trace2: move doc to trace2.h

Move the functions documentation from
Documentation/technical/api-trace2.txt to trace2.h as it's easier for the
developers to find the usage information beside the code instead of looking
for it in another doc file.

Only the functions documentation section is removed from
Documentation/technical/api-trace2.txt as the file is full of
details that seemed more appropriate to be in a separate doc file
as it is, with a link to the doc file added in the trace2.h.
Also the functions doc is removed to avoid having redundandt info which
will be hard to keep syncronized with the documentation in the header file.

Signed-off-by: Heba Waly <heba.waly@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 102 insertions, 22 deletions

diff --git a/trace2.h b/trace2.h
index 050bf3c8c1..e5e81c0533 100644
--- a/trace2.h
+++ b/trace2.h
@@ -1,6 +1,40 @@
 #ifndef TRACE2_H
 #define TRACE2_H
 
+/**
+ * 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.
+ *
+ * For more info about: trace2 targets, conventions for public functions and
+ * macros, trace2 target formats and examples on trace2 API usage refer to
+ * Documentation/technical/api-trace2.txt
+ *
+ */
+
 struct child_process;
 struct repository;
 struct json_writer;
@@ -39,7 +73,12 @@ void trace2_initialize_clock(void);
 /*
  * Initialize TRACE2 tracing facility if any of the builtin TRACE2
  * targets are enabled in the system config or the environment.
- * Emits a 'version' event.
+ * This includes setting up the Trace2 thread local storage (TLS).
+ * 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.
  *
  * Cleanup/Termination is handled automatically by a registered
  * atexit() routine.
@@ -49,7 +88,7 @@ void trace2_initialize_fl(const char *file, int line);
 #define trace2_initialize() trace2_initialize_fl(__FILE__, __LINE__)
 
 /*
- * Return true if trace2 is enabled.
+ * Return 1 if trace2 is enabled (at least one target is active).
  */
 int trace2_is_enabled(void);
 
@@ -114,7 +153,8 @@ void trace2_cmd_mode_fl(const char *file, int line, const char *mode);
 #define trace2_cmd_mode(sv) trace2_cmd_mode_fl(__FILE__, __LINE__, (sv))
 
 /*
- * Emit an 'alias' expansion event.
+ * Emits an "alias" message containing the alias used and the argument
+ * expansion.
  */
 void trace2_cmd_alias_fl(const char *file, int line, const char *alias,
 			 const char **argv);
@@ -123,7 +163,7 @@ void trace2_cmd_alias_fl(const char *file, int line, const char *alias,
 	trace2_cmd_alias_fl(__FILE__, __LINE__, (alias), (argv))
 
 /*
- * Emit one or more 'def_param' events for "interesting" configuration
+ * Emit one or more 'def_param' events for "important" configuration
  * settings.
  *
  * Use the TR2_SYSENV_CFG_PARAM setting to register a comma-separated
@@ -144,7 +184,7 @@ void trace2_cmd_list_config_fl(const char *file, int line);
 
 /*
  * Emit a "def_param" event for the given config key/value pair IF
- * we consider the key to be "interesting".
+ * we consider the key to be "important".
  *
  * Use this for new/updated config settings created/updated after
  * trace2_cmd_list_config() is called.
@@ -155,20 +195,34 @@ void trace2_cmd_set_config_fl(const char *file, int line, const char *key,
 #define trace2_cmd_set_config(k, v) \
 	trace2_cmd_set_config_fl(__FILE__, __LINE__, (k), (v))
 
-/*
- * Emit a 'child_start' event prior to spawning a child process.
+/**
+ * Emits a "child_start" message containing the "child-id",
+ * "child-argv", and "child-classification".
  *
  * Before calling optionally set "cmd->trace2_child_class" to a string
  * describing the type of the child process.  For example, "editor" or
  * "pager".
+ *
+ * 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_start_fl(const char *file, int line,
 			   struct child_process *cmd);
 
 #define trace2_child_start(cmd) trace2_child_start_fl(__FILE__, __LINE__, (cmd))
 
