telemetry analysis.  This format is enabled with the `GIT_TRACE2_EVENT`
environment variable or the `trace2.eventTarget` system or global config
setting.

For example

------------
$ export GIT_TRACE2_EVENT=~/log.event
$ git version
git version 2.20.1.155.g426c96fcdb
------------

or

------------
$ git config --global trace2.eventTarget ~/log.event
$ git version
git version 2.20.1.155.g426c96fcdb
------------

yields

------------
$ cat ~/log.event
{"event":"version","sid":"sid":"20190408T191610.507018Z-H9b68c35f-P000059a8","thread":"main","time":"2019-01-16T17:28:42.620713Z","file":"common-main.c","line":38,"evt":"1","exe":"2.20.1.155.g426c96fcdb"}
{"event":"start","sid":"20190408T191610.507018Z-H9b68c35f-P000059a8","thread":"main","time":"2019-01-16T17:28:42.621027Z","file":"common-main.c","line":39,"t_abs":0.001173,"argv":["git","version"]}
{"event":"cmd_name","sid":"20190408T191610.507018Z-H9b68c35f-P000059a8","thread":"main","time":"2019-01-16T17:28:42.621122Z","file":"git.c","line":432,"name":"version","hierarchy":"version"}
{"event":"exit","sid":"20190408T191610.507018Z-H9b68c35f-P000059a8","thread":"main","time":"2019-01-16T17:28:42.621236Z","file":"git.c","line":662,"t_abs":0.001227,"code":0}
{"event":"atexit","sid":"20190408T191610.507018Z-H9b68c35f-P000059a8","thread":"main","time":"2019-01-16T17:28:42.621268Z","file":"trace2/tr2_tgt_event.c","line":163,"t_abs":0.001265,"code":0}
------------

=== Enabling a Target

To enable a target, set the corresponding environment variable or
system or global config value to one of the following:

include::../trace2-target-values.txt[]

If the target already exists and is a directory, the traces will be
written to files (one per process) underneath the given directory. They
will be named according to the last component of the SID (optionally
followed by a counter to avoid filename collisions).

== Trace2 API

All public Trace2 functions and macros are defined in `trace2.h` and
`trace2.c`.  All public symbols are prefixed with `trace2_`.

There are no public Trace2 data structures.

The Trace2 code also defines a set of private functions and data types
in the `trace2/` directory.  These symbols are prefixed with `tr2_`
and should only be used by functions in `trace2.c`.

== Conventions for Public Functions and Macros

The functions defined by the Trace2 API are declared and documented
in `trace2.h`.  It defines the API functions and wrapper macros for
Trace2.

Some functions have a `_fl()` suffix to indicate that they take `file`
and `line-number` arguments.

Some functions have a `_va_fl()` suffix to indicate that they also
take a `va_list` argument.

Some functions have a `_printf_fl()` suffix to indicate that they also
take a varargs argument.

There are CPP wrapper macros and ifdefs to hide most of these details.
See `trace2.h` for more details.  The following discussion will only
describe the simplified forms.

== Public API

All Trace2 API functions send a messsage to all of the active
Trace2 Targets.  This section describes the set of available
messages.

It helps to divide these functions into groups for discussion
purposes.

=== Basic Command Messages

These are concerned with the lifetime of the overall git process.

`void trace2_initialize_clock()`::

        Initialize the Trace2 start clock and nothing else.  This should
        be called at the very top of main() to capture the process start
        time and reduce startup order dependencies.

`void trace2_initialize()`::

        Determines if any Trace2 Targets should be enabled and
        initializes the Trace2 facility.  This includes setting up the
        Trace2 thread local storage (TLS).
+
This function emits a "version" message containing the version of git
and the Trace2 protocol.
+
This function should be called from `main()` as early as possible in
the life of the process after essential process initialization.

`int trace2_is_enabled()`::

        Returns 1 if Trace2 is enabled (at least one target is
        active).

`void trace2_cmd_start(int argc, const char **argv)`::

        Emits a "start" message containing the process command line
        arguments.

`int trace2_cmd_exit(int exit_code)`::

        Emits an "exit" message containing the process exit-code and
        elapsed time.
+
Returns the exit-code.

`void trace2_cmd_error(const char *fmt, va_list ap)`::

        Emits an "error" message containing a formatted error message.

`void trace2_cmd_path(const char *pathname)`::

        Emits a "cmd_path" message with the full pathname of the
        current process.

=== Command Detail Messages

These are concerned with describing the specific Git command
after the command line, config, and environment are inspected.

`void trace2_cmd_name(const char *name)`::

        Emits a "cmd_name" message with the canonical name of the
        command, for example "status" or "checkout".

`void trace2_cmd_mode(const char *mode)`::

        Emits a "cmd_mode" message with a qualifier name to further
        describe the current git command.
+
This message is intended to be used with git commands having multiple
major modes.  For example, a "checkout" command can checkout a new
branch or it can checkout a single file, so the checkout code could
emit a cmd_mode message of "branch" or "file".

