mailinfo: keep the parsed log message in a strbuf

When mailinfo() is eventually libified, the calling "git am" still
will have to write out the log message in the "msg" file for hooks
and other users of the information, but it does not have to reopen
and reread what it wrote earlier if the function kept it in a strbuf.

This also removes the need for seeking and truncating the output
file when we see a scissors mark in the input, which in turn allows
us to lose two callsites of die_errno().

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/mailinfo.c

@@ -12,7 +12,6 @@
 struct mailinfo {
 	FILE *input;
 	FILE *output;
-	FILE *cmitmsg;
 	FILE *patchfile;
 
 	struct strbuf name;
@@ -36,6 +35,8 @@ struct mailinfo {
 	int header_stage; /* still checking in-body headers? */
 	struct strbuf **p_hdr_data;
 	struct strbuf **s_hdr_data;
+
+	struct strbuf log_message;
 };
 
 static void cleanup_space(struct strbuf *sb)
@@ -676,10 +677,8 @@ static int handle_commit_msg(struct mailinfo *mi, struct strbuf *line)
 
 	if (mi->use_scissors && is_scissors_line(line)) {
 		int i;
-		if (fseek(mi->cmitmsg, 0L, SEEK_SET))
-			die_errno("Could not rewind output message file");
-		if (ftruncate(fileno(mi->cmitmsg), 0))
-			die_errno("Could not truncate output message file at scissors");
+
+		strbuf_setlen(&mi->log_message, 0);
 		mi->header_stage = 1;
 
 		/*
@@ -696,13 +695,12 @@ static int handle_commit_msg(struct mailinfo *mi, struct strbuf *line)
 
 	if (patchbreak(line)) {
 		if (mi->message_id)
-			fprintf(mi->cmitmsg, "Message-Id: %s\n", mi->message_id);
-		fclose(mi->cmitmsg);
-		mi->cmitmsg = NULL;
+			strbuf_addf(&mi->log_message,
+				    "Message-Id: %s\n", mi->message_id);
 		return 1;
 	}
 
-	fputs(line->buf, mi->cmitmsg);
+	strbuf_addbuf(&mi->log_message, line);
 	return 0;
 }
 
@@ -968,18 +966,19 @@ static void handle_info(struct mailinfo *mi)
 
 static int mailinfo(struct mailinfo *mi, const char *msg, const char *patch)
 {
+	FILE *cmitmsg;
 	int peek;
 	struct strbuf line = STRBUF_INIT;
 
-	mi->cmitmsg = fopen(msg, "w");
-	if (!mi->cmitmsg) {
+	cmitmsg = fopen(msg, "w");
+	if (!cmitmsg) {
 		perror(msg);
 		return -1;
 	}
 	mi->patchfile = fopen(patch, "w");
 	if (!mi->patchfile) {
 		perror(patch);
-		fclose(mi->cmitmsg);
+		fclose(cmitmsg);
 		return -1;
 	}
 
@@ -996,6 +995,8 @@ static int mailinfo(struct mailinfo *mi, const char *msg, const char *patch)
 		check_header(mi, &line, mi->p_hdr_data, 1);
 
 	handle_body(mi, &line);
+	fwrite(mi->log_message.buf, 1, mi->log_message.len, cmitmsg);
+	fclose(cmitmsg);
 	fclose(mi->patchfile);
 
 	handle_info(mi);
@@ -1023,6 +1024,7 @@ static void setup_mailinfo(struct mailinfo *mi)
 	strbuf_init(&mi->name, 0);
 	strbuf_init(&mi->email, 0);
 	strbuf_init(&mi->charset, 0);
+	strbuf_init(&mi->log_message, 0);
 	mi->header_stage = 1;
 	mi->use_inbody_headers = 1;
 	mi->content_top = mi->content;
@@ -1049,6 +1051,8 @@ static void clear_mailinfo(struct mailinfo *mi)
 		free(*(mi->content_top));
 		mi->content_top--;
 	}
+
+	strbuf_release(&mi->log_message);
 }
 
 static const char mailinfo_usage[] =