[PATCH v3 2/5] sideband: introduce an "escape hatch" to allow control characters

["Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

The preceding commit fixed the vulnerability whereas sideband messages
(that are under the control of the remote server) could contain ANSI
escape sequences that would be sent to the terminal verbatim.

However, this fix may not be desirable under all circumstances, e.g.
when remote servers deliberately add coloring to their messages to
increase their urgency.

To help with those use cases, give users a way to opt-out of the
protections: `sideband.allowControlCharacters`.

Suggested-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 Documentation/config.txt            |  2 ++
 Documentation/config/sideband.txt   |  5 +++++
 sideband.c                          | 10 ++++++++++
 t/t5409-colorize-remote-messages.sh |  8 +++++++-
 4 files changed, 24 insertions(+), 1 deletion(-)
 create mode 100644 Documentation/config/sideband.txt

diff --git a/Documentation/config.txt b/Documentation/config.txt
index 8c0b3ed807..48870bb588 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -522,6 +522,8 @@ include::config/sequencer.txt[]
 
 include::config/showbranch.txt[]
 
+include::config/sideband.txt[]
+
 include::config/sparse.txt[]
 
 include::config/splitindex.txt[]
diff --git a/Documentation/config/sideband.txt b/Documentation/config/sideband.txt
new file mode 100644
index 0000000000..3fb5045cd7
--- /dev/null
+++ b/Documentation/config/sideband.txt
@@ -0,0 +1,5 @@
+sideband.allowControlCharacters::
+	By default, control characters that are delivered via the sideband
+	are masked, to prevent potentially unwanted ANSI escape sequences
+	from being sent to the terminal. Use this config setting to override
+	this behavior.
diff --git a/sideband.c b/sideband.c
index 3c74f3bdb7..1499587ff6 100644
--- a/sideband.c
+++ b/sideband.c
@@ -25,6 +25,8 @@ static struct keyword_entry keywords[] = {
 	{ "error",	GIT_COLOR_BOLD_RED },
 };
 
+static int allow_control_characters;
+
 /* Returns a color setting (GIT_COLOR_NEVER, etc). */
 static int use_sideband_colors(void)
 {
@@ -38,6 +40,9 @@ static int use_sideband_colors(void)
 	if (use_sideband_colors_cached >= 0)
 		return use_sideband_colors_cached;
 
+	git_config_get_bool("sideband.allowcontrolcharacters",
+			    &allow_control_characters);
+
 	if (!git_config_get_string_tmp(key, &value))
 		use_sideband_colors_cached = git_config_colorbool(key, value);
 	else if (!git_config_get_string_tmp("color.ui", &value))
@@ -67,6 +72,11 @@ void list_config_color_sideband_slots(struct string_list *list, const char *pref
 
 static void strbuf_add_sanitized(struct strbuf *dest, const char *src, int n)
 {
+	if (allow_control_characters) {
+		strbuf_add(dest, src, n);
+		return;
+	}
+
 	strbuf_grow(dest, n);
 	for (; n && *src; src++, n--) {
 		if (!iscntrl(*src) || *src == '\t' || *src == '\n') {
diff --git a/t/t5409-colorize-remote-messages.sh b/t/t5409-colorize-remote-messages.sh
index f4712f4161..e8067df591 100755
--- a/t/t5409-colorize-remote-messages.sh
+++ b/t/t5409-colorize-remote-messages.sh
@@ -106,9 +106,15 @@ test_expect_success 'disallow (color) control sequences in sideband' '
 	EOF
 	test_config_global uploadPack.packObjectsHook ./color-me-surprised &&
 	test_commit need-at-least-one-commit &&
+
 	git clone --no-local . throw-away 2>stderr &&
 	test_decode_color <stderr >decoded &&
-	test_grep ! RED decoded
+	test_grep ! RED decoded &&
+
+	rm -rf throw-away &&
+	git -c sideband.allowControlCharacters clone --no-local . throw-away 2>stderr &&
+	test_decode_color <stderr >decoded &&
+	test_grep RED decoded
 '
 
 test_done
-- 
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