Preserve BOM in web editor (#28935)

The `ToUTF8*` functions were stripping BOM, while BOM is actually valid
in UTF8, so the stripping must be optional depending on use case. This
does:

- Add a options struct to all `ToUTF8*` functions, that by default will
strip BOM to preserve existing behaviour
- Remove `ToUTF8` function, it was dead code
- Rename `ToUTF8WithErr` to `ToUTF8`
- Preserve BOM in Monaco Editor
- Remove a unnecessary newline in the textarea value. Browsers did
ignore it, it seems but it's better not to rely on this behaviour.

Fixes: https://github.com/go-gitea/gitea/issues/28743
Related: https://github.com/go-gitea/gitea/issues/6716 which seems to
have once introduced a mechanism that strips and re-adds the BOM, but
from what I can tell, this mechanism was removed at some point after
that PR.

13 files changed, 69 insertions, 134 deletions

diff --git a/modules/charset/charset.go b/modules/charset/charset.go
index 51152142a5..1855446a98 100644
--- a/modules/charset/charset.go
+++ b/modules/charset/charset.go
@@ -22,17 +22,21 @@ import (
 // UTF8BOM is the utf-8 byte-order marker
 var UTF8BOM = []byte{'\xef', '\xbb', '\xbf'}
 
+type ConvertOpts struct {
+	KeepBOM bool
+}
+
 // ToUTF8WithFallbackReader detects the encoding of content and converts to UTF-8 reader if possible
-func ToUTF8WithFallbackReader(rd io.Reader) io.Reader {
+func ToUTF8WithFallbackReader(rd io.Reader, opts ConvertOpts) io.Reader {
 	buf := make([]byte, 2048)
 	n, err := util.ReadAtMost(rd, buf)
 	if err != nil {
-		return io.MultiReader(bytes.NewReader(RemoveBOMIfPresent(buf[:n])), rd)
+		return io.MultiReader(bytes.NewReader(MaybeRemoveBOM(buf[:n], opts)), rd)
 	}
 
 	charsetLabel, err := DetectEncoding(buf[:n])
 	if err != nil || charsetLabel == "UTF-8" {
-		return io.MultiReader(bytes.NewReader(RemoveBOMIfPresent(buf[:n])), rd)
+		return io.MultiReader(bytes.NewReader(MaybeRemoveBOM(buf[:n], opts)), rd)
 	}
 
 	encoding, _ := charset.Lookup(charsetLabel)
@@ -42,20 +46,20 @@ func ToUTF8WithFallbackReader(rd io.Reader) io.Reader {
 
 	return transform.NewReader(
 		io.MultiReader(
-			bytes.NewReader(RemoveBOMIfPresent(buf[:n])),
+			bytes.NewReader(MaybeRemoveBOM(buf[:n], opts)),
 			rd,
 		),
 		encoding.NewDecoder(),
 	)
 }
 
-// ToUTF8WithErr converts content to UTF8 encoding
-func ToUTF8WithErr(content []byte) (string, error) {
+// ToUTF8 converts content to UTF8 encoding
+func ToUTF8(content []byte, opts ConvertOpts) (string, error) {
 	charsetLabel, err := DetectEncoding(content)
 	if err != nil {
 		return "", err
 	} else if charsetLabel == "UTF-8" {
-		return string(RemoveBOMIfPresent(content)), nil
+		return string(MaybeRemoveBOM(content, opts)), nil
 	}
 
 	encoding, _ := charset.Lookup(charsetLabel)
@@ -70,28 +74,22 @@ func ToUTF8WithErr(content []byte) (string, error) {
 		result = append(result, content[n:]...)
 	}
 
-	result = RemoveBOMIfPresent(result)
+	result = MaybeRemoveBOM(result, opts)
 
 	return string(result), err
 }
 
 // ToUTF8WithFallback detects the encoding of content and converts to UTF-8 if possible
-func ToUTF8WithFallback(content []byte) []byte {
-	bs, _ := io.ReadAll(ToUTF8WithFallbackReader(bytes.NewReader(content)))
+func ToUTF8WithFallback(content []byte, opts ConvertOpts) []byte {
+	bs, _ := io.ReadAll(ToUTF8WithFallbackReader(bytes.NewReader(content), opts))
 	return bs
 }
 
-// ToUTF8 converts content to UTF8 encoding and ignore error
-func ToUTF8(content string) string {
-	res, _ := ToUTF8WithErr([]byte(content))
-	return res
-}
-
 // ToUTF8DropErrors makes sure the return string is valid utf-8; attempts conversion if possible
-func ToUTF8DropErrors(content []byte) []byte {
+func ToUTF8DropErrors(content []byte, opts ConvertOpts) []byte {
 	charsetLabel, err := DetectEncoding(content)
 	if err != nil || charsetLabel == "UTF-8" {
-		return RemoveBOMIfPresent(content)
+		return MaybeRemoveBOM(content, opts)
 	}
 
 	encoding, _ := charset.Lookup(charsetLabel)
@@ -117,11 +115,14 @@ func ToUTF8DropErrors(content []byte) []byte {
 		}
 	}
 
