= 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, nor does it 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 traditional `printf()` format and similar
to the `GIT_TRACE` format.  This format is enabled with the `GIT_TRACE2`
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":"20190408T191610.507018Z-H9b68c35f-P000059a8","thread":"main","time":"2019-01-16T17:28:42.620713Z","file":"common-main.c","line":38,"evt":"4","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[]

When trace files are written to a target directory, they will be named according
to the last component of the SID (optionally followed by a counter to avoid
filename collisions).

== Trace2 API

The Trace2 public API is defined and documented in `trace2.h`; refer to it for
more information.  All public functions and macros are prefixed
with `trace2_` and are implemented in `trace2.c`.

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` (or other private
source files in `trace2/`).

=== Conventions for Public Functions and Macros

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 `printf()` style format with a variable number of arguments.

CPP wrapper macros are defined to hide most of these details.

== 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. It
	should always be the first event in a trace session. The EVENT format
	version will be incremented if new event types are added, if existing
	fields are removed, or if there are significant changes in
	interpretation of existing events or fields. Smaller changes, such as
	adding a new field to an existing event, will not require an increment
	to the EVENT format version.
+
------------
{
	"event":"version",
	...
	"evt":"4",		       # EVENT format version
	"exe":"2.20.1.155.g426c96fcdb" # git version
}
------------

`"too_many_files"`::
	This event is written to the git-trace2-discard sentinel file if there
	are too many files in the target trace directory (see the
	trace2.maxFiles config option).
+
------------
{
	"event":"too_many_files",
	...
}
------------

`"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
	"signo":13         # SIGTERM, SIGINT, etc.
}
------------

`"error"`::
	This event is emitted when one of the `BUG()`, `bug()`, `error()`,
	`die()`, `warning()`, 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_ancestry"`::
	This event contains the text command name for the parent (and earlier
	generations of parents) of the current process, in an array ordered from
	nearest parent to furthest great-grandparent. It may not be implemented
	on all platforms.
+
------------
{
	"event":"cmd_ancestry",
	...
	"ancestry":["bash","tmux: server","systemd"]
}
------------

`"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
process 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.

`"child_ready"`::
	This event is generated after the current process has started
	a background process and released all handles to it.
+
------------
{
	"event":"child_ready",
	...
	"child_id":2,
	"pid":14708,	 # child PID
	"ready":"ready", # child ready state
	"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
process may be a shell script which doesn't have a session-id.)
+
This event is generated after the child is started in the background
and given a little time to boot up and start working.  If the child
starts up normally while the parent is still waiting, the "ready"
field will have the value "ready".
If the child is too slow to start and the parent times out, the field
will have the value "timeout".
If the child starts but the parent is unable to probe it, the field
will have the value "error".
+
After the parent process emits this event, it will release all of its
handles to the child process and treat the child as a background
daemon.  So even if the child does eventually finish booting up,
the parent will not emit an updated event.
+
Note that the `t_rel` field contains the observed run time in seconds
when the parent released the child process into the background.
The child is assumed to be a long-running daemon process and may
outlive the parent process.  So the parent's child event times should
not be compared to the child's atexit times.

`"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 (because
	it needs to access data in the thread's thread-local storage).
+
------------
{
	"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.
+
------------
{
	"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, such as a config
	setting, command-line flag, or environment variable.
+
------------
{
	"event":"def_param",
	...
	"scope":"global",
	"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"]
}
------------

`"th_timer"`::
	This event logs the amount of time that a stopwatch timer was
	running in the thread.  This event is generated when a thread
	exits for timers that requested per-thread events.
+
------------
{
	"event":"th_timer",
	...
	"category":"my_category",
	"name":"my_timer",
	"intervals":5,         # number of time it was started/stopped
	"t_total":0.052741,    # total time in seconds it was running
	"t_min":0.010061,      # shortest interval
	"t_max":0.011648       # longest interval
}
------------

`"timer"`::
	This event logs the amount of time that a stopwatch timer was
	running aggregated across all threads.  This event is generated
	when the process exits.
+
------------
{
	"event":"timer",
	...
	"category":"my_category",
	"name":"my_timer",
	"intervals":5,         # number of time it was started/stopped
	"t_total":0.052741,    # total time in seconds it was running
	"t_min":0.010061,      # shortest interval
	"t_max":0.011648       # longest interval
}
------------

