[PATCH v2 1/2] bugfix: fix broken time_buf paddings for git-blame

[Jiang Xin <worldhello.net@gmail.com>]

When `git blame` shows date field in a fixed width (as long as
blame_date_width characters), if time_str shorter than that, add spaces
for padding.  But there are two bugs in the following codes:

        memcpy(time_buf, time_str, time_len);
        memset(time_buf + time_len, ' ', blame_date_width - time_len);

1. The type of blame_date_width is size_t (unsigned int).  If time_len
   is greater than blame_ate_width, "blame_date_width - time_len" will
   never be a negative number, but a really big positive number, and
   will cause memory overwrite.

   This bug can be triggered if either l10n message for function
   show_date_relative() in date.c is longer then 30 charcters, then
   `git blame --date relative` may fail.

2. When show blame information with relative time, the UTF-8 characters
   in time_str will break the alignment of columns after the date field.
   This is because the time_buf padding with spaces should have a
   constant display width, not a fixed strlen size.  So we should call
   utf8_strwidth() instead of strlen() for calibration.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
---
 builtin/blame.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/builtin/blame.c b/builtin/blame.c
index 88cb799..0a0a858 100644
--- a/builtin/blame.c
+++ b/builtin/blame.c
@@ -1556,22 +1556,31 @@ static void assign_blame(struct scoreboard *sb, int opt)
 static const char *format_time(unsigned long time, const char *tz_str,
 			       int show_raw_time)
 {
-	static char time_buf[128];
+	static struct strbuf time_buf = STRBUF_INIT;
 
+	strbuf_reset(&time_buf);
 	if (show_raw_time) {
-		snprintf(time_buf, sizeof(time_buf), "%lu %s", time, tz_str);
+		strbuf_addf(&time_buf, "%lu %s", time, tz_str);
 	}
 	else {
 		const char *time_str;
+		size_t time_width;
 		int time_len;
 		int tz;
 		tz = atoi(tz_str);
 		time_str = show_date(time, tz, blame_date_mode);
 		time_len = strlen(time_str);
-		memcpy(time_buf, time_str, time_len);
-		memset(time_buf + time_len, ' ', blame_date_width - time_len);
+		strbuf_add(&time_buf, time_str, time_len);
+		/*
+		 * Add space paddings to time_buf to display a fixed width
+		 * string, and use time_width for display width calibration.
+		 */
+		for (time_width = utf8_strwidth(time_str);
+		     time_width < blame_date_width;
+		     time_width++)
+			strbuf_addch(&time_buf, ' ');
 	}
-	return time_buf;
+	return time_buf.buf;
 }
 
 #define OUTPUT_ANNOTATE_COMPAT	001
-- 
2.0.0.rc0.3.g444188f.dirty