utf8: add function to detect a missing UTF-16/32 BOM

larsxschneider · gitster · commit c6e48652f69f · 2018-04-16T11:40:56.000+09:00
If the endianness is not defined in the encoding name, then let's be strict and require a BOM to avoid any encoding confusion. The is_missing_required_utf_bom() function returns true if a required BOM is missing. The Unicode standard instructs to assume big-endian if there in no BOM for UTF-16/32 [1][2]. However, the W3C/WHATWG encoding standard used in HTML5 recommends to assume little-endian to "deal with deployed content" [3]. Strictly requiring a BOM seems to be the safest option for content in Git. This function is used in a subsequent commit. [1] http://unicode.org/faq/utf_bom.html#gen6 [2] http://www.unicode.org/versions/Unicode10.0.0/ch03.pdf Section 3.10, D98, page 132 [3] https://encoding.spec.whatwg.org/#utf-16le Signed-off-by: Lars Schneider <larsxschneider@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/utf8.c b/utf8.c
@@ -586,6 +586,19 @@ int has_prohibited_utf_bom(const char *enc, const char *data, size_t len)
 	);
 }
 
+int is_missing_required_utf_bom(const char *enc, const char *data, size_t len)
+{
+	return (
+	   (same_utf_encoding(enc, "UTF-16")) &&
+	   !(has_bom_prefix(data, len, utf16_be_bom, sizeof(utf16_be_bom)) ||
+	     has_bom_prefix(data, len, utf16_le_bom, sizeof(utf16_le_bom)))
+	) || (
+	   (same_utf_encoding(enc, "UTF-32")) &&
+	   !(has_bom_prefix(data, len, utf32_be_bom, sizeof(utf32_be_bom)) ||
+	     has_bom_prefix(data, len, utf32_le_bom, sizeof(utf32_le_bom)))
+	);
+}
+
 /*
  * Returns first character length in bytes for multi-byte `text` according to
  * `encoding`.
diff --git a/utf8.h b/utf8.h
@@ -79,4 +79,23 @@ void strbuf_utf8_align(struct strbuf *buf, align_type position, unsigned int wid
  */
 int has_prohibited_utf_bom(const char *enc, const char *data, size_t len);
 
+/*
+ * If the endianness is not defined in the encoding name, then we
+ * require a BOM. The function returns true if a required BOM is missing.
+ *
+ * The Unicode standard instructs to assume big-endian if there in no
+ * BOM for UTF-16/32 [1][2]. However, the W3C/WHATWG encoding standard
+ * used in HTML5 recommends to assume little-endian to "deal with
+ * deployed content" [3].
+ *
+ * Therefore, strictly requiring a BOM seems to be the safest option for
+ * content in Git.
+ *
+ * [1] http://unicode.org/faq/utf_bom.html#gen6
+ * [2] http://www.unicode.org/versions/Unicode10.0.0/ch03.pdf
+ *     Section 3.10, D98, page 132
+ * [3] https://encoding.spec.whatwg.org/#utf-16le
+ */
+int is_missing_required_utf_bom(const char *enc, const char *data, size_t len);
+
 #endif
