[RFC PATCH 02/21] C99 support: hard-depend on C99 variadic macros

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Remove the "else" branches of the HAVE_VARIADIC_MACROS macro, which
has been unconditionally set since 765dc168882 (git-compat-util:
always enable variadic macros, 2021-01-28).

That commit went out with v2.31.0, so we've got a few releases already
where we've been explicitly breaking things for compilers that don't
support this C99 feature.

In addition to that we've been emitting extremely verbose warnings
since at least ee4512ed481 (trace2: create new combined trace
facility, 2019-02-22) if HAVE_VARIADIC_MACROS wasn't true. There is no
such thing as a "region_enter_printf" or "region_leave_printf" format,
so at least under GCC and Clang everything that includes
trace.h (almost every file) emits a couple of warnings about that.

There's a large benefit to being able to rely on variadic macros, the
code surrounding usage.c is hard to maintain if we need to write two
implementations of everything, and by relying on "__FILE__" and
"__LINE__" along with "__VA_ARGS__" we can in the future make error(),
die() etc. log where they were called from, without the verbosity of a
dual-implementation.

Let's also update our CodingGuidelines to note that we depend on
this. The added bullet-point starts with lower-case for consistency
with other bullet-points in that section.

The diff in "trace.h" is relatively hard to read, since we need to
retain the existing API docs, which were comments on the code used if
HAVE_VARIADIC_MACROS was not defined.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 Documentation/CodingGuidelines |   3 +
 banned.h                       |   5 --
 git-compat-util.h              |  12 ---
 trace.c                        |  80 +-------------------
 trace.h                        | 133 +++++++++++++--------------------
 trace2.c                       |  39 ----------
 trace2.h                       |  25 -------
 usage.c                        |  15 +---
 8 files changed, 58 insertions(+), 254 deletions(-)

diff --git a/Documentation/CodingGuidelines b/Documentation/CodingGuidelines
index 711cb9171e0..2c217c55502 100644
--- a/Documentation/CodingGuidelines
+++ b/Documentation/CodingGuidelines
@@ -210,6 +210,9 @@ For C programs:
    . since mid 2017 with 512f41cf, we have been using designated
      initializers for array (e.g. "int array[10] = { [5] = 2 }").
 
+   . since late 2021 with 765dc168882, we have been using variadic
+     macros, mostly for printf-like trace and debug macros.
+
    These used to be forbidden, but we have not heard any breakage
    report, and they are assumed to be safe.
 
diff --git a/banned.h b/banned.h
index 7ab4f2e4921..6ccf46bc197 100644
--- a/banned.h
+++ b/banned.h
@@ -21,13 +21,8 @@
 
 #undef sprintf
 #undef vsprintf
-#ifdef HAVE_VARIADIC_MACROS
 #define sprintf(...) BANNED(sprintf)
 #define vsprintf(...) BANNED(vsprintf)
-#else
-#define sprintf(buf,fmt,arg) BANNED(sprintf)
-#define vsprintf(buf,fmt,arg) BANNED(vsprintf)
-#endif
 
 #undef gmtime
 #define gmtime(t) BANNED(gmtime)
diff --git a/git-compat-util.h b/git-compat-util.h
index e86df9769ba..aec64261a96 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -1193,24 +1193,12 @@ static inline int regexec_buf(const regex_t *preg, const char *buf, size_t size,
 #endif
 #endif
 
-/*
- * This is always defined as a first step towards making the use of variadic
- * macros unconditional. If it causes compilation problems on your platform,
- * please report it to the Git mailing list at git@vger.kernel.org.
- */
-#define HAVE_VARIADIC_MACROS 1
-
 /* usage.c: only to be used for testing BUG() implementation (see test-tool) */
 extern int BUG_exit_code;
 
-#ifdef HAVE_VARIADIC_MACROS
 __attribute__((format (printf, 3, 4))) NORETURN
 void BUG_fl(const char *file, int line, const char *fmt, ...);
 #define BUG(...) BUG_fl(__FILE__, __LINE__, __VA_ARGS__)
-#else
-__attribute__((format (printf, 1, 2))) NORETURN
-void BUG(const char *fmt, ...);
-#endif
 
