Detect truncated utf-8 characters at the end of content as still representing utf...

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>

diff --git a/modules/charset/charset.go b/modules/charset/charset.go
index cf8aa0cb75e67379c21c706965a0e97be63ad68d..89bbf1f8c93a811eb0f7522b67091a7695fdb24c 100644 (file)
--- 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 8376a0698af8ae07b99e312f8187942c931abc9b..cfd5fb569663c4e566814a7369b20440f11c4bfe 100644 (file)
--- 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{
+       0x69, 0x6d, 0x70, 0x6f, 0x72, 0x74, 0x20, 0x6f, 0x72, 0x67, 0x2e, 0x61, 0x70, 0x61, 0x63, 0x68, 0x65, 0x2e, 0x74, 0x6f,
+       0x6f, 0x6c, 0x73, 0x2e, 0x61, 0x6e, 0x74, 0x2e, 0x74, 0x61, 0x73, 0x6b, 0x64, 0x65, 0x66, 0x73, 0x2e, 0x63, 0x6f, 0x6e,
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