format-patch: add --interdiff option to embed diff in cover letter

When submitting a revised version of a patch series, it can be helpful
(to reviewers) to include a summary of changes since the previous
attempt in the form of an interdiff, however, doing so involves manually
copy/pasting the diff into the cover letter.

Add an --interdiff option to automate this process. The argument to
--interdiff specifies the tip of the previous attempt against which to
generate the interdiff. For example:

    git format-patch --cover-letter --interdiff=v1 -3 v2

The previous attempt and the patch series being formatted must share a
common base.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

7 files changed, 78 insertions, 2 deletions

diff --git a/Documentation/git-format-patch.txt b/Documentation/git-format-patch.txt
index b41e1329a7..a1b1bafee7 100644
--- a/Documentation/git-format-patch.txt
+++ b/Documentation/git-format-patch.txt
@@ -23,6 +23,7 @@ SYNOPSIS
 		   [(--reroll-count|-v) <n>]
 		   [--to=<email>] [--cc=<email>]
 		   [--[no-]cover-letter] [--quiet] [--notes[=<ref>]]
+		   [--interdiff=<previous>]
 		   [--progress]
 		   [<common diff options>]
 		   [ <since> | <revision range> ]
@@ -228,6 +229,14 @@ feeding the result to `git send-email`.
 	containing the branch description, shortlog and the overall diffstat.  You can
 	fill in a description in the file before sending it out.
 
+--interdiff=<previous>::
+	As a reviewer aid, insert an interdiff into the cover letter showing
+	the differences between the previous version of the patch series and
+	the series currently being formatted. `previous` is a single revision
+	naming the tip of the previous series which shares a common base with
+	the series being formatted (for example `git format-patch
+	--cover-letter --interdiff=feature/v1 -3 feature/v2`).
+
 --notes[=<ref>]::
 	Append the notes (see linkgit:git-notes[1]) for the commit
 	after the three-dash line.
diff --git a/Makefile b/Makefile
index 08e5c54549..b2685190e1 100644
--- a/Makefile
+++ b/Makefile
@@ -871,6 +871,7 @@ LIB_OBJS += hashmap.o
 LIB_OBJS += help.o
 LIB_OBJS += hex.o
 LIB_OBJS += ident.o
+LIB_OBJS += interdiff.o
 LIB_OBJS += kwset.o
 LIB_OBJS += levenshtein.o
 LIB_OBJS += line-log.o
diff --git a/builtin/log.c b/builtin/log.c
index 873aabcf40..1020b78477 100644
--- a/builtin/log.c
+++ b/builtin/log.c
@@ -30,6 +30,7 @@
 #include "gpg-interface.h"
 #include "progress.h"
 #include "commit-slab.h"
+#include "interdiff.h"
 
 #define MAIL_DEFAULT_WRAP 72
 
@@ -1082,6 +1083,11 @@ static void make_cover_letter(struct rev_info *rev, int use_stdout,
 	/* We can only do diffstat with a unique reference point */
 	if (origin)
 		show_diffstat(rev, origin, head);
+
+	if (rev->idiff_oid1) {
+		fprintf_ln(rev->diffopt.file, "%s", _("Interdiff:"));
+		show_interdiff(rev);
+	}
 }
 
 static const char *clean_message_id(const char *msg_id)
@@ -1448,6 +1454,7 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 	struct base_tree_info bases;
 	int show_progress = 0;
 	struct progress *progress = NULL;
+	struct oid_array idiff_prev = OID_ARRAY_INIT;
 
 	const struct option builtin_format_patch_options[] = {
 		{ OPTION_CALLBACK, 'n', "numbered", &numbered, NULL,
@@ -1521,6 +1528,9 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 		OPT__QUIET(&quiet, N_("don't print the patch filenames")),
 		OPT_BOOL(0, "progress", &show_progress,
 			 N_("show progress while generating patches")),
+		OPT_CALLBACK(0, "interdiff", &idiff_prev, N_("rev"),
+			     N_("show changes against <rev> in cover letter"),
+			     parse_opt_object_name),
 		OPT_END()
 	};
 