 /*
  * Preserves errno, prints a message, but gives no warning for ENOENT.
diff --git a/trace.c b/trace.c
index f726686fd92..794a087c21e 100644
--- a/trace.c
+++ b/trace.c
@@ -108,16 +108,11 @@ static int prepare_trace_line(const char *file, int line,
 	gettimeofday(&tv, NULL);
 	secs = tv.tv_sec;
 	localtime_r(&secs, &tm);
-	strbuf_addf(buf, "%02d:%02d:%02d.%06ld ", tm.tm_hour, tm.tm_min,
-		    tm.tm_sec, (long) tv.tv_usec);
-
-#ifdef HAVE_VARIADIC_MACROS
-	/* print file:line */
-	strbuf_addf(buf, "%s:%d ", file, line);
+	strbuf_addf(buf, "%02d:%02d:%02d.%06ld %s:%d", tm.tm_hour, tm.tm_min,
+		    tm.tm_sec, (long) tv.tv_usec, file, line);
 	/* align trace output (column 40 catches most files names in git) */
 	while (buf->len < 40)
 		strbuf_addch(buf, ' ');
-#endif
 
 	return 1;
 }
@@ -229,74 +224,6 @@ static void trace_performance_vprintf_fl(const char *file, int line,
 	strbuf_release(&buf);
 }
 
-#ifndef HAVE_VARIADIC_MACROS
-
-void trace_printf(const char *format, ...)
-{
-	va_list ap;
-	va_start(ap, format);
-	trace_vprintf_fl(NULL, 0, &trace_default_key, format, ap);
-	va_end(ap);
-}
-
-void trace_printf_key(struct trace_key *key, const char *format, ...)
-{
-	va_list ap;
-	va_start(ap, format);
-	trace_vprintf_fl(NULL, 0, key, format, ap);
-	va_end(ap);
-}
-
-void trace_argv_printf(const char **argv, const char *format, ...)
-{
-	va_list ap;
-	va_start(ap, format);
-	trace_argv_vprintf_fl(NULL, 0, argv, format, ap);
-	va_end(ap);
-}
-
-void trace_strbuf(struct trace_key *key, const struct strbuf *data)
-{
-	trace_strbuf_fl(NULL, 0, key, data);
-}
-
-void trace_performance(uint64_t nanos, const char *format, ...)
-{
-	va_list ap;
-	va_start(ap, format);
-	trace_performance_vprintf_fl(NULL, 0, nanos, format, ap);
-	va_end(ap);
-}
-
-void trace_performance_since(uint64_t start, const char *format, ...)
-{
-	va_list ap;
-	va_start(ap, format);
-	trace_performance_vprintf_fl(NULL, 0, getnanotime() - start,
-				     format, ap);
-	va_end(ap);
-}
-
-void trace_performance_leave(const char *format, ...)
-{
-	va_list ap;
-	uint64_t since;
-
-	if (perf_indent)
-		perf_indent--;
-
-	if (!format) /* Allow callers to leave without tracing anything */
-		return;
-
-	since = perf_start_times[perf_indent];
-	va_start(ap, format);
-	trace_performance_vprintf_fl(NULL, 0, getnanotime() - since,
-				     format, ap);
-	va_end(ap);
-}
-
-#else
-
 void trace_printf_key_fl(const char *file, int line, struct trace_key *key,
 			 const char *format, ...)
 {
@@ -342,9 +269,6 @@ void trace_performance_leave_fl(const char *file, int line,
 	va_end(ap);
 }
 
-#endif /* HAVE_VARIADIC_MACROS */
-
-
 static const char *quote_crnl(const char *path)
 {
 	static struct strbuf new_path = STRBUF_INIT;
diff --git a/trace.h b/trace.h
index e25984051aa..7ad2f03d07c 100644
--- a/trace.h
+++ b/trace.h
@@ -126,85 +126,10 @@ 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.
+ * Macros to add the file:line of the calling code, instead of that of
+ * the trace function itself.
  *
- * 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.
- *
- * 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, ...);
-
-__attribute__((format (printf, 1, 2)))
-void trace_performance_leave(const char *format, ...);
-
-#else
-
-/*
- * Macros to add file:line - see above for C-style declarations of how these
- * should be used.
- */
-
-/*
- * TRACE_CONTEXT may be set to __FUNCTION__ if the compiler supports it. The
- * default is __FILE__, as it is consistent with assert(), and static function
- * names are not necessarily unique.
- *
- * __FILE__ ":" __FUNCTION__ doesn't work with GNUC, as __FILE__ is supplied
- * by the preprocessor as a string literal, and __FUNCTION__ is filled in by
- * the compiler as a string constant.
- */
-#ifndef TRACE_CONTEXT
-# define TRACE_CONTEXT __FILE__
-#endif
-
-/*
  * Note: with C99 variadic macros, __VA_ARGS__ must include the last fixed
  * parameter ('format' in this case). Otherwise, a call without variable
  * arguments will have a surplus ','. E.g.:
@@ -218,7 +143,23 @@ void trace_performance_leave(const char *format, ...);
  *
  * which is invalid (note the ',)'). With GNUC, '##__VA_ARGS__' drops the
  * comma, but this is non-standard.
+ *
+ * TRACE_CONTEXT may be set to __FUNCTION__ if the compiler supports it. The
+ * default is __FILE__, as it is consistent with assert(), and static function
+ * names are not necessarily unique.
+ *
+ * __FILE__ ":" __FUNCTION__ doesn't work with GNUC, as __FILE__ is supplied
+ * by the preprocessor as a string literal, and __FUNCTION__ is filled in by
+ * the compiler as a string constant.
+ */
+#ifndef TRACE_CONTEXT
+# define TRACE_CONTEXT __FILE__
+#endif
+
+/**
+ * Prints a formatted message, similar to printf.
  */
+#define trace_printf(...) trace_printf_key(&trace_default_key, __VA_ARGS__)
 