-	return RemoveBOMIfPresent(decoded)
+	return MaybeRemoveBOM(decoded, opts)
 }
 
-// RemoveBOMIfPresent removes a UTF-8 BOM from a []byte
-func RemoveBOMIfPresent(content []byte) []byte {
+// MaybeRemoveBOM removes a UTF-8 BOM from a []byte when opts.KeepBOM is false
+func MaybeRemoveBOM(content []byte, opts ConvertOpts) []byte {
+	if opts.KeepBOM {
+		return content
+	}
 	if len(content) > 2 && bytes.Equal(content[0:3], UTF8BOM) {
 		return content[3:]
 	}
diff --git a/modules/charset/charset_test.go b/modules/charset/charset_test.go
index fc56799b47..829844a976 100644
--- a/modules/charset/charset_test.go
+++ b/modules/charset/charset_test.go
@@ -30,15 +30,15 @@ func resetDefaultCharsetsOrder() {
 	}
 }
 
-func TestRemoveBOMIfPresent(t *testing.T) {
-	res := RemoveBOMIfPresent([]byte{0xc3, 0xa1, 0xc3, 0xa9, 0xc3, 0xad, 0xc3, 0xb3, 0xc3, 0xba})
+func TestMaybeRemoveBOM(t *testing.T) {
+	res := MaybeRemoveBOM([]byte{0xc3, 0xa1, 0xc3, 0xa9, 0xc3, 0xad, 0xc3, 0xb3, 0xc3, 0xba}, ConvertOpts{})
 	assert.Equal(t, []byte{0xc3, 0xa1, 0xc3, 0xa9, 0xc3, 0xad, 0xc3, 0xb3, 0xc3, 0xba}, res)
 
-	res = RemoveBOMIfPresent([]byte{0xef, 0xbb, 0xbf, 0xc3, 0xa1, 0xc3, 0xa9, 0xc3, 0xad, 0xc3, 0xb3, 0xc3, 0xba})
+	res = MaybeRemoveBOM([]byte{0xef, 0xbb, 0xbf, 0xc3, 0xa1, 0xc3, 0xa9, 0xc3, 0xad, 0xc3, 0xb3, 0xc3, 0xba}, ConvertOpts{})
 	assert.Equal(t, []byte{0xc3, 0xa1, 0xc3, 0xa9, 0xc3, 0xad, 0xc3, 0xb3, 0xc3, 0xba}, res)
 }
 
-func TestToUTF8WithErr(t *testing.T) {
+func TestToUTF8(t *testing.T) {
 	resetDefaultCharsetsOrder()
 	var res string
 	var err error
@@ -47,53 +47,53 @@ func TestToUTF8WithErr(t *testing.T) {
 	// locale, so some conversions might behave differently. For that reason, we don't
 	// depend on particular conversions but in expected behaviors.
 
-	res, err = ToUTF8WithErr([]byte{0x41, 0x42, 0x43})
+	res, err = ToUTF8([]byte{0x41, 0x42, 0x43}, ConvertOpts{})
 	assert.NoError(t, err)
 	assert.Equal(t, "ABC", res)
 
 	// "áéíóú"
-	res, err = ToUTF8WithErr([]byte{0xc3, 0xa1, 0xc3, 0xa9, 0xc3, 0xad, 0xc3, 0xb3, 0xc3, 0xba})
+	res, err = ToUTF8([]byte{0xc3, 0xa1, 0xc3, 0xa9, 0xc3, 0xad, 0xc3, 0xb3, 0xc3, 0xba}, ConvertOpts{})
 	assert.NoError(t, err)
 	assert.Equal(t, []byte{0xc3, 0xa1, 0xc3, 0xa9, 0xc3, 0xad, 0xc3, 0xb3, 0xc3, 0xba}, []byte(res))
 
 	// "áéíóú"
-	res, err = ToUTF8WithErr([]byte{
+	res, err = ToUTF8([]byte{
 		0xef, 0xbb, 0xbf, 0xc3, 0xa1, 0xc3, 0xa9, 0xc3, 0xad, 0xc3, 0xb3,
 		0xc3, 0xba,
-	})
+	}, ConvertOpts{})
 	assert.NoError(t, err)
 	assert.Equal(t, []byte{0xc3, 0xa1, 0xc3, 0xa9, 0xc3, 0xad, 0xc3, 0xb3, 0xc3, 0xba}, []byte(res))
 