-/*
- * Emit a 'child_exit' event after the child process completes.
+/**
+ * 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.
  */
 void trace2_child_exit_fl(const char *file, int line, struct child_process *cmd,
 			  int child_exit_code);
@@ -176,21 +230,22 @@ void trace2_child_exit_fl(const char *file, int line, struct child_process *cmd,
 #define trace2_child_exit(cmd, code) \
 	trace2_child_exit_fl(__FILE__, __LINE__, (cmd), (code))
 
-/*
+/**
  * Emit an 'exec' event prior to calling one of exec(), execv(),
  * execvp(), and etc.  On Unix-derived systems, this will be the
  * last event emitted for the current process, unless the exec
  * fails.  On Windows, exec() behaves like 'child_start' and a
  * waitpid(), so additional events may be emitted.
  *
- * Returns the "exec_id".
+ * Returns a unique "exec-id".  This value is used later
+ * if the exec() fails and a "exec-result" message is necessary.
  */
 int trace2_exec_fl(const char *file, int line, const char *exe,
 		   const char **argv);
 
 #define trace2_exec(exe, argv) trace2_exec_fl(__FILE__, __LINE__, (exe), (argv))
 
-/*
+/**
  * Emit an 'exec_result' when possible.  On Unix-derived systems,
  * this should be called after exec() returns (which only happens
  * when there is an error starting the new process).  On Windows,
@@ -226,11 +281,12 @@ void trace2_thread_exit_fl(const char *file, int line);
 #define trace2_thread_exit() trace2_thread_exit_fl(__FILE__, __LINE__)
 
 /*
- * Emit a 'param' event.
+ * Emits a "def_param" message containing a key/value pair.
  *
- * Write a "<param> = <value>" pair describing some aspect of the
- * run such as an important configuration setting or command line
- * option that significantly changes command behavior.
+ * 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_def_param_fl(const char *file, int line, const char *param,
 			 const char *value);
@@ -243,18 +299,35 @@ void trace2_def_param_fl(const char *file, int line, const char *param,
  * a trace2-repo-id to be used in subsequent activity events.
  *
  * Emits a 'worktree' event for this repo instance.
+ *
+ * Region and data messages 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.
  */
 void trace2_def_repo_fl(const char *file, int line, struct repository *repo);
 
 #define trace2_def_repo(repo) trace2_def_repo_fl(__FILE__, __LINE__, repo)
 
-/*
+/**
  * Emit a 'region_enter' event for <category>.<label> with optional
  * repo-id and printf message.
  *
- * Enter a new nesting level on the current thread and remember the
- * current time.  This controls the indenting of all subsequent events
- * on this thread.
+ * 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_enter_fl(const char *file, int line, const char *category,
 			    const char *label, const struct repository *repo, ...);
@@ -289,12 +362,17 @@ void trace2_region_enter_printf(const char *category, const char *label,
 /* clang-format on */
 #endif
 
-/*
+/**
  * Emit a 'region_leave' event for <category>.<label> with optional
  * repo-id and printf message.
  *
  * Leave current nesting level and report the elapsed time spent
  * in this nesting level.
+ *
+ * The `category`, `label`, and `repo` fields are the same as
+ * trace2_region_enter_fl. The `category` and `label` do not
+ * need to match the corresponding "region_enter" message,
+ * but it makes the data stream easier to understand.
  */
 void trace2_region_leave_fl(const char *file, int line, const char *category,
 			    const char *label, const struct repository *repo, ...);
@@ -329,10 +407,12 @@ void trace2_region_leave_printf(const char *category, const char *label,
 /* clang-format on */
 #endif
 
-/*
+/**
  * Emit a key-value pair 'data' event of the form <category>.<key> = <value>.
  * This event implicitly contains information about thread, nesting region,
  * and optional repo-id.
+ * This could be used to print the number of files in a directory during
+ * a multi-threaded recursive tree walk.
  *
  * On event-based TRACE2 targets, this generates a 'data' event suitable
  * for post-processing.  On printf-based TRACE2 targets, this is converted