date: make DATE_MODE thread-safe

date_mode_from_type() modifies a static variable and returns a pointer
to it.  This is not thread-safe.  Most callers of date_mode_from_type()
use it via the macro DATE_MODE and pass its result on to functions like
show_date(), which take a const pointer and don't modify the struct.

Avoid the static storage by putting the variable on the stack and
returning the whole struct date_mode.  Change functions that take a
constant pointer to expect the whole struct instead.

Reduce the cost of passing struct date_mode around on 64-bit systems
by reordering its members to close the hole between the 32-bit wide
.type and the 64-bit aligned .strftime_fmt as well as the alignment
hole at the end.  sizeof reports 24 before and 16 with this change
on x64.  Keep .type at the top to still allow initialization without
designator -- though that's only done in a single location, in
builtin/blame.c.

Signed-off-by: René Scharfe <l.s.r@web.de>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 3 insertions, 3 deletions

diff --git a/date.h b/date.h
index 6136212a19..0747864fd7 100644
--- a/date.h
+++ b/date.h
@@ -22,8 +22,8 @@ enum date_mode_type {
 
 struct date_mode {
 	enum date_mode_type type;
-	const char *strftime_fmt;
 	int local;
+	const char *strftime_fmt;
 };
 
 #define DATE_MODE_INIT { \
@@ -36,14 +36,14 @@ struct date_mode {
  *   show_date(t, tz, DATE_MODE(NORMAL));
  */
 #define DATE_MODE(t) date_mode_from_type(DATE_##t)
-struct date_mode *date_mode_from_type(enum date_mode_type type);
+struct date_mode date_mode_from_type(enum date_mode_type type);
 
 /**
  * Format <'time', 'timezone'> into static memory according to 'mode'
  * and return it. The mode is an initialized "struct date_mode"
  * (usually from the DATE_MODE() macro).
  */
-const char *show_date(timestamp_t time, int timezone, const struct date_mode *mode);
+const char *show_date(timestamp_t time, int timezone, struct date_mode mode);
 
 /**
  * Parse a date format for later use with show_date().