[PATCH] strbuf: let strbuf_addftime handle %z and %Z itself

["René Scharfe" <l.s.r@web.de>]

There is no portable way to pass timezone information to strftime.  Add
parameters for timezone offset and name to strbuf_addftime and let it
handle the timezone-related format specifiers %z and %Z internally.
Callers can opt out by passing NULL as timezone name.

Use an empty string as timezone name in show_date (the only current
caller) for now because we only have the timezone offset in non-local
mode.  POSIX allows %Z to resolve to nothing in case of missing info.

Helped-by: Ulrich Mueller <ulm@gentoo.org>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Rene Scharfe <l.s.r@web.de>
---
Duplicates strbuf_expand to a certain extent, but not too badly, I
think.  Leaves the door open for letting strftime handle the local
case.

 date.c                     |  2 +-
 strbuf.c                   | 42 ++++++++++++++++++++++++++++++++++++++----
 strbuf.h                   | 11 ++++++++---
 t/t6006-rev-list-format.sh | 12 ++++++++++++
 4 files changed, 59 insertions(+), 8 deletions(-)

diff --git a/date.c b/date.c
index 63fa99685e..5580577334 100644
--- a/date.c
+++ b/date.c
@@ -246,7 +246,7 @@ const char *show_date(timestamp_t time, int tz, const struct date_mode *mode)
 			month_names[tm->tm_mon], tm->tm_year + 1900,
 			tm->tm_hour, tm->tm_min, tm->tm_sec, tz);
 	else if (mode->type == DATE_STRFTIME)
-		strbuf_addftime(&timebuf, mode->strftime_fmt, tm);
+		strbuf_addftime(&timebuf, mode->strftime_fmt, tm, tz, "");
 	else
 		strbuf_addf(&timebuf, "%.3s %.3s %d %02d:%02d:%02d %d%c%+05d",
 				weekday_names[tm->tm_wday],
diff --git a/strbuf.c b/strbuf.c
index 00457940cf..b880107370 100644
--- a/strbuf.c
+++ b/strbuf.c
@@ -785,14 +785,47 @@ char *xstrfmt(const char *fmt, ...)
 	return ret;
 }
 
-void strbuf_addftime(struct strbuf *sb, const char *fmt, const struct tm *tm)
+void strbuf_addftime(struct strbuf *sb, const char *fmt, const struct tm *tm,
+		     int tz_offset, const char *tz_name)
 {
+	struct strbuf munged_fmt = STRBUF_INIT;
 	size_t hint = 128;
 	size_t len;
 
 	if (!*fmt)
 		return;
 
+	/*
+	 * There is no portable way to pass timezone information to
+	 * strftime, so we handle %z and %Z here.
+	 */
+	if (tz_name) {
+		for (;;) {
+			const char *percent = strchrnul(fmt, '%');
+			strbuf_add(&munged_fmt, fmt, percent - fmt);
+			if (!*percent)
+				break;
+			fmt = percent + 1;
+			switch (*fmt) {
+			case '%':
+				strbuf_addstr(&munged_fmt, "%%");
+				fmt++;
+				break;
+			case 'z':
+				strbuf_addf(&munged_fmt, "%+05d", tz_offset);
+				fmt++;
+				break;
+			case 'Z':
+				strbuf_addstr(&munged_fmt, tz_name);
+				fmt++;
+				break;
+			default:
+				strbuf_addch(&munged_fmt, '%');
+			}
+		}
+		fmt = munged_fmt.buf;
+	}
+
 	strbuf_grow(sb, hint);
 	len = strftime(sb->buf + sb->len, sb->alloc - sb->len, fmt, tm);
 
@@ -804,17 +837,18 @@ void strbuf_addftime(struct strbuf *sb, const char *fmt, const struct tm *tm)
 		 * output contains at least one character, and then drop the extra
 		 * character before returning.
 		 */
-		struct strbuf munged_fmt = STRBUF_INIT;
-		strbuf_addf(&munged_fmt, "%s ", fmt);
+		if (fmt != munged_fmt.buf)
+			strbuf_addstr(&munged_fmt, fmt);
+		strbuf_addch(&munged_fmt, ' ');
 		while (!len) {
 			hint *= 2;
 			strbuf_grow(sb, hint);
 			len = strftime(sb->buf + sb->len, sb->alloc - sb->len,
 				       munged_fmt.buf, tm);
 		}
-		strbuf_release(&munged_fmt);
 		len--; /* drop munged space */
 	}
+	strbuf_release(&munged_fmt);
 	strbuf_setlen(sb, sb->len + len);
 }
 
diff --git a/strbuf.h b/strbuf.h
index 80047b1bb7..39d5836abd 100644
--- a/strbuf.h
+++ b/strbuf.h
@@ -339,9 +339,14 @@ __attribute__((format (printf,2,0)))
 extern void strbuf_vaddf(struct strbuf *sb, const char *fmt, va_list ap);
 
 /**
- * Add the time specified by `tm`, as formatted by `strftime`.
- */
-extern void strbuf_addftime(struct strbuf *sb, const char *fmt, const struct tm *tm);
+ * Add the time specified by `tm`, as formatted by `strftime`.  `tz_offset`
+ * and `tz_name` are used to expand %z and %Z internally, unless `tz_name`
+ * is NULL.  `tz_offset` is in decimal hhmm format, e.g. -600 means six
+ * hours west of Greenwich.
+ */
+extern void strbuf_addftime(struct strbuf *sb, const char *fmt,
+			    const struct tm *tm, int tz_offset,
+			    const char *tz_name);
 
 /**
  * Read a given size of data from a FILE* pointer to the buffer.
diff --git a/t/t6006-rev-list-format.sh b/t/t6006-rev-list-format.sh
index a1dcdb81d7..dc0bed8d90 100755
--- a/t/t6006-rev-list-format.sh
+++ b/t/t6006-rev-list-format.sh
@@ -483,4 +483,16 @@ test_expect_success 'unused %G placeholders are passed through' '
 	test_cmp expect actual
 '
 
+test_expect_success 'date format "%F %T %z" is the same as iso' '
+	git log -1 --format="%ad" --date=iso >expect &&
+	git log -1 --format="%ad" --date="format:%F %T %z" >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'date format "%%z" expands to percent zed' '
+	echo "%z" >expect &&
+	git log -1 --format="%ad" --date="format:%%z" >actual &&
+	test_cmp expect actual
+'
+
 test_done
-- 
2.13.0