`void trace2_cmd_alias(const char *alias, const char **argv_expansion)`::

        Emits an "alias" message containing the alias used and the
        argument expansion.

`void trace2_def_param(const char *parameter, const char *value)`::

        Emits a "def_param" message containing a key/value pair.
+
This message is intended to report some global aspect of the current
command, such as a configuration setting or command line switch that
significantly affects program performance or behavior, such as
`core.abbrev`, `status.showUntrackedFiles`, or `--no-ahead-behind`.

`void trace2_cmd_list_config()`::

        Emits a "def_param" messages for "important" configuration
        settings.
+
The environment variable `GIT_TRACE2_CONFIG_PARAMS` or the `trace2.configParams`
config value can be set to a
list of patterns of important configuration settings, for example:
`core.*,remote.*.url`.  This function will iterate over all config
settings and emit a "def_param" message for each match.

`void trace2_cmd_set_config(const char *key, const char *value)`::

        Emits a "def_param" message for a new or updated key/value
        pair IF `key` is considered important.
+
This is used to hook into `git_config_set()` and catch any
configuration changes and update a value previously reported by
`trace2_cmd_list_config()`.

`void trace2_def_repo(struct repository *repo)`::

        Registers a repository with the Trace2 layer.  Assigns a
        unique "repo-id" to `repo->trace2_repo_id`.
+
Emits a "worktree" messages containing the repo-id and the worktree
pathname.
+
Region and data messages (described later) may refer to this repo-id.
+
The main/top-level repository will have repo-id value 1 (aka "r1").
+
The repo-id field is in anticipation of future in-proc submodule
repositories.

=== Child Process Messages

These are concerned with the various spawned child processes,
including shell scripts, git commands, editors, pagers, and hooks.

`void trace2_child_start(struct child_process *cmd)`::

        Emits a "child_start" message containing the "child-id",
        "child-argv", and "child-classification".
+
Before calling this, set `cmd->trace2_child_class` to a name
describing the type of child process, for example "editor".
+
This function assigns a unique "child-id" to `cmd->trace2_child_id`.
This field is used later during the "child_exit" message to associate
it with the "child_start" message.
+
This function should be called before spawning the child process.

`void trace2_child_exit(struct child_proess *cmd, int child_exit_code)`::

        Emits a "child_exit" message containing the "child-id",
        the child's elapsed time and exit-code.
+
The reported elapsed time includes the process creation overhead and
time spend waiting for it to exit, so it may be slightly longer than
the time reported by the child itself.
+
This function should be called after reaping the child process.

`int trace2_exec(const char *exe, const char **argv)`::

        Emits a "exec" message containing the "exec-id" and the
        argv of the new process.
+
This function should be called before calling one of the `exec()`
variants, such as `execvp()`.
+
This function returns a unique "exec-id".  This value is used later
if the exec() fails and a "exec-result" message is necessary.

`void trace2_exec_result(int exec_id, int error_code)`::

        Emits a "exec_result" message containing the "exec-id"
        and the error code.
+
On Unix-based systems, `exec()` does not return if successful.
This message is used to indicate that the `exec()` failed and
that the current program is continuing.

=== Git Thread Messages

These messages are concerned with Git thread usage.

`void trace2_thread_start(const char *thread_name)`::

        Emits a "thread_start" message.
+
The `thread_name` field should be a descriptive name, such as the
unique name of the thread-proc.  A unique "thread-id" will be added
to the name to uniquely identify thread instances.
+
Region and data messages (described later) may refer to this thread
name.
+
This function must be called by the thread-proc of the new thread
(so that TLS data is properly initialized) and not by the caller
of `pthread_create()`.

`void trace2_thread_exit()`::

        Emits a "thread_exit" message containing the thread name
        and the thread elapsed time.
+
This function must be called by the thread-proc before it returns
(so that the coorect TLS data is used and cleaned up.  It should
not be called by the caller of `pthread_join()`.

=== Region and Data Messages

These are concerned with recording performance data
over regions or spans of code.

`void trace2_region_enter(const char *category, const char *label, const struct repository *repo)`::

`void trace2_region_enter_printf(const char *category, const char *label, const struct repository *repo, const char *fmt, ...)`::

`void trace2_region_enter_printf_va(const char *category, const char *label, const struct repository *repo, const char *fmt, va_list ap)`::

        Emits a thread-relative "region_enter" message with optional
        printf string.
+
This function pushes a new region nesting stack level on the current
thread and starts a clock for the new stack frame.
+
The `category` field is an arbitrary category name used to classify
regions by feature area, such as "status" or "index".  At this time
it is only just printed along with the rest of the message.  It may
be used in the future to filter messages.
+
The `label` field is an arbitrary label used to describe the activity
being started, such as "read_recursive" or "do_read_index".
+
The `repo` field, if set, will be used to get the "repo-id", so that
recursive oerations can be attributed to the correct repository.