-	res, err = ToUTF8WithErr([]byte{
+	res, err = ToUTF8([]byte{
 		0x48, 0x6F, 0x6C, 0x61, 0x2C, 0x20, 0x61, 0x73, 0xED, 0x20, 0x63,
 		0xF3, 0x6D, 0x6F, 0x20, 0xF1, 0x6F, 0x73, 0x41, 0x41, 0x41, 0x2e,
-	})
+	}, ConvertOpts{})
 	assert.NoError(t, err)
 	stringMustStartWith(t, "Hola,", res)
 	stringMustEndWith(t, "AAA.", res)
 
-	res, err = ToUTF8WithErr([]byte{
+	res, err = ToUTF8([]byte{
 		0x48, 0x6F, 0x6C, 0x61, 0x2C, 0x20, 0x61, 0x73, 0xED, 0x20, 0x63,
 		0xF3, 0x6D, 0x6F, 0x20, 0x07, 0xA4, 0x6F, 0x73, 0x41, 0x41, 0x41, 0x2e,
-	})
+	}, ConvertOpts{})
 	assert.NoError(t, err)
 	stringMustStartWith(t, "Hola,", res)
 	stringMustEndWith(t, "AAA.", res)
 
-	res, err = ToUTF8WithErr([]byte{
+	res, err = ToUTF8([]byte{
 		0x48, 0x6F, 0x6C, 0x61, 0x2C, 0x20, 0x61, 0x73, 0xED, 0x20, 0x63,
 		0xF3, 0x6D, 0x6F, 0x20, 0x81, 0xA4, 0x6F, 0x73, 0x41, 0x41, 0x41, 0x2e,
-	})
+	}, ConvertOpts{})
 	assert.NoError(t, err)
 	stringMustStartWith(t, "Hola,", res)
 	stringMustEndWith(t, "AAA.", res)
 
 	// Japanese (Shift-JIS)
 	// 日属秘ぞしちゅ。
-	res, err = ToUTF8WithErr([]byte{
+	res, err = ToUTF8([]byte{
 		0x93, 0xFA, 0x91, 0xAE, 0x94, 0xE9, 0x82, 0xBC, 0x82, 0xB5, 0x82,
 		0xBF, 0x82, 0xE3, 0x81, 0x42,
-	})
+	}, ConvertOpts{})
 	assert.NoError(t, err)
 	assert.Equal(t, []byte{
 		0xE6, 0x97, 0xA5, 0xE5, 0xB1, 0x9E, 0xE7, 0xA7, 0x98, 0xE3,
@@ -101,7 +101,7 @@ func TestToUTF8WithErr(t *testing.T) {
 	},
 		[]byte(res))
 
-	res, err = ToUTF8WithErr([]byte{0x00, 0x00, 0x00, 0x00})
+	res, err = ToUTF8([]byte{0x00, 0x00, 0x00, 0x00}, ConvertOpts{})
 	assert.NoError(t, err)
 	assert.Equal(t, []byte{0x00, 0x00, 0x00, 0x00}, []byte(res))
 }
@@ -109,22 +109,22 @@ func TestToUTF8WithErr(t *testing.T) {
 func TestToUTF8WithFallback(t *testing.T) {
 	resetDefaultCharsetsOrder()
 	// "ABC"
-	res := ToUTF8WithFallback([]byte{0x41, 0x42, 0x43})
+	res := ToUTF8WithFallback([]byte{0x41, 0x42, 0x43}, ConvertOpts{})
 	assert.Equal(t, []byte{0x41, 0x42, 0x43}, res)
 
 	// "áéíóú"
-	res = ToUTF8WithFallback([]byte{0xc3, 0xa1, 0xc3, 0xa9, 0xc3, 0xad, 0xc3, 0xb3, 0xc3, 0xba})
+	res = ToUTF8WithFallback([]byte{0xc3, 0xa1, 0xc3, 0xa9, 0xc3, 0xad, 0xc3, 0xb3, 0xc3, 0xba}, ConvertOpts{})
 	assert.Equal(t, []byte{0xc3, 0xa1, 0xc3, 0xa9, 0xc3, 0xad, 0xc3, 0xb3, 0xc3, 0xba}, res)
 
 	// UTF8 BOM + "áéíóú"
-	res = ToUTF8WithFallback([]byte{0xef, 0xbb, 0xbf, 0xc3, 0xa1, 0xc3, 0xa9, 0xc3, 0xad, 0xc3, 0xb3, 0xc3, 0xba})
+	res = ToUTF8WithFallback([]byte{0xef, 0xbb, 0xbf, 0xc3, 0xa1, 0xc3, 0xa9, 0xc3, 0xad, 0xc3, 0xb3, 0xc3, 0xba}, ConvertOpts{})
 	assert.Equal(t, []byte{0xc3, 0xa1, 0xc3, 0xa9, 0xc3, 0xad, 0xc3, 0xb3, 0xc3, 0xba}, res)
 
