Detect truncated utf-8 characters at the end of content as still representing utf-8 (#19773)

Our character detection algorithm can potentially incorrectly detect utf-8 as iso-8859-x
if there is a truncated character at the end of the partially read file.

This PR changes the detection algorithm to truncated utf8 characters at the end of the
buffer.

Fix #19743

Signed-off-by: Andrew Thornton <art27@cantab.net>

2 files changed, 164 insertions, 1 deletions

diff --git a/modules/charset/charset.go b/modules/charset/charset.go
index cf8aa0cb75..89bbf1f8c9 100644
--- a/modules/charset/charset.go
+++ b/modules/charset/charset.go
@@ -131,7 +131,26 @@ func RemoveBOMIfPresent(content []byte) []byte {
 
 // DetectEncoding detect the encoding of content
 func DetectEncoding(content []byte) (string, error) {
-	if utf8.Valid(content) {
+	// First we check if the content represents valid utf8 content excepting a truncated character at the end.
+
+	// Now we could decode all the runes in turn but this is not necessarily the cheapest thing to do
+	// instead we walk backwards from the end to trim off a the incomplete character
+	toValidate := content
+	end := len(toValidate) - 1
+
+	if end < 0 {
+		// no-op
+	} else if toValidate[end]>>5 == 0b110 {
+		// Incomplete 1 byte extension e.g. © <c2><a9> which has been truncated to <c2>
+		toValidate = toValidate[:end]
+	} else if end > 0 && toValidate[end]>>6 == 0b10 && toValidate[end-1]>>4 == 0b1110 {
+		// Incomplete 2 byte extension e.g. ⛔ <e2><9b><94> which has been truncated to <e2><9b>
+		toValidate = toValidate[:end-1]
+	} else if end > 1 && toValidate[end]>>6 == 0b10 && toValidate[end-1]>>6 == 0b10 && toValidate[end-2]>>3 == 0b11110 {
+		// Incomplete 3 byte extension e.g. 💩 <f0><9f><92><a9> which has been truncated to <f0><9f><92>
+		toValidate = toValidate[:end-2]
+	}
+	if utf8.Valid(toValidate) {
 		log.Debug("Detected encoding: utf-8 (fast)")
 		return "UTF-8", nil
 	}
diff --git a/modules/charset/charset_test.go b/modules/charset/charset_test.go
index 8376a0698a..cfd5fb5696 100644
--- a/modules/charset/charset_test.go
+++ b/modules/charset/charset_test.go
@@ -5,6 +5,8 @@
 package charset
 
 import (
+	"bytes"
+	"io"
 	"strings"
 	"testing"
 
@@ -304,3 +306,145 @@ func stringMustEndWith(t *testing.T, expected, value string) {
 func bytesMustStartWith(t *testing.T, expected, value []byte) {
 	assert.Equal(t, expected, value[:len(expected)])
 }
+
+func TestToUTF8WithFallbackReader(t *testing.T) {
+	resetDefaultCharsetsOrder()
+
+	for testLen := 0; testLen < 2048; testLen++ {
+		pattern := "    test { () }\n"
+		input := ""
+		for len(input) < testLen {
+			input += pattern
+		}
+		input = input[:testLen]
+		input += "// Выключаем"
+		rd := ToUTF8WithFallbackReader(bytes.NewReader([]byte(input)))
+		r, _ := io.ReadAll(rd)
+		assert.EqualValuesf(t, input, string(r), "testing string len=%d", testLen)
+	}
+
+	truncatedOneByteExtension := failFastBytes
+	encoding, _ := DetectEncoding(truncatedOneByteExtension)
+	assert.Equal(t, "UTF-8", encoding)
+
+	truncatedTwoByteExtension := failFastBytes
+	truncatedTwoByteExtension[len(failFastBytes)-1] = 0x9b
+	truncatedTwoByteExtension[len(failFastBytes)-2] = 0xe2
+
+	encoding, _ = DetectEncoding(truncatedTwoByteExtension)
+	assert.Equal(t, "UTF-8", encoding)
+
+	truncatedThreeByteExtension := failFastBytes
+	truncatedThreeByteExtension[len(failFastBytes)-1] = 0x92
+	truncatedThreeByteExtension[len(failFastBytes)-2] = 0x9f
+	truncatedThreeByteExtension[len(failFastBytes)-3] = 0xf0
+
+	encoding, _ = DetectEncoding(truncatedThreeByteExtension)
+	assert.Equal(t, "UTF-8", encoding)
+}
+
+var failFastBytes = []byte{
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