`void trace2_region_leave(const char *category, const char *label, const struct repository *repo)`::

`void trace2_region_leave_printf(const char *category, const char *label, const struct repository *repo, const char *fmt, ...)`::

`void trace2_region_leave_printf_va(const char *category, const char *label, const struct repository *repo, const char *fmt, va_list ap)`::

        Emits a thread-relative "region_leave" message with optional
        printf string.
+
This function pops the region nesting stack on the current thread
and reports the elapsed time of the stack frame.
+
The `category`, `label`, and `repo` fields are the same as above.
The `category` and `label` do not need to match the correpsonding
"region_enter" message, but it makes the data stream easier to
understand.

`void trace2_data_string(const char *category, const struct repository *repo, const char *key, const char * value)`::

`void trace2_data_intmax(const char *category, const struct repository *repo, const char *key, intmax value)`::

`void trace2_data_json(const char *category, const struct repository *repo, const char *key, const struct json_writer *jw)`::

        Emits a region- and thread-relative "data" or "data_json" message.
+
This is a key/value pair message containing information about the
current thread, region stack, and repository.  This could be used
to print the number of files in a directory during a multi-threaded
recursive tree walk.

`void trace2_printf(const char *fmt, ...)`::

`void trace2_printf_va(const char *fmt, va_list ap)`::

        Emits a region- and thread-relative "printf" message.

== Trace2 Target Formats

=== NORMAL Format

Events are written as lines of the form:

------------
[<time> SP <filename>:<line> SP+] <event-name> [[SP] <event-message>] LF
------------

`<event-name>`::

        is the event name.

`<event-message>`::
        is a free-form printf message intended for human consumption.
+
Note that this may contain embedded LF or CRLF characters that are
not escaped, so the event may spill across multiple lines.

If `GIT_TRACE2_BRIEF` or `trace2.normalBrief` is true, the `time`, `filename`,
and `line` fields are omitted.

This target is intended to be more of a summary (like GIT_TRACE) and
less detailed than the other targets.  It ignores thread, region, and
data messages, for example.

=== PERF Format

Events are written as lines of the form:

------------
[<time> SP <filename>:<line> SP+
    BAR SP] d<depth> SP
    BAR SP <thread-name> SP+
    BAR SP <event-name> SP+
    BAR SP [r<repo-id>] SP+
    BAR SP [<t_abs>] SP+
    BAR SP [<t_rel>] SP+
    BAR SP [<category>] SP+
    BAR SP DOTS* <perf-event-message>
    LF
------------

`<depth>`::
        is the git process depth.  This is the number of parent
        git processes.  A top-level git command has depth value "d0".
        A child of it has depth value "d1".  A second level child
        has depth value "d2" and so on.

`<thread-name>`::
        is a unique name for the thread.  The primary thread
        is called "main".  Other thread names are of the form "th%d:%s"
        and include a unique number and the name of the thread-proc.

`<event-name>`::
        is the event name.

`<repo-id>`::
        when present, is a number indicating the repository
        in use.  A `def_repo` event is emitted when a repository is
        opened.  This defines the repo-id and associated worktree.
        Subsequent repo-specific events will reference this repo-id.
+
Currently, this is always "r1" for the main repository.
This field is in anticipation of in-proc submodules in the future.

`<t_abs>`::
        when present, is the absolute time in seconds since the
        program started.

`<t_rel>`::
        when present, is time in seconds relative to the start of
        the current region.  For a thread-exit event, it is the elapsed
        time of the thread.

`<category>`::
        is present on region and data events and is used to
        indicate a broad category, such as "index" or "status".

`<perf-event-message>`::
        is a free-form printf message intended for human consumption.

------------
15:33:33.532712 wt-status.c:2310                  | d0 | main                     | region_enter | r1  |  0.126064 |           | status     | label:print
15:33:33.532712 wt-status.c:2331                  | d0 | main                     | region_leave | r1  |  0.127568 |  0.001504 | status     | label:print
------------

If `GIT_TRACE2_PERF_BRIEF` or `trace2.perfBrief` is true, the `time`, `file`,
and `line` fields are omitted.

------------
d0 | main                     | region_leave | r1  |  0.011717 |  0.009122 | index      | label:preload
------------

The PERF target is intended for interactive performance analysis
during development and is quite noisy.

=== EVENT Format

Each event is a JSON-object containing multiple key/value pairs
written as a single line and followed by a LF.

------------
'{' <key> ':' <value> [',' <key> ':' <value>]* '}' LF
------------

Some key/value pairs are common to all events and some are
event-specific.

==== Common Key/Value Pairs

The following key/value pairs are common to all events:

------------
{
        "event":"version",
        "sid":"20190408T191827.272759Z-H9b68c35f-P00003510",
        "thread":"main",
        "time":"2019-04-08T19:18:27.282761Z",
        "file":"common-main.c",
        "line":42,
        ...
}
------------

`"event":<event>`::
        is the event name.

`"sid":<sid>`::
        is the session-id.  This is a unique string to identify the
        process instance to allow all events emitted by a process to
        be identified.  A session-id is used instead of a PID because
        PIDs are recycled by the OS.  For child git processes, the
        session-id is prepended with the session-id of the parent git
        process to allow parent-child relationships to be identified
        during post-processing.

`"thread":<thread>`::
        is the thread name.

