TMBCSEncoding with CP_UTF8 and MB_ERR_INVALID_CHARS decodes malformed UTF-8 instead of raising EEncodingError
## Summary
`TEncoding.UTF8.GetString` and `TMBCSEncoding.Create(CP_UTF8, MB_ERR_INVALID_CHARS, 0).GetString` appear to decode malformed UTF-8 input leniently instead of raising an encoding error.
In the repro below, the byte sequence `4C 75 C4` represents `Lu` followed by a lone UTF-8 lead byte `$C4`. This is malformed UTF-8 because `$C4` requires a continuation byte.
Delphi raises `EEncodingError` for both `TEncoding.UTF8` and `TMBCSEncoding` with `MB_ERR_INVALID_CHARS`.
Free Pascal instead decodes the invalid byte as `?` and returns successfully.
## System Information
* **Operating system:** Windows
* **Processor architecture:** i386-win32 / win64 target
* **Compiler version:** Lazarus 4.4 (rev `lazarus_4_4`), FPC 3.2.2 `i386-win32-win32/win64`
* **Device:** Computer
## Steps to reproduce
1. Save the following program as `Utf8StrictDecodeRepro.dpr`.
2. Compile with Free Pascal:
```bash
fpc -Mdelphi Utf8StrictDecodeRepro.dpr
```
3. Run the compiled executable.
4. Compare the output with Delphi, for example:
```bash
dcc32 Utf8StrictDecodeRepro.dpr
```
## Example Project
```pascal
program Utf8StrictDecodeRepro;
// Minimal repro: FPC leniently decodes lone UTF-8 lead byte $C4 (from CN=Lu\C4).
// Compile: fpc -Mdelphi Utf8StrictDecodeRepro.dpr
// dcc32 Utf8StrictDecodeRepro.dpr
{$APPTYPE CONSOLE}
{$IFDEF FPC}
{$MODE DELPHI}
{$ENDIF}
uses
SysUtils;
const
CP_UTF8 = 65001;
MB_ERR_INVALID_CHARS = 8;
function MakeLuC4Bytes: TBytes;
begin
// Bytes flushed from DN "CN=Lu\C4" (\C4 => $C4).
SetLength(Result, 3);
Result[0] := $4C; // 'L'
Result[1] := $75; // 'u'
Result[2] := $C4; // lone UTF-8 lead byte (malformed)
end;
procedure HexDump(const ABytes: TBytes);
var
I: Integer;
begin
for I := 0 to High(ABytes) do
begin
if I > 0 then
Write(' ');
Write(IntToHex(ABytes[I], 2));
end;
end;
procedure TryGetString(const ALabel: String; AEncoding: TEncoding;
const ABytes: TBytes);
var
S: String;
I: Integer;
begin
WriteLn(ALabel);
try
{$IFDEF FPC}
S := String(AEncoding.GetString(ABytes));
{$ELSE}
S := AEncoding.GetString(ABytes);
{$ENDIF}
WriteLn(' decoded OK');
WriteLn(' QuotedStr = ', QuotedStr(S));
WriteLn(' Length = ', Length(S));
for I := 1 to Length(S) do
WriteLn(' [', I, '] U+', IntToHex(Ord(S[I]), 4));
except
on E: Exception do
WriteLn(' ', E.ClassName, ': ', E.Message);
end;
end;
var
LBytes: TBytes;
LStrictUtf8: TEncoding;
begin
LBytes := MakeLuC4Bytes;
WriteLn('Malformed UTF-8 byte run (from CN=Lu\C4):');
Write(' ');
HexDump(LBytes);
WriteLn;
WriteLn;
{$IFDEF FPC}
WriteLn('Platform: Free Pascal');
{$ELSE}
WriteLn('Platform: Delphi');
{$ENDIF}
WriteLn;
TryGetString('1) TEncoding.UTF8', TEncoding.UTF8, LBytes);
WriteLn;
LStrictUtf8 := TMBCSEncoding.Create(CP_UTF8, MB_ERR_INVALID_CHARS, 0);
try
TryGetString('2) TMBCSEncoding.Create(CP_UTF8, MB_ERR_INVALID_CHARS, 0)',
LStrictUtf8, LBytes);
finally
LStrictUtf8.Free;
end;
WriteLn;
WriteLn('Delphi: both should throw EEncodingError.');
WriteLn('FPC: both may decode leniently (replacement char) instead of throwing.');
WriteLn;
Write('Press Enter to exit...');
ReadLn;
end.
```
## What is the current bug behavior?
Free Pascal decodes the malformed UTF-8 byte sequence successfully and substitutes the invalid byte with `?`.
FPC output:
```text
Malformed UTF-8 byte run (from CN=Lu\C4):
4C 75 C4
Platform: Free Pascal
1) TEncoding.UTF8
decoded OK
QuotedStr = 'Lu?'
Length = 3
[1] U+004C
[2] U+0075
[3] U+003F
2) TMBCSEncoding.Create(CP_UTF8, MB_ERR_INVALID_CHARS, 0)
decoded OK
QuotedStr = 'Lu?'
Length = 3
[1] U+004C
[2] U+0075
[3] U+003F
Delphi: both should throw EEncodingError.
FPC: both may decode leniently (replacement char) instead of throwing.
Press Enter to exit...
```
## What is the expected (correct) behavior?
When decoding malformed UTF-8, especially when using:
```pascal
TMBCSEncoding.Create(CP_UTF8, MB_ERR_INVALID_CHARS, 0)
```
Free Pascal should raise an encoding exception instead of silently replacing the invalid byte.
Delphi output for the same repro:
```text
Malformed UTF-8 byte run (from CN=Lu\C4):
4C 75 C4
Platform: Delphi
1) TEncoding.UTF8
EEncodingError: No mapping for the Unicode character exists in the target multi-byte code page
2) TMBCSEncoding.Create(CP_UTF8, MB_ERR_INVALID_CHARS, 0)
EEncodingError: No mapping for the Unicode character exists in the target multi-byte code page
Delphi: both should throw EEncodingError.
FPC: both may decode leniently (replacement char) instead of throwing.
Press Enter to exit...
```
## Relevant logs and/or screenshots
The important malformed byte sequence is:
```text
4C 75 C4
```
This is:
```text
4C = L
75 = u
C4 = lone UTF-8 lead byte, missing required continuation byte
```
So the input should not be accepted as valid UTF-8.
## Possible fixes
I am not sure where the exact fix belongs, but the behavior suggests that the UTF-8 decoding path used by `TEncoding.UTF8` / `TMBCSEncoding` may be ignoring `MB_ERR_INVALID_CHARS` or otherwise falling back to replacement-character behavior instead of strict error behavior.
Expected behavior would be to detect the malformed UTF-8 sequence and raise `EEncodingError` when strict decoding is requested.
issue
GitLab AI Context
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