revision: improve commit_rewrite_person()

The function, commit_rewrite_person(), is designed to find and replace
an ident string in the header part, and the way it avoids a random
occurrence of "author A U Thor <author@example.com" in the text is by
insisting "author" to appear at the beginning of line by passing
"\nauthor " as "what".

The implementation also doesn't make any effort to limit itself to the
commit header by locating the blank line that appears after the header
part and stopping the search there. Also, the interface forces the
caller to make multiple calls if it wants to rewrite idents on multiple
headers. It shouldn't be the case.

To support the existing caller better, update commit_rewrite_person()
to:
- Make a single pass in the input buffer to locate headers named
  "author" and "committer" and replace idents on them.
- Stop at the end of the header, ensuring that nothing in the body of
  the commit object is modified.

The return type of the function commit_rewrite_person() has also been
changed from int to void. This has been done because the caller of the
function doesn't do anything with the return value of the function.

By simplifying the interface of the commit_rewrite_person(), we also
intend to expose it as a public function. We will also be renaming the
function in a future commit to a different name which clearly tells that
the function replaces idents in the header of the commit buffer.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: John Cai <johncai86@gmail.com>
Helped-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Siddharth Asthana <siddharthasthana31@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

revision.c

@@ -3755,25 +3755,18 @@ int rewrite_parents(struct rev_info *revs, struct commit *commit,
 	return 0;
 }
 
-static int commit_rewrite_person(struct strbuf *buf, const char *what, struct string_list *mailmap)
+/*
+ * Returns the difference between the new and old length of the ident line.
+ */
+static ssize_t rewrite_ident_line(const char *person, size_t len,
+				   struct strbuf *buf,
+				   struct string_list *mailmap)
 {
-	char *person, *endp;
-	size_t len, namelen, maillen;
+	size_t namelen, maillen;
 	const char *name;
 	const char *mail;
 	struct ident_split ident;
 
-	person = strstr(buf->buf, what);
-	if (!person)
-		return 0;
-
-	person += strlen(what);
-	endp = strchr(person, '\n');
-	if (!endp)
-		return 0;
-
-	len = endp - person;
-
 	if (split_ident_line(&ident, person, len))
 		return 0;
 
@@ -3784,6 +3777,7 @@ static int commit_rewrite_person(struct strbuf *buf, const char *what, struct st
 
 	if (map_user(mailmap, &mail, &maillen, &name, &namelen)) {
 		struct strbuf namemail = STRBUF_INIT;
+		size_t newlen;
 
 		strbuf_addf(&namemail, "%.*s <%.*s>",
 			    (int)namelen, name, (int)maillen, mail);
@@ -3791,15 +3785,50 @@ static int commit_rewrite_person(struct strbuf *buf, const char *what, struct st
 		strbuf_splice(buf, ident.name_begin - buf->buf,
 			      ident.mail_end - ident.name_begin + 1,
 			      namemail.buf, namemail.len);
+		newlen = namemail.len;
 
 		strbuf_release(&namemail);
 
-		return 1;
+		return newlen - (ident.mail_end - ident.name_begin);
 	}
 
 	return 0;
 }
 
+static void commit_rewrite_person(struct strbuf *buf, const char **header,
+				   struct string_list *mailmap)
+{
+	size_t buf_offset = 0;
+
+	if (!mailmap)
+		return;
+
+	for (;;) {
+		const char *person, *line;
+		size_t i;
+		int found_header = 0;
+
+		line = buf->buf + buf_offset;
+		if (!*line || *line == '\n')
+			return; /* End of headers */
+
+		for (i = 0; header[i]; i++)
+			if (skip_prefix(line, header[i], &person)) {
+				const char *endp = strchrnul(person, '\n');
+				found_header = 1;
+				buf_offset += endp - line;
+				buf_offset += rewrite_ident_line(person, endp - person, buf, mailmap);
+				break;
+			}
+
+		if (!found_header) {
+			buf_offset = strchrnul(line, '\n') - buf->buf;
+			if (buf->buf[buf_offset] == '\n')
+				buf_offset++;
+		}
+	}
+}
+
 static int commit_match(struct commit *commit, struct rev_info *opt)
 {
 	int retval;
@@ -3832,11 +3861,12 @@ static int commit_match(struct commit *commit, struct rev_info *opt)
 		strbuf_addstr(&buf, message);
 
 	if (opt->grep_filter.header_list && opt->mailmap) {
+		const char *commit_headers[] = { "author ", "committer ", NULL };
+
 		if (!buf.len)
 			strbuf_addstr(&buf, message);
 
-		commit_rewrite_person(&buf, "\nauthor ", opt->mailmap);
-		commit_rewrite_person(&buf, "\ncommitter ", opt->mailmap);
+		commit_rewrite_person(&buf, commit_headers, opt->mailmap);
 	}
 
 	/* Append "fake" message parts as needed */