`"th_counter"`::
	This event logs the value of a counter variable in a thread.
	This event is generated when a thread exits for counters that
	requested per-thread events.
+
------------
{
	"event":"th_counter",
	...
	"category":"my_category",
	"name":"my_counter",
	"count":23
}
------------

`"counter"`::
	This event logs the value of a counter variable across all threads.
	This event is generated when the process exits.  The total value
	reported here is the sum across all threads.
+
------------
{
	"event":"counter",
	...
	"category":"my_category",
	"name":"my_counter",
	"count":23
}
------------

`"printf"`::
	This event logs a human-readable message with no particular formatting
	guidelines.
+
------------
{
	"event":"printf",
	...
	"t_abs":0.015905,      # elapsed time in seconds
	"msg":"Hello world"    # optional
}
------------


== 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 used 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 corresponding 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 multi-threaded preload-index code can be
instrumented with a region around the thread pool and then
per-thread start and exit events within the thread-proc.
+
----------------
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 items
at offset 508.
+
This example also shows that thread names are assigned in a racy manner
as each thread starts.

Config (def param) Events::

	  Dump "interesting" config values to trace2 log.
+
We can optionally emit configuration events, see
`trace2.configparams` in linkgit:git-config[1] for how to enable
it.
+
----------------
$ git config --system color.ui never
$ git config --global color.ui always
$ git config --local color.ui auto
$ git config --list --show-scope | grep 'color.ui'
system  color.ui=never
global  color.ui=always
local   color.ui=auto
----------------
+
Then, mark the config `color.ui` as "interesting" config with
`GIT_TRACE2_CONFIG_PARAMS`:
+
----------------
$ export GIT_TRACE2_PERF_BRIEF=1
$ export GIT_TRACE2_PERF=~/log.perf
$ export GIT_TRACE2_CONFIG_PARAMS=color.ui
$ git version
...
$ cat ~/log.perf
d0 | main                     | version      |     |           |           |              | ...
d0 | main                     | start        |     |  0.001642 |           |              | /usr/local/bin/git version
d0 | main                     | cmd_name     |     |           |           |              | version (version)
d0 | main                     | def_param    |     |           |           | scope:system | color.ui:never
d0 | main                     | def_param    |     |           |           | scope:global | color.ui:always
d0 | main                     | def_param    |     |           |           | scope:local  | color.ui:auto
d0 | main                     | data         | r0  |  0.002100 |  0.002100 | fsync        | fsync/writeout-only:0
d0 | main                     | data         | r0  |  0.002126 |  0.002126 | fsync        | fsync/hardware-flush:0
d0 | main                     | exit         |     |  0.000470 |           |              | code:0
d0 | main                     | atexit       |     |  0.000477 |           |              | code:0
----------------

Stopwatch Timer Events::

	Measure the time spent in a function call or span of code
	that might be called from many places within the code
	throughout the life of the process.
+
----------------
static void expensive_function(void)
{
	trace2_timer_start(TRACE2_TIMER_ID_TEST1);
	...
	sleep_millisec(1000); // Do something expensive
	...
	trace2_timer_stop(TRACE2_TIMER_ID_TEST1);
}

static int ut_100timer(int argc, const char **argv)
{
	...

	expensive_function();

	// Do something else 1...

	expensive_function();

	// Do something else 2...

	expensive_function();

	return 0;
}
----------------
+
In this example, we measure the total time spent in
`expensive_function()` regardless of when it is called
in the overall flow of the program.
+
----------------
$ export GIT_TRACE2_PERF_BRIEF=1
$ export GIT_TRACE2_PERF=~/log.perf
$ t/helper/test-tool trace2 100timer 3 1000
...
$ cat ~/log.perf
d0 | main                     | version      |     |           |           |              | ...
d0 | main                     | start        |     |  0.001453 |           |              | t/helper/test-tool trace2 100timer 3 1000
d0 | main                     | cmd_name     |     |           |           |              | trace2 (trace2)
d0 | main                     | exit         |     |  3.003667 |           |              | code:0
d0 | main                     | timer        |     |           |           | test         | name:test1 intervals:3 total:3.001686 min:1.000254 max:1.000929
d0 | main                     | atexit       |     |  3.003796 |           |              | code:0
----------------


== 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").
