log --format: teach %C(auto,black) to respect color config

Traditionally, %C(color attr) always emitted the ANSI color
sequence; it was up to the scripts that wanted to conditionally
color their output to omit %C(...) specifier when they do not want
colors.

Optionally allow "auto," to be prefixed to the color, so that the
output is colored iff we would color regular "log" output
(e.g., taking into account color.* and --color command line
options).

Tests and pretty_context bits by Jeff King <peff@peff.net>.

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

5 files changed, 72 insertions, 4 deletions

diff --git a/Documentation/pretty-formats.txt b/Documentation/pretty-formats.txt
index d9eddedc72..105f18a6f9 100644
--- a/Documentation/pretty-formats.txt
+++ b/Documentation/pretty-formats.txt
@@ -144,7 +144,11 @@ The placeholders are:
 - '%Cgreen': switch color to green
 - '%Cblue': switch color to blue
 - '%Creset': reset color
-- '%C(...)': color specification, as described in color.branch.* config option
+- '%C(...)': color specification, as described in color.branch.* config option;
+  adding `auto,` at the beginning will emit color only when colors are
+  enabled for log output (by `color.diff`, `color.ui`, or `--color`, and
+  respecting the `auto` settings of the former if we are going to a
+  terminal)
 - '%m': left, right or boundary mark
 - '%n': newline
 - '%%': a raw '%'
diff --git a/commit.h b/commit.h
index 9f2131318d..73fd399b09 100644
--- a/commit.h
+++ b/commit.h
@@ -89,6 +89,7 @@ struct pretty_print_context {
 	int show_notes;
 	struct reflog_walk_info *reflog_info;
 	const char *output_encoding;
+	int color;
 };
 
 struct userformat_want {
diff --git a/log-tree.c b/log-tree.c
index c894930c18..7185bd65f2 100644
--- a/log-tree.c
+++ b/log-tree.c
@@ -660,6 +660,7 @@ void show_log(struct rev_info *opt)
 	ctx.preserve_subject = opt->preserve_subject;
 	ctx.reflog_info = opt->reflog_info;
 	ctx.fmt = opt->commit_format;
+	ctx.color = opt->diffopt.use_color;
 	pretty_print_commit(&ctx, commit, &msgbuf);
 
 	if (opt->add_signoff)
diff --git a/pretty.c b/pretty.c
index dba682828c..63350af614 100644
--- a/pretty.c
+++ b/pretty.c
@@ -960,12 +960,19 @@ static size_t format_commit_one(struct strbuf *sb, const char *placeholder,
 	switch (placeholder[0]) {
 	case 'C':
 		if (placeholder[1] == '(') {
-			const char *end = strchr(placeholder + 2, ')');
+			const char *begin = placeholder + 2;
+			const char *end = strchr(begin, ')');
 			char color[COLOR_MAXLEN];
+
 			if (!end)
 				return 0;
-			color_parse_mem(placeholder + 2,
-					end - (placeholder + 2),
+			if (!memcmp(begin, "auto,", 5)) {
+				if (!want_color(c->pretty_ctx->color))
+					return end - placeholder + 1;
+				begin += 5;
+			}
+			color_parse_mem(begin,
+					end - begin,
 					"--pretty format", color);
 			strbuf_addstr(sb, color);
 			return end - placeholder + 1;
diff --git a/t/t6006-rev-list-format.sh b/t/t6006-rev-list-format.sh
index c0c62c9da0..3fc3b74c8e 100755
--- a/t/t6006-rev-list-format.sh
+++ b/t/t6006-rev-list-format.sh
@@ -3,6 +3,7 @@
 test_description='git rev-list --pretty=format test'
 
 . ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-terminal.sh
 
 test_tick
 test_expect_success 'setup' '
@@ -19,6 +20,18 @@ test_format () {
 	"
 }
 
+# Feed to --format to provide predictable colored sequences.
+AUTO_COLOR='%C(auto,red)foo%C(auto,reset)'
+has_color () {
+	printf '\033[31mfoo\033[m\n' >expect &&
+	test_cmp expect "$1"
+}
+
+has_no_color () {
+	echo foo >expect &&
+	test_cmp expect "$1"
+}
+
 test_format percent %%h <<'EOF'
 commit 131a310eb913d107dd3c09a65d1651175898735d
 %h
@@ -124,6 +137,48 @@ commit 86c75cfd708a0e5868dc876ed5b8bb66c80b4873
 foo
 EOF
 
+test_expect_success '%C(auto) does not enable color by default' '
+	git log --format=$AUTO_COLOR -1 >actual &&
+	has_no_color actual
+'
+
+test_expect_success '%C(auto) enables colors for color.diff' '
+	git -c color.diff=always log --format=$AUTO_COLOR -1 >actual &&
+	has_color actual
+'
+
+test_expect_success '%C(auto) enables colors for color.ui' '
+	git -c color.ui=always log --format=$AUTO_COLOR -1 >actual &&
+	has_color actual
+'
+
+test_expect_success '%C(auto) respects --color' '
+	git log --format=$AUTO_COLOR -1 --color >actual &&
+	has_color actual
+'
+
+test_expect_success '%C(auto) respects --no-color' '
+	git -c color.ui=always log --format=$AUTO_COLOR -1 --no-color >actual &&
+	has_no_color actual
+'
+
+test_expect_success TTY '%C(auto) respects --color=auto (stdout is tty)' '
+	(
+		TERM=vt100 && export TERM &&
+		test_terminal \
+			git log --format=$AUTO_COLOR -1 --color=auto >actual &&
+		has_color actual
+	)
+'
+
+test_expect_success '%C(auto) respects --color=auto (stdout not tty)' '
+	(
+		TERM=vt100 && export TERM &&
+		git log --format=$AUTO_COLOR -1 --color=auto >actual &&
+		has_no_color actual
+	)
+'
+
 cat >commit-msg <<'EOF'
 Test printing of complex bodies