@@ -1706,7 +1716,7 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 		if (rev.pending.nr == 2) {
 			struct object_array_entry *o = rev.pending.objects;
 			if (oidcmp(&o[0].item->oid, &o[1].item->oid) == 0)
-				return 0;
+				goto done;
 		}
 		get_patch_ids(&rev, &ids);
 	}
@@ -1730,7 +1740,7 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 	}
 	if (nr == 0)
 		/* nothing to do */
-		return 0;
+		goto done;
 	total = nr;
 	if (cover_letter == -1) {
 		if (config_cover_letter == COVER_AUTO)
@@ -1743,6 +1753,13 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 	if (numbered)
 		rev.total = total + start_number - 1;
 
+	if (idiff_prev.nr) {
+		if (!cover_letter)
+			die(_("--interdiff requires --cover-letter"));
+		rev.idiff_oid1 = &idiff_prev.oid[idiff_prev.nr - 1];
+		rev.idiff_oid2 = get_commit_tree_oid(list[0]);
+	}
+
 	if (!signature) {
 		; /* --no-signature inhibits all signatures */
 	} else if (signature && signature != git_version_string) {
@@ -1860,6 +1877,9 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 	string_list_clear(&extra_hdr, 0);
 	if (ignore_if_in_upstream)
 		free_patch_ids(&ids);
+
+done:
+	oid_array_clear(&idiff_prev);
 	return 0;
 }
 
diff --git a/interdiff.c b/interdiff.c
new file mode 100644
index 0000000000..d0fac10c7c
--- /dev/null
+++ b/interdiff.c
@@ -0,0 +1,17 @@
+#include "cache.h"
+#include "commit.h"
+#include "revision.h"
+#include "interdiff.h"
+
+void show_interdiff(struct rev_info *rev)
+{
+	struct diff_options opts;
+
+	memcpy(&opts, &rev->diffopt, sizeof(opts));
+	opts.output_format = DIFF_FORMAT_PATCH;
+	diff_setup_done(&opts);
+
+	diff_tree_oid(rev->idiff_oid1, rev->idiff_oid2, "", &opts);
+	diffcore_std(&opts);
+	diff_flush(&opts);
+}
diff --git a/interdiff.h b/interdiff.h
new file mode 100644
index 0000000000..793c0144fe
--- /dev/null
+++ b/interdiff.h
@@ -0,0 +1,8 @@
+#ifndef INTERDIFF_H
+#define INTERDIFF_H
+
+struct rev_info;
+
+void show_interdiff(struct rev_info *);
+
+#endif
diff --git a/revision.h b/revision.h
index bf2239f876..61931fbac5 100644
--- a/revision.h
+++ b/revision.h
@@ -212,6 +212,10 @@ struct rev_info {
 	/* notes-specific options: which refs to show */
 	struct display_notes_opt notes_opt;
 
+	/* interdiff */
+	const struct object_id *idiff_oid1;
+	const struct object_id *idiff_oid2;
+
 	/* commit counts */
 	int count_left;
 	int count_right;
diff --git a/t/t4014-format-patch.sh b/t/t4014-format-patch.sh
index 53880da7bb..57b46322aa 100755
--- a/t/t4014-format-patch.sh
+++ b/t/t4014-format-patch.sh
@@ -1717,4 +1717,21 @@ test_expect_success 'format-patch --pretty=mboxrd' '
 	test_cmp expect actual
 '
 
+test_expect_success 'interdiff: setup' '
+	git checkout -b boop master &&
+	test_commit fnorp blorp &&
+	test_commit fleep blorp
+'
+
+test_expect_success 'interdiff: cover-letter' '
+	sed "y/q/ /" >expect <<-\EOF &&
+	+fleep
+	--q
+	EOF
+	git format-patch --cover-letter --interdiff=boop~2 -1 boop &&
+	test_i18ngrep "^Interdiff:$" 0000-cover-letter.patch &&
+	sed "1,/^@@ /d; /^-- $/q" <0000-cover-letter.patch >actual &&
+	test_cmp expect actual
+'
+
 test_done