trace: move doc to trace.h

Move the documentation from Documentation/technical/api-trace.txt
to trace.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.

Documentation/technical/api-trace.txt is removed because the
information it has is now redundant and it'll be hard to keep it up to
date and synchronized with the documentation in the header file.

Signed-off-by: Heba Waly <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: HebaWaly

Documentation/technical/api-trace.txt

@@ -4,6 +4,82 @@
 #include "git-compat-util.h"
 #include "strbuf.h"
 
+/**
+ * The trace API can be used to print debug messages to stderr or a file. Trace
+ * code is inactive unless explicitly enabled by setting `GIT_TRACE*` environment
+ * variables.
+ *
+ * The trace implementation automatically adds `timestamp file:line ... \n` to
+ * all trace messages. E.g.:
+ *
+ * ------------
+ * 23:59:59.123456 git.c:312               trace: built-in: git 'foo'
+ * 00:00:00.000001 builtin/foo.c:99        foo: some message
+ * ------------
+ *
+ * Bugs & Caveats
+ * --------------
+ *
+ * GIT_TRACE_* environment variables can be used to tell Git to show
+ * trace output to its standard error stream. Git can often spawn a pager
+ * internally to run its subcommand and send its standard output and
+ * standard error to it.
+ *
+ * Because GIT_TRACE_PERFORMANCE trace is generated only at the very end
+ * of the program with atexit(), which happens after the pager exits, it
+ * would not work well if you send its log to the standard error output
+ * and let Git spawn the pager at the same time.
+ *
+ * As a work around, you can for example use '--no-pager', or set
+ * GIT_TRACE_PERFORMANCE to another file descriptor which is redirected
+ * to stderr, or set GIT_TRACE_PERFORMANCE to a file specified by its
+ * absolute path.
+ *
+ * For example instead of the following command which by default may not
+ * print any performance information:
+ *
+ * ------------
+ * GIT_TRACE_PERFORMANCE=2 git log -1
+ * ------------
+ *
+ * you may want to use:
+ *
+ * ------------
+ * GIT_TRACE_PERFORMANCE=2 git --no-pager log -1
+ * ------------
+ *
+ * or:
+ *
+ * ------------
+ * GIT_TRACE_PERFORMANCE=3 3>&2 git log -1
+ * ------------
+ *
+ * or:
+ *
+ * ------------
+ * GIT_TRACE_PERFORMANCE=/path/to/log/file git log -1
+ * ------------
+ *
+ */
+
+/**
+ * Defines a trace key (or category). The default (for API functions that
+ * don't take a key) is `GIT_TRACE`.
+ *
+ * E.g. to define a trace key controlled by environment variable `GIT_TRACE_FOO`:
+ *
+ * ------------
+ * static struct trace_key trace_foo = TRACE_KEY_INIT(FOO);
+ *
+ * static void trace_print_foo(const char *message)
+ * {
+ * 	trace_printf_key(&trace_foo, "%s", message);
+ * }
+ * ------------
+ *
+ * Note: don't use `const` as the trace implementation stores internal state in
+ * the `trace_key` structure.
+ */
 struct trace_key {
 	const char * const key;
 	int fd;
@@ -18,31 +94,84 @@ extern struct trace_key trace_perf_key;
 extern struct trace_key trace_setup_key;
 
 void trace_repo_setup(const char *prefix);
+
+/**
+ * Checks whether the trace key is enabled. Used to prevent expensive
+ * string formatting before calling one of the printing APIs.
+ */
 int trace_want(struct trace_key *key);
+
+/**
+ * Disables tracing for the specified key, even if the environment variable
+ * was set.
+ */
 void trace_disable(struct trace_key *key);
+
+/**
+ * Returns nanoseconds since the epoch (01/01/1970), typically used
+ * for performance measurements.
+ * Currently there are high precision timer implementations for Linux (using
+ * `clock_gettime(CLOCK_MONOTONIC)`) and Windows (`QueryPerformanceCounter`).
+ * Other platforms use `gettimeofday` as time source.
+ */
 uint64_t getnanotime(void);
+
 void trace_command_performance(const char **argv);
 void trace_verbatim(struct trace_key *key, const void *buf, unsigned len);
 uint64_t trace_performance_enter(void);
 
 #ifndef HAVE_VARIADIC_MACROS
 
+/**
+ * Prints a formatted message, similar to printf.
+ */
 __attribute__((format (printf, 1, 2)))
 void trace_printf(const char *format, ...);
 
 __attribute__((format (printf, 2, 3)))
 void trace_printf_key(struct trace_key *key, const char *format, ...);
 
+/**
+ * Prints a formatted message, followed by a quoted list of arguments.
+ */
 __attribute__((format (printf, 2, 3)))
 void trace_argv_printf(const char **argv, const char *format, ...);
 
+/**
+ * Prints the strbuf, without additional formatting (i.e. doesn't
+ * choke on `%` or even `\0`).
+ */
 void trace_strbuf(struct trace_key *key, const struct strbuf *data);
 
-/* Prints elapsed time (in nanoseconds) if GIT_TRACE_PERFORMANCE is enabled. */
+/**
+ * Prints elapsed time (in nanoseconds) if GIT_TRACE_PERFORMANCE is enabled.
+ *
+ * Example:
+ * ------------
+ * uint64_t t = 0;
+ * for (;;) {
+ * 	// ignore
+ * t -= getnanotime();
+ * // code section to measure
+ * t += getnanotime();
+ * // ignore
+ * }
+ * trace_performance(t, "frotz");
+ * ------------
+ */
 __attribute__((format (printf, 2, 3)))
 void trace_performance(uint64_t nanos, const char *format, ...);
 
-/* Prints elapsed time since 'start' if GIT_TRACE_PERFORMANCE is enabled. */
+/**
+ * Prints elapsed time since 'start' if GIT_TRACE_PERFORMANCE is enabled.
+ *
+ * Example:
+ * ------------
+ * uint64_t start = getnanotime();
+ * // code section to measure
+ * trace_performance_since(start, "foobar");
+ * ------------
+ */
 __attribute__((format (printf, 2, 3)))
 void trace_performance_since(uint64_t start, const char *format, ...);