`"time":<time>`::
        is the UTC time of the event.

`"file":<filename>`::
        is source file generating the event.

`"line":<line-number>`::
        is the integer source line number generating the event.

`"repo":<repo-id>`::
        when present, is the integer repo-id as described previously.

If `GIT_TRACE2_EVENT_BRIEF` or `trace2.eventBrief` is true, the `file`
and `line` fields are omitted from all events and the `time` field is
only present on the "start" and "atexit" events.

==== Event-Specific Key/Value Pairs

`"version"`::
        This event gives the version of the executable and the EVENT format.
+
------------
{
        "event":"version",
        ...
        "evt":"1",                     # EVENT format version
        "exe":"2.20.1.155.g426c96fcdb" # git version
}
------------

`"start"`::
        This event contains the complete argv received by main().
+
------------
{
        "event":"start",
        ...
        "t_abs":0.001227, # elapsed time in seconds
        "argv":["git","version"]
}
------------

`"exit"`::
        This event is emitted when git calls `exit()`.
+
------------
{
        "event":"exit",
        ...
        "t_abs":0.001227, # elapsed time in seconds
        "code":0          # exit code
}
------------

`"atexit"`::
        This event is emitted by the Trace2 `atexit` routine during
        final shutdown.  It should be the last event emitted by the
        process.
+
(The elapsed time reported here is greater than the time reported in
the "exit" event because it runs after all other atexit tasks have
completed.)
+
------------
{
        "event":"atexit",
        ...
        "t_abs":0.001227, # elapsed time in seconds
        "code":0          # exit code
}
------------

`"signal"`::
        This event is emitted when the program is terminated by a user
        signal.  Depending on the platform, the signal event may
        prevent the "atexit" event from being generated.
+
------------
{
        "event":"signal",
        ...
        "t_abs":0.001227,  # elapsed time in seconds
        "signal":13        # SIGTERM, SIGINT, etc.
}
------------

`"error"`::
        This event is emitted when one of the `error()`, `die()`,
        or `usage()` functions are called.
+
------------
{
        "event":"error",
        ...
        "msg":"invalid option: --cahced", # formatted error message
        "fmt":"invalid option: %s"        # error format string
}
------------
+
The error event may be emitted more than once.  The format string
allows post-processors to group errors by type without worrying
about specific error arguments.

`"cmd_path"`::
        This event contains the discovered full path of the git
        executable (on platforms that are configured to resolve it).
+
------------
{
        "event":"cmd_path",
        ...
        "path":"C:/work/gfw/git.exe"
}
------------

`"cmd_name"`::
        This event contains the command name for this git process
        and the hierarchy of commands from parent git processes.
+
------------
{
        "event":"cmd_name",
        ...
        "name":"pack-objects",
        "hierarchy":"push/pack-objects"
}
------------
+
Normally, the "name" field contains the canonical name of the
command.  When a canonical name is not available, one of
these special values are used:
+
------------
"_query_"            # "git --html-path"
"_run_dashed_"       # when "git foo" tries to run "git-foo"
"_run_shell_alias_"  # alias expansion to a shell command
"_run_git_alias_"    # alias expansion to a git command
"_usage_"            # usage error
------------

`"cmd_mode"`::
        This event, when present, describes the command variant This
        event may be emitted more than once.
+
------------
{
        "event":"cmd_mode",
        ...
        "name":"branch"
}
------------
+
The "name" field is an arbitrary string to describe the command mode.
For example, checkout can checkout a branch or an individual file.
And these variations typically have different performance
characteristics that are not comparable.

`"alias"`::
        This event is present when an alias is expanded.
+
------------
{
        "event":"alias",
        ...
        "alias":"l",             # registered alias
        "argv":["log","--graph"] # alias expansion
}
------------

`"child_start"`::
        This event describes a child process that is about to be
        spawned.
+
------------
{
        "event":"child_start",
        ...
        "child_id":2,
        "child_class":"?",
        "use_shell":false,
        "argv":["git","rev-list","--objects","--stdin","--not","--all","--quiet"]

        "hook_name":"<hook_name>"  # present when child_class is "hook"
        "cd":"<path>"              # present when cd is required
}
------------
+
The "child_id" field can be used to match this child_start with the
corresponding child_exit event.
+
The "child_class" field is a rough classification, such as "editor",
"pager", "transport/*", and "hook".  Unclassified children are classified
with "?".

`"child_exit"`::
        This event is generated after the current process has returned
        from the waitpid() and collected the exit information from the
        child.
+
------------
{
        "event":"child_exit",
        ...
        "child_id":2,
        "pid":14708,     # child PID
        "code":0,        # child exit-code
        "t_rel":0.110605 # observed run-time of child process
}
------------
+
Note that the session-id of the child process is not available to
the current/spawning process, so the child's PID is reported here as
a hint for post-processing.  (But it is only a hint because the child
proces may be a shell script which doesn't have a session-id.)
+
Note that the `t_rel` field contains the observed run time in seconds
for the child process (starting before the fork/exec/spawn and
stopping after the waitpid() and includes OS process creation overhead).
So this time will be slightly larger than the atexit time reported by
the child process itself.

