format-patch: make cover letters always text/plain

When formatting a series of patches using --attach and --cover-letter,
the cover letter lacks the closing MIME boundary, violating RFC 2046.
Certain clients, such as Thunderbird, discard the message body in such a
case.

Since the cover letter is just one part and sending it as
multipart/mixed is not very useful, always emit it as text/plain,
avoiding the boundary problem altogether.

Reported-by: Patrick Hemmer <git@stormcloud9.net>
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/log.c

@@ -1019,7 +1019,7 @@ static void make_cover_letter(struct rev_info *rev, int use_stdout,
 	    open_next_file(NULL, rev->numbered_files ? NULL : "cover-letter", rev, quiet))
 		return;
 
-	log_write_email_headers(rev, head, &pp.after_subject, &need_8bit_cte);
+	log_write_email_headers(rev, head, &pp.after_subject, &need_8bit_cte, 0);
 
 	for (i = 0; !need_8bit_cte && i < nr; i++) {
 		const char *buf = get_commit_buffer(list[i], NULL);

log-tree.c

@@ -362,7 +362,8 @@ void fmt_output_email_subject(struct strbuf *sb, struct rev_info *opt)
 
 void log_write_email_headers(struct rev_info *opt, struct commit *commit,
 			     const char **extra_headers_p,
-			     int *need_8bit_cte_p)
+			     int *need_8bit_cte_p,
+			     int maybe_multipart)
 {
 	const char *extra_headers = opt->extra_headers;
 	const char *name = oid_to_hex(opt->zero_commit ?
@@ -385,7 +386,7 @@ void log_write_email_headers(struct rev_info *opt, struct commit *commit,
 			       opt->ref_message_ids->items[i].string);
 		graph_show_oneline(opt->graph);
 	}
-	if (opt->mime_boundary) {
+	if (opt->mime_boundary && maybe_multipart) {
 		static char subject_buffer[1024];
 		static char buffer[1024];
 		struct strbuf filename =  STRBUF_INIT;
@@ -610,7 +611,7 @@ void show_log(struct rev_info *opt)
 
 	if (cmit_fmt_is_mail(opt->commit_format)) {
 		log_write_email_headers(opt, commit, &extra_headers,
-					&ctx.need_8bit_cte);
+					&ctx.need_8bit_cte, 1);
 		ctx.rev = opt;
 		ctx.print_email_subject = 1;
 	} else if (opt->commit_format != CMIT_FMT_USERFORMAT) {

log-tree.h

@@ -27,7 +27,8 @@ void format_decorations_extended(struct strbuf *sb, const struct commit *commit,
 void show_decorations(struct rev_info *opt, struct commit *commit);
 void log_write_email_headers(struct rev_info *opt, struct commit *commit,
 			     const char **extra_headers_p,
-			     int *need_8bit_cte_p);
+			     int *need_8bit_cte_p,
+			     int maybe_multipart);
 void load_ref_decorations(struct decoration_filter *filter, int flags);
 
 #define FORMAT_PATCH_NAME_MAX 64

t/t4014-format-patch.sh

@@ -1661,6 +1661,15 @@ test_expect_success 'format-patch --base with --attach' '
 	test_write_lines 1 2 >expect &&
 	test_cmp expect actual
 '
+test_expect_success 'format-patch --attach cover-letter only is non-multipart' '
+	test_when_finished "rm -fr patches" &&
+	git format-patch -o patches --cover-letter --attach=mimemime --base=HEAD~ -1 &&
+	! egrep "^--+mimemime" patches/0000*.patch &&
+	egrep "^--+mimemime$" patches/0001*.patch >output &&
+	test_line_count = 2 output &&
+	egrep "^--+mimemime--$" patches/0001*.patch >output &&
+	test_line_count = 1 output
+'
 
 test_expect_success 'format-patch --pretty=mboxrd' '
 	sp=" " &&