 	// "Hola, así cómo ños"
 	res = ToUTF8WithFallback([]byte{
 		0x48, 0x6F, 0x6C, 0x61, 0x2C, 0x20, 0x61, 0x73, 0xED, 0x20, 0x63,
 		0xF3, 0x6D, 0x6F, 0x20, 0xF1, 0x6F, 0x73,
-	})
+	}, ConvertOpts{})
 	assert.Equal(t, []byte{
 		0x48, 0x6F, 0x6C, 0x61, 0x2C, 0x20, 0x61, 0x73, 0xC3, 0xAD, 0x20, 0x63,
 		0xC3, 0xB3, 0x6D, 0x6F, 0x20, 0xC3, 0xB1, 0x6F, 0x73,
@@ -133,126 +133,65 @@ func TestToUTF8WithFallback(t *testing.T) {
 	// "Hola, así cómo "
 	minmatch := []byte{0x48, 0x6F, 0x6C, 0x61, 0x2C, 0x20, 0x61, 0x73, 0xC3, 0xAD, 0x20, 0x63, 0xC3, 0xB3, 0x6D, 0x6F, 0x20}
 
-	res = ToUTF8WithFallback([]byte{0x48, 0x6F, 0x6C, 0x61, 0x2C, 0x20, 0x61, 0x73, 0xED, 0x20, 0x63, 0xF3, 0x6D, 0x6F, 0x20, 0x07, 0xA4, 0x6F, 0x73})
+	res = ToUTF8WithFallback([]byte{0x48, 0x6F, 0x6C, 0x61, 0x2C, 0x20, 0x61, 0x73, 0xED, 0x20, 0x63, 0xF3, 0x6D, 0x6F, 0x20, 0x07, 0xA4, 0x6F, 0x73}, ConvertOpts{})
 	// Do not fail for differences in invalid cases, as the library might change the conversion criteria for those
 	assert.Equal(t, minmatch, res[0:len(minmatch)])
 
-	res = ToUTF8WithFallback([]byte{0x48, 0x6F, 0x6C, 0x61, 0x2C, 0x20, 0x61, 0x73, 0xED, 0x20, 0x63, 0xF3, 0x6D, 0x6F, 0x20, 0x81, 0xA4, 0x6F, 0x73})
+	res = ToUTF8WithFallback([]byte{0x48, 0x6F, 0x6C, 0x61, 0x2C, 0x20, 0x61, 0x73, 0xED, 0x20, 0x63, 0xF3, 0x6D, 0x6F, 0x20, 0x81, 0xA4, 0x6F, 0x73}, ConvertOpts{})
 	// Do not fail for differences in invalid cases, as the library might change the conversion criteria for those
 	assert.Equal(t, minmatch, res[0:len(minmatch)])
 
 	// Japanese (Shift-JIS)
 	// "日属秘ぞしちゅ。"
-	res = ToUTF8WithFallback([]byte{0x93, 0xFA, 0x91, 0xAE, 0x94, 0xE9, 0x82, 0xBC, 0x82, 0xB5, 0x82, 0xBF, 0x82, 0xE3, 0x81, 0x42})
+	res = ToUTF8WithFallback([]byte{0x93, 0xFA, 0x91, 0xAE, 0x94, 0xE9, 0x82, 0xBC, 0x82, 0xB5, 0x82, 0xBF, 0x82, 0xE3, 0x81, 0x42}, ConvertOpts{})
 	assert.Equal(t, []byte{
 		0xE6, 0x97, 0xA5, 0xE5, 0xB1, 0x9E, 0xE7, 0xA7, 0x98, 0xE3,
 		0x81, 0x9E, 0xE3, 0x81, 0x97, 0xE3, 0x81, 0xA1, 0xE3, 0x82, 0x85, 0xE3, 0x80, 0x82,
 	}, res)
 
-	res = ToUTF8WithFallback([]byte{0x00, 0x00, 0x00, 0x00})
+	res = ToUTF8WithFallback([]byte{0x00, 0x00, 0x00, 0x00}, ConvertOpts{})
 	assert.Equal(t, []byte{0x00, 0x00, 0x00, 0x00}, res)
 }
 