`"exec"`::
        This event is generated before git attempts to `exec()`
        another command rather than starting a child process.
+
------------
{
        "event":"exec",
        ...
        "exec_id":0,
        "exe":"git",
        "argv":["foo", "bar"]
}
------------
+
The "exec_id" field is a command-unique id and is only useful if the
`exec()` fails and a corresponding exec_result event is generated.

`"exec_result"`::
        This event is generated if the `exec()` fails and control
        returns to the current git command.
+
------------
{
        "event":"exec_result",
        ...
        "exec_id":0,
        "code":1      # error code (errno) from exec()
}
------------

`"thread_start"`::
        This event is generated when a thread is started.  It is
        generated from *within* the new thread's thread-proc (for TLS
        reasons).
+
------------
{
        "event":"thread_start",
        ...
        "thread":"th02:preload_thread" # thread name
}
------------

`"thread_exit"`::
        This event is generated when a thread exits.  It is generated
        from *within* the thread's thread-proc (for TLS reasons).
+
------------
{
        "event":"thread_exit",
        ...
        "thread":"th02:preload_thread", # thread name
        "t_rel":0.007328                # thread elapsed time
}
------------

`"def_param"`::
        This event is generated to log a global parameter.
+
------------
{
        "event":"def_param",
        ...
        "param":"core.abbrev",
        "value":"7"
}
------------

`"def_repo"`::
        This event defines a repo-id and associates it with the root
        of the worktree.
+
------------
{
        "event":"def_repo",
        ...
        "repo":1,
        "worktree":"/Users/jeffhost/work/gfw"
}
------------
+
As stated earlier, the repo-id is currently always 1, so there will
only be one def_repo event.  Later, if in-proc submodules are
supported, a def_repo event should be emitted for each submodule
visited.

`"region_enter"`::
        This event is generated when entering a region.
+
------------
{
        "event":"region_enter",
        ...
        "repo":1,                # optional
        "nesting":1,             # current region stack depth
        "category":"index",      # optional
        "label":"do_read_index", # optional
        "msg":".git/index"       # optional
}
------------
+
The `category` field may be used in a future enhancement to
do category-based filtering.
+
`GIT_TRACE2_EVENT_NESTING` or `trace2.eventNesting` can be used to
filter deeply nested regions and data events.  It defaults to "2".

`"region_leave"`::
        This event is generated when leaving a region.
+
------------
{
        "event":"region_leave",
        ...
        "repo":1,                # optional
        "t_rel":0.002876,        # time spent in region in seconds
        "nesting":1,             # region stack depth
        "category":"index",      # optional
        "label":"do_read_index", # optional
        "msg":".git/index"       # optional
}
------------

`"data"`::
        This event is generated to log a thread- and region-local
        key/value pair.
+
------------
{
        "event":"data",
        ...
        "repo":1,              # optional
        "t_abs":0.024107,      # absolute elapsed time
        "t_rel":0.001031,      # elapsed time in region/thread
        "nesting":2,           # region stack depth
        "category":"index",
        "key":"read/cache_nr",
        "value":"3552"
}
------------
+
The "value" field may be an integer or a string.

`"data-json"`::
        This event is generated to log a pre-formatted JSON string
        containing structured data.
+
------------
{
        "event":"data_json",
        ...
        "repo":1,              # optional
        "t_abs":0.015905,
        "t_rel":0.015905,
        "nesting":1,
        "category":"process",
        "key":"windows/ancestry",
        "value":["bash.exe","bash.exe"]
}
------------

== Example Trace2 API Usage

Here is a hypothetical usage of the Trace2 API showing the intended
usage (without worrying about the actual Git details).

Initialization::

        Initialization happens in `main()`.  Behind the scenes, an
        `atexit` and `signal` handler are registered.
+
----------------
int main(int argc, const char **argv)
{
        int exit_code;

        trace2_initialize();
        trace2_cmd_start(argv);

        exit_code = cmd_main(argc, argv);

        trace2_cmd_exit(exit_code);

        return exit_code;
}
----------------

Command Details::

        After the basics are established, additional command
        information can be sent to Trace2 as it is discovered.
+
----------------
int cmd_checkout(int argc, const char **argv)
{
        trace2_cmd_name("checkout");
        trace2_cmd_mode("branch");
        trace2_def_repo(the_repository);

        // emit "def_param" messages for "interesting" config settings.
        trace2_cmd_list_config();

        if (do_something())
            trace2_cmd_error("Path '%s': cannot do something", path);

        return 0;
}
----------------

Child Processes::

