Change semantics of interpolate to work like snprintf.

  Also fix many off-by-ones and a useless memset.

Signed-off-by: Pierre Habouzit <madcoder@debian.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -923,15 +923,14 @@ long format_commit_message(const struct commit *commit, const void *format,
 
 	do {
 		char *buf = *buf_p;
-		unsigned long space = *space_p;
+		unsigned long len;
 
-		space = interpolate(buf, space, format,
+		len = interpolate(buf, *space_p, format,
 				    table, ARRAY_SIZE(table));
-		if (!space)
+		if (len < *space_p)
 			break;
-		buf = xrealloc(buf, space);
+		ALLOC_GROW(buf, len + 1, *space_p);
 		*buf_p = buf;
-		*space_p = space;
 	} while (1);
 	interp_clear_table(table, ARRAY_SIZE(table));
 

interpolate.c

@@ -44,9 +44,8 @@ void interp_clear_table(struct interp *table, int ninterps)
  *        { "%%", "%"},
  *    }
  *
- * Returns 0 on a successful substitution pass that fits in result,
- * Returns a number of bytes needed to hold the full substituted
- * string otherwise.
+ * Returns the length of the substituted string (not including the final \0).
+ * Like with snprintf, if the result is >= reslen, then it overflowed.
  */
 
 unsigned long interpolate(char *result, unsigned long reslen,
@@ -61,8 +60,6 @@ unsigned long interpolate(char *result, unsigned long reslen,
 	int i;
 	char c;
 
-        memset(result, 0, reslen);
-
 	while ((c = *src)) {
 		if (c == '%') {
 			/* Try to match an interpolation string. */
@@ -78,9 +75,9 @@ unsigned long interpolate(char *result, unsigned long reslen,
 				value = interps[i].value;
 				valuelen = strlen(value);
 
-				if (newlen + valuelen + 1 < reslen) {
+				if (newlen + valuelen < reslen) {
 					/* Substitute. */
-					strncpy(dest, value, valuelen);
+					memcpy(dest, value, valuelen);
 					dest += valuelen;
 				}
 				newlen += valuelen;
@@ -95,8 +92,9 @@ unsigned long interpolate(char *result, unsigned long reslen,
 		newlen++;
 	}
 
-	if (newlen + 1 < reslen)
-		return 0;
-	else
-		return newlen + 2;
+	/* XXX: the previous loop always keep room for the ending NUL,
+	   we just need to check if there was room for a NUL in the first place */
+	if (reslen > 0)
+		*dest = '\0';
+	return newlen;
 }