-func TestToUTF8(t *testing.T) {
-	resetDefaultCharsetsOrder()
-	// Note: golang compiler seems so behave differently depending on the current
-	// locale, so some conversions might behave differently. For that reason, we don't
-	// depend on particular conversions but in expected behaviors.
-
-	res := ToUTF8(string([]byte{0x41, 0x42, 0x43}))
-	assert.Equal(t, "ABC", res)
-
-	// "áéíóú"
-	res = ToUTF8(string([]byte{0xc3, 0xa1, 0xc3, 0xa9, 0xc3, 0xad, 0xc3, 0xb3, 0xc3, 0xba}))
-	assert.Equal(t, []byte{0xc3, 0xa1, 0xc3, 0xa9, 0xc3, 0xad, 0xc3, 0xb3, 0xc3, 0xba}, []byte(res))
-
-	// BOM + "áéíóú"
-	res = ToUTF8(string([]byte{
-		0xef, 0xbb, 0xbf, 0xc3, 0xa1, 0xc3, 0xa9, 0xc3, 0xad, 0xc3, 0xb3,
-		0xc3, 0xba,
-	}))
-	assert.Equal(t, []byte{0xc3, 0xa1, 0xc3, 0xa9, 0xc3, 0xad, 0xc3, 0xb3, 0xc3, 0xba}, []byte(res))
-
-	// Latin1
-	// Hola, así cómo ños
-	res = ToUTF8(string([]byte{
-		0x48, 0x6F, 0x6C, 0x61, 0x2C, 0x20, 0x61, 0x73, 0xED, 0x20, 0x63,
-		0xF3, 0x6D, 0x6F, 0x20, 0xF1, 0x6F, 0x73,
-	}))
-	assert.Equal(t, []byte{
-		0x48, 0x6f, 0x6c, 0x61, 0x2c, 0x20, 0x61, 0x73, 0xc3, 0xad, 0x20, 0x63,
-		0xc3, 0xb3, 0x6d, 0x6f, 0x20, 0xc3, 0xb1, 0x6f, 0x73,
-	}, []byte(res))
-
-	// Latin1
-	// Hola, así cómo \x07ños
-	res = ToUTF8(string([]byte{
-		0x48, 0x6F, 0x6C, 0x61, 0x2C, 0x20, 0x61, 0x73, 0xED, 0x20, 0x63,
-		0xF3, 0x6D, 0x6F, 0x20, 0x07, 0xA4, 0x6F, 0x73,
-	}))
-	// Hola,
-	bytesMustStartWith(t, []byte{0x48, 0x6F, 0x6C, 0x61, 0x2C}, []byte(res))
-
-	// This test FAILS
-	// res = ToUTF8("Hola, así cómo \x81ños")
-	// Do not fail for differences in invalid cases, as the library might change the conversion criteria for those
-	// assert.Regexp(t, "^Hola, así cómo", res)
-
-	// Japanese (Shift-JIS)
-	// 日属秘ぞしちゅ。
-	res = ToUTF8(string([]byte{
-		0x93, 0xFA, 0x91, 0xAE, 0x94, 0xE9, 0x82, 0xBC, 0x82, 0xB5, 0x82,
-		0xBF, 0x82, 0xE3, 0x81, 0x42,
-	}))
-	assert.Equal(t, []byte{
-		0xE6, 0x97, 0xA5, 0xE5, 0xB1, 0x9E, 0xE7, 0xA7, 0x98, 0xE3,
-		0x81, 0x9E, 0xE3, 0x81, 0x97, 0xE3, 0x81, 0xA1, 0xE3, 0x82, 0x85, 0xE3, 0x80, 0x82,
-	},
-		[]byte(res))
-
-	res = ToUTF8("\x00\x00\x00\x00")
-	assert.Equal(t, []byte{0x00, 0x00, 0x00, 0x00}, []byte(res))
-}
-
 func TestToUTF8DropErrors(t *testing.T) {
 	resetDefaultCharsetsOrder()
 	// "ABC"
-	res := ToUTF8DropErrors([]byte{0x41, 0x42, 0x43})
+	res := ToUTF8DropErrors([]byte{0x41, 0x42, 0x43}, ConvertOpts{})
 	assert.Equal(t, []byte{0x41, 0x42, 0x43}, res)
 
 	// "áéíóú"
-	res = ToUTF8DropErrors([]byte{0xc3, 0xa1, 0xc3, 0xa9, 0xc3, 0xad, 0xc3, 0xb3, 0xc3, 0xba})
+	res = ToUTF8DropErrors([]byte{0xc3, 0xa1, 0xc3, 0xa9, 0xc3, 0xad, 0xc3, 0xb3, 0xc3, 0xba}, ConvertOpts{})
 	assert.Equal(t, []byte{0xc3, 0xa1, 0xc3, 0xa9, 0xc3, 0xad, 0xc3, 0xb3, 0xc3, 0xba}, res)
 
 	// UTF8 BOM + "áéíóú"
-	res = ToUTF8DropErrors([]byte{0xef, 0xbb, 0xbf, 0xc3, 0xa1, 0xc3, 0xa9, 0xc3, 0xad, 0xc3, 0xb3, 0xc3, 0xba})
+	res = ToUTF8DropErrors([]byte{0xef, 0xbb, 0xbf, 0xc3, 0xa1, 0xc3, 0xa9, 0xc3, 0xad, 0xc3, 0xb3, 0xc3, 0xba}, ConvertOpts{})
 	assert.Equal(t, []byte{0xc3, 0xa1, 0xc3, 0xa9, 0xc3, 0xad, 0xc3, 0xb3, 0xc3, 0xba}, res)
 