        Wrap code spawning child processes.
+
----------------
void run_child(...)
{
        int child_exit_code;
        struct child_process cmd = CHILD_PROCESS_INIT;
        ...
        cmd.trace2_child_class = "editor";

        trace2_child_start(&cmd);
        child_exit_code = spawn_child_and_wait_for_it();
        trace2_child_exit(&cmd, child_exit_code);
}
----------------
+
For example, the following fetch command spawned ssh, index-pack,
rev-list, and gc.  This example also shows that fetch took
5.199 seconds and of that 4.932 was in ssh.
+
----------------
$ export GIT_TRACE2_BRIEF=1
$ export GIT_TRACE2=~/log.normal
$ git fetch origin
...
----------------
+
----------------
$ cat ~/log.normal
version 2.20.1.vfs.1.1.47.g534dbe1ad1
start git fetch origin
worktree /Users/jeffhost/work/gfw
cmd_name fetch (fetch)
child_start[0] ssh git@github.com ...
child_start[1] git index-pack ...
... (Trace2 events from child processes omitted)
child_exit[1] pid:14707 code:0 elapsed:0.076353
child_exit[0] pid:14706 code:0 elapsed:4.931869
child_start[2] git rev-list ...
... (Trace2 events from child process omitted)
child_exit[2] pid:14708 code:0 elapsed:0.110605
child_start[3] git gc --auto
... (Trace2 events from child process omitted)
child_exit[3] pid:14709 code:0 elapsed:0.006240
exit elapsed:5.198503 code:0
atexit elapsed:5.198541 code:0
----------------
+
When a git process is a (direct or indirect) child of another
git process, it inherits Trace2 context information.  This
allows the child to print the command hierarchy.  This example
shows gc as child[3] of fetch.  When the gc process reports
its name as "gc", it also reports the hierarchy as "fetch/gc".
(In this example, trace2 messages from the child process is
indented for clarity.)
+
----------------
$ export GIT_TRACE2_BRIEF=1
$ export GIT_TRACE2=~/log.normal
$ git fetch origin
...
----------------
+
----------------
$ cat ~/log.normal
version 2.20.1.160.g5676107ecd.dirty
start git fetch official
worktree /Users/jeffhost/work/gfw
cmd_name fetch (fetch)
...
child_start[3] git gc --auto
    version 2.20.1.160.g5676107ecd.dirty
    start /Users/jeffhost/work/gfw/git gc --auto
    worktree /Users/jeffhost/work/gfw
    cmd_name gc (fetch/gc)
    exit elapsed:0.001959 code:0
    atexit elapsed:0.001997 code:0
child_exit[3] pid:20303 code:0 elapsed:0.007564
exit elapsed:3.868938 code:0
atexit elapsed:3.868970 code:0
----------------

Regions::

        Regions can be use to time an interesting section of code.
+
----------------
void wt_status_collect(struct wt_status *s)
{
        trace2_region_enter("status", "worktrees", s->repo);
        wt_status_collect_changes_worktree(s);
        trace2_region_leave("status", "worktrees", s->repo);

        trace2_region_enter("status", "index", s->repo);
        wt_status_collect_changes_index(s);
        trace2_region_leave("status", "index", s->repo);

        trace2_region_enter("status", "untracked", s->repo);
        wt_status_collect_untracked(s);
        trace2_region_leave("status", "untracked", s->repo);
}

void wt_status_print(struct wt_status *s)
{
        trace2_region_enter("status", "print", s->repo);
        switch (s->status_format) {
            ...
        }
        trace2_region_leave("status", "print", s->repo);
}
----------------
+
In this example, scanning for untracked files ran from +0.012568 to
+0.027149 (since the process started) and took 0.014581 seconds.
+
----------------
$ export GIT_TRACE2_PERF_BRIEF=1
$ export GIT_TRACE2_PERF=~/log.perf
$ git status
...

$ cat ~/log.perf
d0 | main                     | version      |     |           |           |            | 2.20.1.160.g5676107ecd.dirty
d0 | main                     | start        |     |  0.001173 |           |            | git status
d0 | main                     | def_repo     | r1  |           |           |            | worktree:/Users/jeffhost/work/gfw
d0 | main                     | cmd_name     |     |           |           |            | status (status)
...
d0 | main                     | region_enter | r1  |  0.010988 |           | status     | label:worktrees
d0 | main                     | region_leave | r1  |  0.011236 |  0.000248 | status     | label:worktrees
d0 | main                     | region_enter | r1  |  0.011260 |           | status     | label:index
d0 | main                     | region_leave | r1  |  0.012542 |  0.001282 | status     | label:index
d0 | main                     | region_enter | r1  |  0.012568 |           | status     | label:untracked
d0 | main                     | region_leave | r1  |  0.027149 |  0.014581 | status     | label:untracked
d0 | main                     | region_enter | r1  |  0.027411 |           | status     | label:print
d0 | main                     | region_leave | r1  |  0.028741 |  0.001330 | status     | label:print
d0 | main                     | exit         |     |  0.028778 |           |            | code:0
d0 | main                     | atexit       |     |  0.028809 |           |            | code:0
----------------
+
Regions may be nested.  This causes messages to be indented in the
PERF target, for example.
Elapsed times are relative to the start of the correpsonding nesting
level as expected.  For example, if we add region message to:
+
----------------
static enum path_treatment read_directory_recursive(struct dir_struct *dir,
        struct index_state *istate, const char *base, int baselen,
        struct untracked_cache_dir *untracked, int check_only,
        int stop_at_first_file, const struct pathspec *pathspec)
{
        enum path_treatment state, subdir_state, dir_state = path_none;