 #define trace_printf_key(key, ...)					    \
 	do {								    \
@@ -227,8 +168,9 @@ void trace_performance_leave(const char *format, ...);
 					    __VA_ARGS__);		    \
 	} while (0)
 
-#define trace_printf(...) trace_printf_key(&trace_default_key, __VA_ARGS__)
-
+/**
+ * Prints a formatted message, followed by a quoted list of arguments.
+ */
 #define trace_argv_printf(argv, ...)					    \
 	do {								    \
 		if (trace_pass_fl(&trace_default_key))			    \
@@ -236,12 +178,32 @@ void trace_performance_leave(const char *format, ...);
 					    argv, __VA_ARGS__);		    \
 	} while (0)
 
+/**
+ * Prints the strbuf, without additional formatting (i.e. doesn't
+ * choke on `%` or even `\0`).
+ */
 #define trace_strbuf(key, data)						    \
 	do {								    \
 		if (trace_pass_fl(key))					    \
 			trace_strbuf_fl(TRACE_CONTEXT, __LINE__, key, data);\
 	} while (0)
 
+/**
+ * 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");
+ * ------------
+ */
 #define trace_performance(nanos, ...)					    \
 	do {								    \
 		if (trace_pass_fl(&trace_perf_key))			    \
@@ -249,6 +211,16 @@ void trace_performance_leave(const char *format, ...);
 					     __VA_ARGS__);		    \
 	} while (0)
 
+/**
+ * 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");
+ * ------------
+ */
 #define trace_performance_since(start, ...)				    \
 	do {								    \
 		if (trace_pass_fl(&trace_perf_key))			    \
@@ -265,6 +237,7 @@ void trace_performance_leave(const char *format, ...);
 						   __VA_ARGS__);	    \
 	} while (0)
 
+
 /* backend functions, use non-*fl macros instead */
 __attribute__((format (printf, 4, 5)))
 void trace_printf_key_fl(const char *file, int line, struct trace_key *key,
@@ -285,6 +258,4 @@ static inline int trace_pass_fl(struct trace_key *key)
 	return key->fd || !key->initialized;
 }
 
-#endif /* HAVE_VARIADIC_MACROS */
-
 #endif /* TRACE_H */
diff --git a/trace2.c b/trace2.c
index b2d471526fd..179caa72cfe 100644
--- a/trace2.c
+++ b/trace2.c
@@ -641,20 +641,6 @@ void trace2_region_enter_printf_fl(const char *file, int line,
 	va_end(ap);
 }
 
-#ifndef HAVE_VARIADIC_MACROS
-void trace2_region_enter_printf(const char *category, const char *label,
-				const struct repository *repo, const char *fmt,
-				...)
-{
-	va_list ap;
-
-	va_start(ap, fmt);
-	trace2_region_enter_printf_va_fl(NULL, 0, category, label, repo, fmt,
-					 ap);
-	va_end(ap);
-}
-#endif
-
 void trace2_region_leave_printf_va_fl(const char *file, int line,
 				      const char *category, const char *label,
 				      const struct repository *repo,
@@ -717,20 +703,6 @@ void trace2_region_leave_printf_fl(const char *file, int line,
 	va_end(ap);
 }
 
-#ifndef HAVE_VARIADIC_MACROS
-void trace2_region_leave_printf(const char *category, const char *label,
-				const struct repository *repo, const char *fmt,
-				...)
-{
-	va_list ap;
-
-	va_start(ap, fmt);
-	trace2_region_leave_printf_va_fl(NULL, 0, category, label, repo, fmt,
-					 ap);
-	va_end(ap);
-}
-#endif
-
 void trace2_data_string_fl(const char *file, int line, const char *category,
 			   const struct repository *repo, const char *key,
 			   const char *value)
@@ -826,17 +798,6 @@ void trace2_printf_fl(const char *file, int line, const char *fmt, ...)
 	va_end(ap);
 }
 