 	// "Hola, así cómo ños"
-	res = ToUTF8DropErrors([]byte{0x48, 0x6F, 0x6C, 0x61, 0x2C, 0x20, 0x61, 0x73, 0xED, 0x20, 0x63, 0xF3, 0x6D, 0x6F, 0x20, 0xF1, 0x6F, 0x73})
+	res = ToUTF8DropErrors([]byte{0x48, 0x6F, 0x6C, 0x61, 0x2C, 0x20, 0x61, 0x73, 0xED, 0x20, 0x63, 0xF3, 0x6D, 0x6F, 0x20, 0xF1, 0x6F, 0x73}, ConvertOpts{})
 	assert.Equal(t, []byte{0x48, 0x6F, 0x6C, 0x61, 0x2C, 0x20, 0x61, 0x73}, res[:8])
 	assert.Equal(t, []byte{0x73}, res[len(res)-1:])
 
 	// "Hola, así cómo "
 	minmatch := []byte{0x48, 0x6F, 0x6C, 0x61, 0x2C, 0x20, 0x61, 0x73, 0xC3, 0xAD, 0x20, 0x63, 0xC3, 0xB3, 0x6D, 0x6F, 0x20}
 
-	res = ToUTF8DropErrors([]byte{0x48, 0x6F, 0x6C, 0x61, 0x2C, 0x20, 0x61, 0x73, 0xED, 0x20, 0x63, 0xF3, 0x6D, 0x6F, 0x20, 0x07, 0xA4, 0x6F, 0x73})
+	res = ToUTF8DropErrors([]byte{0x48, 0x6F, 0x6C, 0x61, 0x2C, 0x20, 0x61, 0x73, 0xED, 0x20, 0x63, 0xF3, 0x6D, 0x6F, 0x20, 0x07, 0xA4, 0x6F, 0x73}, ConvertOpts{})
 	// Do not fail for differences in invalid cases, as the library might change the conversion criteria for those
 	assert.Equal(t, minmatch, res[0:len(minmatch)])
 
-	res = ToUTF8DropErrors([]byte{0x48, 0x6F, 0x6C, 0x61, 0x2C, 0x20, 0x61, 0x73, 0xED, 0x20, 0x63, 0xF3, 0x6D, 0x6F, 0x20, 0x81, 0xA4, 0x6F, 0x73})
+	res = ToUTF8DropErrors([]byte{0x48, 0x6F, 0x6C, 0x61, 0x2C, 0x20, 0x61, 0x73, 0xED, 0x20, 0x63, 0xF3, 0x6D, 0x6F, 0x20, 0x81, 0xA4, 0x6F, 0x73}, ConvertOpts{})
 	// Do not fail for differences in invalid cases, as the library might change the conversion criteria for those
 	assert.Equal(t, minmatch, res[0:len(minmatch)])
 
 	// Japanese (Shift-JIS)
 	// "日属秘ぞしちゅ。"
-	res = ToUTF8DropErrors([]byte{0x93, 0xFA, 0x91, 0xAE, 0x94, 0xE9, 0x82, 0xBC, 0x82, 0xB5, 0x82, 0xBF, 0x82, 0xE3, 0x81, 0x42})
+	res = ToUTF8DropErrors([]byte{0x93, 0xFA, 0x91, 0xAE, 0x94, 0xE9, 0x82, 0xBC, 0x82, 0xB5, 0x82, 0xBF, 0x82, 0xE3, 0x81, 0x42}, ConvertOpts{})
 	assert.Equal(t, []byte{
 		0xE6, 0x97, 0xA5, 0xE5, 0xB1, 0x9E, 0xE7, 0xA7, 0x98, 0xE3,
 		0x81, 0x9E, 0xE3, 0x81, 0x97, 0xE3, 0x81, 0xA1, 0xE3, 0x82, 0x85, 0xE3, 0x80, 0x82,
 	}, res)
 
-	res = ToUTF8DropErrors([]byte{0x00, 0x00, 0x00, 0x00})
+	res = ToUTF8DropErrors([]byte{0x00, 0x00, 0x00, 0x00}, ConvertOpts{})
 	assert.Equal(t, []byte{0x00, 0x00, 0x00, 0x00}, res)
 }
 
@@ -302,10 +241,6 @@ func stringMustEndWith(t *testing.T, expected, value string) {
 	assert.Equal(t, expected, value[len(value)-len(expected):])
 }
 
-func bytesMustStartWith(t *testing.T, expected, value []byte) {
-	assert.Equal(t, expected, value[:len(expected)])
-}
-
 func TestToUTF8WithFallbackReader(t *testing.T) {
 	resetDefaultCharsetsOrder()
 