        trace2_region_enter_printf("dir", "read_recursive", NULL, "%.*s", baselen, base);
        ...
        trace2_region_leave_printf("dir", "read_recursive", NULL, "%.*s", baselen, base);
        return dir_state;
}
----------------
+
We can further investigate the time spent scanning for untracked files.
+
----------------
$ export GIT_TRACE2_PERF_BRIEF=1
$ export GIT_TRACE2_PERF=~/log.perf
$ git status
...
$ cat ~/log.perf
d0 | main                     | version      |     |           |           |            | 2.20.1.162.gb4ccea44db.dirty
d0 | main                     | start        |     |  0.001173 |           |            | git status
d0 | main                     | def_repo     | r1  |           |           |            | worktree:/Users/jeffhost/work/gfw
d0 | main                     | cmd_name     |     |           |           |            | status (status)
...
d0 | main                     | region_enter | r1  |  0.015047 |           | status     | label:untracked
d0 | main                     | region_enter |     |  0.015132 |           | dir        | ..label:read_recursive
d0 | main                     | region_enter |     |  0.016341 |           | dir        | ....label:read_recursive vcs-svn/
d0 | main                     | region_leave |     |  0.016422 |  0.000081 | dir        | ....label:read_recursive vcs-svn/
d0 | main                     | region_enter |     |  0.016446 |           | dir        | ....label:read_recursive xdiff/
d0 | main                     | region_leave |     |  0.016522 |  0.000076 | dir        | ....label:read_recursive xdiff/
d0 | main                     | region_enter |     |  0.016612 |           | dir        | ....label:read_recursive git-gui/
d0 | main                     | region_enter |     |  0.016698 |           | dir        | ......label:read_recursive git-gui/po/
d0 | main                     | region_enter |     |  0.016810 |           | dir        | ........label:read_recursive git-gui/po/glossary/
d0 | main                     | region_leave |     |  0.016863 |  0.000053 | dir        | ........label:read_recursive git-gui/po/glossary/
...
d0 | main                     | region_enter |     |  0.031876 |           | dir        | ....label:read_recursive builtin/
d0 | main                     | region_leave |     |  0.032270 |  0.000394 | dir        | ....label:read_recursive builtin/
d0 | main                     | region_leave |     |  0.032414 |  0.017282 | dir        | ..label:read_recursive
d0 | main                     | region_leave | r1  |  0.032454 |  0.017407 | status     | label:untracked
...
d0 | main                     | exit         |     |  0.034279 |           |            | code:0
d0 | main                     | atexit       |     |  0.034322 |           |            | code:0
----------------
+
Trace2 regions are similar to the existing trace_performance_enter()
and trace_performance_leave() routines, but are thread safe and
maintain per-thread stacks of timers.

Data Messages::

        Data messages added to a region.
+
----------------
int read_index_from(struct index_state *istate, const char *path,
        const char *gitdir)
{
        trace2_region_enter_printf("index", "do_read_index", the_repository, "%s", path);

        ...

        trace2_data_intmax("index", the_repository, "read/version", istate->version);
        trace2_data_intmax("index", the_repository, "read/cache_nr", istate->cache_nr);

        trace2_region_leave_printf("index", "do_read_index", the_repository, "%s", path);
}
----------------
+
This example shows that the index contained 3552 entries.
+
----------------
$ export GIT_TRACE2_PERF_BRIEF=1
$ export GIT_TRACE2_PERF=~/log.perf
$ git status
...
$ cat ~/log.perf
d0 | main                     | version      |     |           |           |            | 2.20.1.156.gf9916ae094.dirty
d0 | main                     | start        |     |  0.001173 |           |            | git status
d0 | main                     | def_repo     | r1  |           |           |            | worktree:/Users/jeffhost/work/gfw
d0 | main                     | cmd_name     |     |           |           |            | status (status)
d0 | main                     | region_enter | r1  |  0.001791 |           | index      | label:do_read_index .git/index
d0 | main                     | data         | r1  |  0.002494 |  0.000703 | index      | ..read/version:2
d0 | main                     | data         | r1  |  0.002520 |  0.000729 | index      | ..read/cache_nr:3552
d0 | main                     | region_leave | r1  |  0.002539 |  0.000748 | index      | label:do_read_index .git/index
...
----------------

Thread Events::

        Thread messages added to a thread-proc.
+
For example, the multithreaded preload-index code can be
instrumented with a region around the thread pool and then
per-thread start and exit events within the threadproc.
+
----------------
static void *preload_thread(void *_data)
{
        // start the per-thread clock and emit a message.
        trace2_thread_start("preload_thread");

        // report which chunk of the array this thread was assigned.
        trace2_data_intmax("index", the_repository, "offset", p->offset);
        trace2_data_intmax("index", the_repository, "count", nr);

        do {
            ...
        } while (--nr > 0);
        ...

