logmsg_reencode: return const buffer

The return value from logmsg_reencode may be either a newly
allocated buffer or a pointer to the existing commit->buffer.
We would not want the caller to accidentally free() or
modify the latter, so let's mark it as const.  We can cast
away the constness in logmsg_free, but only once we have
determined that it is a free-able buffer.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/blame.c

@@ -1405,7 +1405,7 @@ static void get_commit_info(struct commit *commit,
 {
 	int len;
 	const char *subject, *encoding;
-	char *message;
+	const char *message;
 
 	commit_info_init(ret);
 

builtin/reset.c

@@ -93,7 +93,7 @@ static int reset_index(const unsigned char *sha1, int reset_type, int quiet)
 static void print_new_head_line(struct commit *commit)
 {
 	const char *hex, *body;
-	char *msg;
+	const char *msg;
 
 	hex = find_unique_abbrev(commit->object.sha1, DEFAULT_ABBREV);
 	printf(_("HEAD is now at %s"), hex);

commit.h

@@ -115,10 +115,10 @@ struct userformat_want {
 
 extern int has_non_ascii(const char *text);
 struct rev_info; /* in revision.h, it circularly uses enum cmit_fmt */
-extern char *logmsg_reencode(const struct commit *commit,
+extern const char *logmsg_reencode(const struct commit *commit,
-			     char **commit_encoding,
+				   char **commit_encoding,
-			     const char *output_encoding);
+				   const char *output_encoding);
-extern void logmsg_free(char *msg, const struct commit *commit);
+extern void logmsg_free(const char *msg, const struct commit *commit);
 extern void get_commit_format(const char *arg, struct rev_info *);
 extern const char *format_subject(struct strbuf *sb, const char *msg,
 				  const char *line_separator);

pretty.c

@@ -606,9 +606,9 @@ static char *replace_encoding_header(char *buf, const char *encoding)
 	return strbuf_detach(&tmp, NULL);
 }
 
-char *logmsg_reencode(const struct commit *commit,
+const char *logmsg_reencode(const struct commit *commit,
-		      char **commit_encoding,
+			    char **commit_encoding,
-		      const char *output_encoding)
+			    const char *output_encoding)
 {
 	static const char *utf8 = "UTF-8";
 	const char *use_encoding;
@@ -687,10 +687,10 @@ char *logmsg_reencode(const struct commit *commit,
 	return out ? out : msg;
 }
 
-void logmsg_free(char *msg, const struct commit *commit)
+void logmsg_free(const char *msg, const struct commit *commit)
 {
 	if (msg != commit->buffer)
-		free(msg);
+		free((void *)msg);
 }
 
 static int mailmap_name(const char **email, size_t *email_len,
@@ -796,7 +796,7 @@ struct format_commit_context {
 	struct signature_check signature_check;
 	enum flush_type flush_type;
 	enum trunc_type truncate;
-	char *message;
+	const char *message;
 	char *commit_encoding;
 	size_t width, indent1, indent2;
 	int auto_color;
@@ -1700,7 +1700,7 @@ void pretty_print_commit(struct pretty_print_context *pp,
 	unsigned long beginning_of_body;
 	int indent = 4;
 	const char *msg;
-	char *reencoded;
+	const char *reencoded;
 	const char *encoding;
 	int need_8bit_cte = pp->need_8bit_cte;
 

revision.c

@@ -2788,7 +2788,7 @@ static int commit_match(struct commit *commit, struct rev_info *opt)
 {
 	int retval;
 	const char *encoding;
-	char *message;
+	const char *message;
 	struct strbuf buf = STRBUF_INIT;
 
 	if (!opt->grep_filter.pattern_list && !opt->grep_filter.header_list)
@@ -2830,12 +2830,19 @@ static int commit_match(struct commit *commit, struct rev_info *opt)
 		format_display_notes(commit->object.sha1, &buf, encoding, 1);
 	}
 
-	/* Find either in the original commit message, or in the temporary */
+	/*
+	 * Find either in the original commit message, or in the temporary.
+	 * Note that we cast away the constness of "message" here. It is
+	 * const because it may come from the cached commit buffer. That's OK,
+	 * because we know that it is modifiable heap memory, and that while
+	 * grep_buffer may modify it for speed, it will restore any
+	 * changes before returning.
+	 */
 	if (buf.len)
 		retval = grep_buffer(&opt->grep_filter, buf.buf, buf.len);
 	else
 		retval = grep_buffer(&opt->grep_filter,
-				     message, strlen(message));
+				     (char *)message, strlen(message));
 	strbuf_release(&buf);
 	logmsg_free(message, commit);
 	return retval;