@@ -317,7 +252,7 @@ func TestToUTF8WithFallbackReader(t *testing.T) {
 		}
 		input = input[:testLen]
 		input += "// Выключаем"
-		rd := ToUTF8WithFallbackReader(bytes.NewReader([]byte(input)))
+		rd := ToUTF8WithFallbackReader(bytes.NewReader([]byte(input)), ConvertOpts{})
 		r, _ := io.ReadAll(rd)
 		assert.EqualValuesf(t, input, string(r), "testing string len=%d", testLen)
 	}
diff --git a/modules/indexer/code/bleve/bleve.go b/modules/indexer/code/bleve/bleve.go
index 0bfd85cb3f..8ba50ed77c 100644
--- a/modules/indexer/code/bleve/bleve.go
+++ b/modules/indexer/code/bleve/bleve.go
@@ -174,7 +174,7 @@ func (b *Indexer) addUpdate(ctx context.Context, batchWriter git.WriteCloserErro
 	return batch.Index(id, &RepoIndexerData{
 		RepoID:    repo.ID,
 		CommitID:  commitSha,
-		Content:   string(charset.ToUTF8DropErrors(fileContents)),
+		Content:   string(charset.ToUTF8DropErrors(fileContents, charset.ConvertOpts{})),
 		Language:  analyze.GetCodeLanguage(update.Filename, fileContents),
 		UpdatedAt: time.Now().UTC(),
 	})
diff --git a/modules/indexer/code/elasticsearch/elasticsearch.go b/modules/indexer/code/elasticsearch/elasticsearch.go
index e7e3429a39..2fadbfeb06 100644
--- a/modules/indexer/code/elasticsearch/elasticsearch.go
+++ b/modules/indexer/code/elasticsearch/elasticsearch.go
@@ -135,7 +135,7 @@ func (b *Indexer) addUpdate(ctx context.Context, batchWriter git.WriteCloserErro
 			Id(id).
 			Doc(map[string]any{
 				"repo_id":    repo.ID,
-				"content":    string(charset.ToUTF8DropErrors(fileContents)),
+				"content":    string(charset.ToUTF8DropErrors(fileContents, charset.ConvertOpts{})),
 				"commit_id":  sha,
 				"language":   analyze.GetCodeLanguage(update.Filename, fileContents),
 				"updated_at": timeutil.TimeStampNow(),
diff --git a/routers/web/repo/commit.go b/routers/web/repo/commit.go
index abb39caa57..00157d44c9 100644
--- a/routers/web/repo/commit.go
+++ b/routers/web/repo/commit.go
@@ -384,7 +384,7 @@ func Diff(ctx *context.Context) {
 			Metas:   ctx.Repo.Repository.ComposeMetas(ctx),
 			GitRepo: ctx.Repo.GitRepo,
 			Ctx:     ctx,
-		}, template.HTMLEscapeString(string(charset.ToUTF8WithFallback(note.Message))))
+		}, template.HTMLEscapeString(string(charset.ToUTF8WithFallback(note.Message, charset.ConvertOpts{}))))
 		if err != nil {
 			ctx.ServerError("RenderCommitMessage", err)
 			return
diff --git a/routers/web/repo/compare.go b/routers/web/repo/compare.go
index 042b8ed692..5ae48e1ce8 100644
--- a/routers/web/repo/compare.go
+++ b/routers/web/repo/compare.go
@@ -142,7 +142,7 @@ func setCsvCompareContext(ctx *context.Context) {
 				return nil, nil, err
 			}
 
-			csvReader, err := csv_module.CreateReaderAndDetermineDelimiter(ctx, charset.ToUTF8WithFallbackReader(reader))
+			csvReader, err := csv_module.CreateReaderAndDetermineDelimiter(ctx, charset.ToUTF8WithFallbackReader(reader, charset.ConvertOpts{}))
 			return csvReader, reader, err
 		}
 
diff --git a/routers/web/repo/editor.go b/routers/web/repo/editor.go
index 5e7cd1caa3..85d40e7820 100644
--- a/routers/web/repo/editor.go
+++ b/routers/web/repo/editor.go
@@ -166,8 +166,8 @@ func editFile(ctx *context.Context, isNewFile bool) {
 		}
 
 		buf = append(buf, d...)
-		if content, err := charset.ToUTF8WithErr(buf); err != nil {
-			log.Error("ToUTF8WithErr: %v", err)
+		if content, err := charset.ToUTF8(buf, charset.ConvertOpts{KeepBOM: true}); err != nil {
+			log.Error("ToUTF8: %v", err)
 			ctx.Data["FileContent"] = string(buf)
 		} else {
 			ctx.Data["FileContent"] = content
diff --git a/routers/web/repo/render.go b/routers/web/repo/render.go
index f2c6ab3f8f..7eb5a42aa4 100644
--- a/routers/web/repo/render.go
+++ b/routers/web/repo/render.go
@@ -43,7 +43,7 @@ func RenderFile(ctx *context.Context) {
 	st := typesniffer.DetectContentType(buf)
 	isTextFile := st.IsText()
 