        // report elapsed time taken by this thread.
        trace2_thread_exit();
        return NULL;
}

void preload_index(struct index_state *index,
        const struct pathspec *pathspec,
        unsigned int refresh_flags)
{
        trace2_region_enter("index", "preload", the_repository);

        for (i = 0; i < threads; i++) {
            ... /* create thread */
        }

        for (i = 0; i < threads; i++) {
            ... /* join thread */
        }

        trace2_region_leave("index", "preload", the_repository);
}
----------------
+
In this example preload_index() was executed by the `main` thread
and started the `preload` region.  Seven threads, named
`th01:preload_thread` through `th07:preload_thread`, were started.
Events from each thread are atomically appended to the shared target
stream as they occur so they may appear in random order with respect
other threads. Finally, the main thread waits for the threads to
finish and leaves the region.
+
Data events are tagged with the active thread name.  They are used
to report the per-thread parameters.
+
----------------
$ export GIT_TRACE2_PERF_BRIEF=1
$ export GIT_TRACE2_PERF=~/log.perf
$ git status
...
$ cat ~/log.perf
...
d0 | main                     | region_enter | r1  |  0.002595 |           | index      | label:preload
d0 | th01:preload_thread      | thread_start |     |  0.002699 |           |            |
d0 | th02:preload_thread      | thread_start |     |  0.002721 |           |            |
d0 | th01:preload_thread      | data         | r1  |  0.002736 |  0.000037 | index      | offset:0
d0 | th02:preload_thread      | data         | r1  |  0.002751 |  0.000030 | index      | offset:2032
d0 | th03:preload_thread      | thread_start |     |  0.002711 |           |            |
d0 | th06:preload_thread      | thread_start |     |  0.002739 |           |            |
d0 | th01:preload_thread      | data         | r1  |  0.002766 |  0.000067 | index      | count:508
d0 | th06:preload_thread      | data         | r1  |  0.002856 |  0.000117 | index      | offset:2540
d0 | th03:preload_thread      | data         | r1  |  0.002824 |  0.000113 | index      | offset:1016
d0 | th04:preload_thread      | thread_start |     |  0.002710 |           |            |
d0 | th02:preload_thread      | data         | r1  |  0.002779 |  0.000058 | index      | count:508
d0 | th06:preload_thread      | data         | r1  |  0.002966 |  0.000227 | index      | count:508
d0 | th07:preload_thread      | thread_start |     |  0.002741 |           |            |
d0 | th07:preload_thread      | data         | r1  |  0.003017 |  0.000276 | index      | offset:3048
d0 | th05:preload_thread      | thread_start |     |  0.002712 |           |            |
d0 | th05:preload_thread      | data         | r1  |  0.003067 |  0.000355 | index      | offset:1524
d0 | th05:preload_thread      | data         | r1  |  0.003090 |  0.000378 | index      | count:508
d0 | th07:preload_thread      | data         | r1  |  0.003037 |  0.000296 | index      | count:504
d0 | th03:preload_thread      | data         | r1  |  0.002971 |  0.000260 | index      | count:508
d0 | th04:preload_thread      | data         | r1  |  0.002983 |  0.000273 | index      | offset:508
d0 | th04:preload_thread      | data         | r1  |  0.007311 |  0.004601 | index      | count:508
d0 | th05:preload_thread      | thread_exit  |     |  0.008781 |  0.006069 |            |
d0 | th01:preload_thread      | thread_exit  |     |  0.009561 |  0.006862 |            |
d0 | th03:preload_thread      | thread_exit  |     |  0.009742 |  0.007031 |            |
d0 | th06:preload_thread      | thread_exit  |     |  0.009820 |  0.007081 |            |
d0 | th02:preload_thread      | thread_exit  |     |  0.010274 |  0.007553 |            |
d0 | th07:preload_thread      | thread_exit  |     |  0.010477 |  0.007736 |            |
d0 | th04:preload_thread      | thread_exit  |     |  0.011657 |  0.008947 |            |
d0 | main                     | region_leave | r1  |  0.011717 |  0.009122 | index      | label:preload
...
d0 | main                     | exit         |     |  0.029996 |           |            | code:0
d0 | main                     | atexit       |     |  0.030027 |           |            | code:0
----------------
+
In this example, the preload region took 0.009122 seconds.  The 7 threads
took between 0.006069 and 0.008947 seconds to work on their portion of
the index.  Thread "th01" worked on 508 items at offset 0.  Thread "th02"
worked on 508 items at offset 2032.  Thread "th04" worked on 508 itemts
at offset 508.
+
This example also shows that thread names are assigned in a racy manner
as each thread starts and allocates TLS storage.

== Future Work

=== Relationship to the Existing Trace Api (api-trace.txt)

There are a few issues to resolve before we can completely
switch to Trace2.

* Updating existing tests that assume GIT_TRACE format messages.

* How to best handle custom GIT_TRACE_<key> messages?

** The GIT_TRACE_<key> mechanism allows each <key> to write to a
different file (in addition to just stderr).

** Do we want to maintain that ability or simply write to the existing
Trace2 targets (and convert <key> to a "category").