[PATCH 2/2] utf8: add comment explaining why BOMs are rejected

From: brian m. carlson <sandals@crustytoothpaste.net>
Date: Dec 27, 2018, 02:17 UTC
Message-ID: <20181227021734.528629-3-sandals@crustytoothpaste.net>
In-Reply-To: <20181227021734.528629-1-sandals@crustytoothpaste.net>

A source of confusion for many Git users is why UTF-16LE and UTF-16BE do not allow a BOM, instead treating it as a ZWNBSP, according to the Unicode FAQ[0]. Explain in a comment why we cannot allow that to occur due to our use of UTF-8 internally.

[0] https://unicode.org/faq/utf_bom.html#bom9

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
---
 utf8.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/utf8.c b/utf8.c
index eb78587504..22af2c485a 100644
--- a/utf8.c
+++ b/utf8.c
@@ -571,6 +571,13 @@ static const char utf16_le_bom[] = {'\xFF', '\xFE'};
 static const char utf32_be_bom[] = {'\0', '\0', '\xFE', '\xFF'};
 static const char utf32_le_bom[] = {'\xFF', '\xFE', '\0', '\0'};
 
+/*
+ * We check here for a forbidden BOM. When using UTF-16BE or UTF-16LE, a BOM is
+ * not allowed by RFC 2781, and any U+FEFF would be treated as a ZWNBSP, not a
+ * BOM. However, because we encode into UTF-8 internally, we cannot allow that
+ * character to occur as a ZWNBSP, since when encoded into UTF-8 it would be
+ * interpreted as a BOM.
+ */
 int has_prohibited_utf_bom(const char *enc, const char *data, size_t len)
 {
 	return (