-	rd := charset.ToUTF8WithFallbackReader(io.MultiReader(bytes.NewReader(buf), dataRc))
+	rd := charset.ToUTF8WithFallbackReader(io.MultiReader(bytes.NewReader(buf), dataRc), charset.ConvertOpts{})
 
 	if markupType := markup.Type(blob.Name()); markupType == "" {
 		if isTextFile {
diff --git a/routers/web/repo/setting/lfs.go b/routers/web/repo/setting/lfs.go
index edf1298c20..cd0f11d548 100644
--- a/routers/web/repo/setting/lfs.go
+++ b/routers/web/repo/setting/lfs.go
@@ -303,7 +303,7 @@ func LFSFileGet(ctx *context.Context) {
 			break
 		}
 
-		rd := charset.ToUTF8WithFallbackReader(io.MultiReader(bytes.NewReader(buf), dataRc))
+		rd := charset.ToUTF8WithFallbackReader(io.MultiReader(bytes.NewReader(buf), dataRc), charset.ConvertOpts{})
 
 		// Building code view blocks with line number on server side.
 		escapedContent := &bytes.Buffer{}
diff --git a/routers/web/repo/view.go b/routers/web/repo/view.go
index 00fd47b650..af3021da11 100644
--- a/routers/web/repo/view.go
+++ b/routers/web/repo/view.go
@@ -303,7 +303,7 @@ func renderReadmeFile(ctx *context.Context, subfolder string, readmeFile *git.Tr
 		return
 	}
 
-	rd := charset.ToUTF8WithFallbackReader(io.MultiReader(bytes.NewReader(buf), dataRc))
+	rd := charset.ToUTF8WithFallbackReader(io.MultiReader(bytes.NewReader(buf), dataRc), charset.ConvertOpts{})
 
 	if markupType := markup.Type(readmeFile.Name()); markupType != "" {
 		ctx.Data["IsMarkup"] = true
@@ -492,7 +492,7 @@ func renderFile(ctx *context.Context, entry *git.TreeEntry) {
 			break
 		}
 
-		rd := charset.ToUTF8WithFallbackReader(io.MultiReader(bytes.NewReader(buf), dataRc))
+		rd := charset.ToUTF8WithFallbackReader(io.MultiReader(bytes.NewReader(buf), dataRc), charset.ConvertOpts{})
 
 		shouldRenderSource := ctx.FormString("display") == "source"
 		readmeExist := util.IsReadmeFileName(blob.Name())
diff --git a/templates/repo/editor/edit.tmpl b/templates/repo/editor/edit.tmpl
index 58ed6f356e..cfc266731b 100644
--- a/templates/repo/editor/edit.tmpl
+++ b/templates/repo/editor/edit.tmpl
@@ -38,8 +38,7 @@
 						data-url="{{.Repository.Link}}/markup"
 						data-context="{{.RepoLink}}"
 						data-previewable-extensions="{{.PreviewableExtensions}}"
-						data-line-wrap-extensions="{{.LineWrapExtensions}}">
-{{.FileContent}}</textarea>
+						data-line-wrap-extensions="{{.LineWrapExtensions}}">{{.FileContent}}</textarea>
 					<div class="editor-loading is-loading"></div>
 				</div>
 				<div class="ui bottom attached tab segment markup" data-tab="preview">
diff --git a/tests/integration/migration-test/migration_test.go b/tests/integration/migration-test/migration_test.go
index 556a54015b..40fcf95705 100644
--- a/tests/integration/migration-test/migration_test.go
+++ b/tests/integration/migration-test/migration_test.go
@@ -141,7 +141,7 @@ func readSQLFromFile(version string) (string, error) {
 	if err != nil {
 		return "", err
 	}
-	return string(charset.RemoveBOMIfPresent(bytes)), nil
+	return string(charset.MaybeRemoveBOM(bytes, charset.ConvertOpts{})), nil
 }
 
 func restoreOldDB(t *testing.T, version string) bool {
diff --git a/web_src/js/features/codeeditor.js b/web_src/js/features/codeeditor.js
index ae54268d56..fceb2f7620 100644
--- a/web_src/js/features/codeeditor.js
+++ b/web_src/js/features/codeeditor.js
@@ -114,7 +114,7 @@ export async function createMonaco(textarea, filename, editorOpts) {
 
   const model = editor.getModel();
   model.onDidChangeContent(() => {
-    textarea.value = editor.getValue();
+    textarea.value = editor.getValue({preserveBOM: true});
     textarea.dispatchEvent(new Event('change')); // seems to be needed for jquery-are-you-sure
   });