-#ifndef HAVE_VARIADIC_MACROS
-void trace2_printf(const char *fmt, ...)
-{
-	va_list ap;
-
-	va_start(ap, fmt);
-	trace2_printf_va_fl(NULL, 0, fmt, ap);
-	va_end(ap);
-}
-#endif
-
 const char *trace2_session_id(void)
 {
 	return tr2_sid_get();
diff --git a/trace2.h b/trace2.h
index 0cc7b5f5312..1b109f57d0a 100644
--- a/trace2.h
+++ b/trace2.h
@@ -397,18 +397,9 @@ void trace2_region_enter_printf_fl(const char *file, int line,
 				   const struct repository *repo,
 				   const char *fmt, ...);
 
-#ifdef HAVE_VARIADIC_MACROS
 #define trace2_region_enter_printf(category, label, repo, ...)                 \
 	trace2_region_enter_printf_fl(__FILE__, __LINE__, (category), (label), \
 				      (repo), __VA_ARGS__)
-#else
-/* clang-format off */
-__attribute__((format (region_enter_printf, 4, 5)))
-void trace2_region_enter_printf(const char *category, const char *label,
-				const struct repository *repo, const char *fmt,
-				...);
-/* clang-format on */
-#endif
 
 /**
  * Emit a 'region_leave' event for <category>.<label> with optional
@@ -442,18 +433,9 @@ void trace2_region_leave_printf_fl(const char *file, int line,
 				   const struct repository *repo,
 				   const char *fmt, ...);
 
-#ifdef HAVE_VARIADIC_MACROS
 #define trace2_region_leave_printf(category, label, repo, ...)                 \
 	trace2_region_leave_printf_fl(__FILE__, __LINE__, (category), (label), \
 				      (repo), __VA_ARGS__)
-#else
-/* clang-format off */
-__attribute__((format (region_leave_printf, 4, 5)))
-void trace2_region_leave_printf(const char *category, const char *label,
-				const struct repository *repo, const char *fmt,
-				...);
-/* clang-format on */
-#endif
 
 /**
  * Emit a key-value pair 'data' event of the form <category>.<key> = <value>.
@@ -506,14 +488,7 @@ void trace2_printf_va_fl(const char *file, int line, const char *fmt,
 
 void trace2_printf_fl(const char *file, int line, const char *fmt, ...);
 
-#ifdef HAVE_VARIADIC_MACROS
 #define trace2_printf(...) trace2_printf_fl(__FILE__, __LINE__, __VA_ARGS__)
-#else
-/* clang-format off */
-__attribute__((format (printf, 1, 2)))
-void trace2_printf(const char *fmt, ...);
-/* clang-format on */
-#endif
 
 /*
  * Optional platform-specific code to dump information about the
diff --git a/usage.c b/usage.c
index c7d233b0de9..1128cf000d5 100644
--- a/usage.c
+++ b/usage.c
@@ -265,10 +265,7 @@ static NORETURN void BUG_vfl(const char *file, int line, const char *fmt, va_lis
 	va_copy(params_copy, params);
 
 	/* truncation via snprintf is OK here */
-	if (file)
-		snprintf(prefix, sizeof(prefix), "BUG: %s:%d: ", file, line);
-	else
-		snprintf(prefix, sizeof(prefix), "BUG: ");
+	snprintf(prefix, sizeof(prefix), "BUG: %s:%d: ", file, line);
 
 	vreportf(prefix, fmt, params);
 
@@ -283,7 +280,6 @@ static NORETURN void BUG_vfl(const char *file, int line, const char *fmt, va_lis
 	abort();
 }
 
-#ifdef HAVE_VARIADIC_MACROS
 NORETURN void BUG_fl(const char *file, int line, const char *fmt, ...)
 {
 	va_list ap;
@@ -291,15 +287,6 @@ NORETURN void BUG_fl(const char *file, int line, const char *fmt, ...)
 	BUG_vfl(file, line, fmt, ap);
 	va_end(ap);
 }
-#else
-NORETURN void BUG(const char *fmt, ...)
-{
-	va_list ap;
-	va_start(ap, fmt);
-	BUG_vfl(NULL, 0, fmt, ap);
-	va_end(ap);
-}
-#endif
 
 #ifdef SUPPRESS_ANNOTATED_LEAKS
 void unleak_memory(const void *ptr, size_t len)
-- 
2.34.0.rc2.809.g11e21d44b24