为什么我会在 EOutOfResources 中泄漏内存？

Question

我尝试围绕 this jpeg 解码器库（Arnaud Bouchez 原创）实现一个包装器。该库非常快，但它不支持所有 jpeg！

对于非常大的 jpg 文件，它会失败（正如预期的那样）并出现 EOutOfResources 异常。
所以我试着默默地跳过这些文件。它可以工作，但是当我关闭应用程序时，FastMM 指示内存泄漏。

function FastJpgDecode(FileName: string; OUT ErrorType: string): TBitmap;
var Img: PJpegDecode;
    res: TJpegDecodeError;
    Stream: TMemoryStream;
begin
  Result:= NIL;
  Stream:= TMemoryStream.Create;
  TRY
    if Length(FileName) > MAX_PATH then { TMemoryStream does not support long paths }
     begin
      ErrorType:= 'File name too long!';
      Exit;
     end;

    Stream.LoadFromFile(FileName);
    Stream.Position:= 0;
    res:= JpegDecode(Stream.Memory, Stream.Size, Img);       
    case res of
     JPEG_SUCCESS:
      begin
       try
        Result:= Img.ToBitmap; // This will raise an EOutOfResources for large files!
       except
        on EOutOfResources do
          ErrorType:= 'JPEG_OUTOFMEM!';
       end;
      end;

     JPEG_EOF                : ErrorType:= 'JPEG_EOF!';
     JPEG_OUTOFMEM           : ErrorType:= 'JPEG_OUTOFMEM!';
     JPEG_CPUNOTSUPPORTED    : ErrorType:= 'JPEG_CPUNOTSUPPORTED!';
     JPEG_BADFILE            : ErrorType:= 'JPEG_BADFILE!';
     JPEG_FORMATNOTSUPPORTED : ErrorType:= 'JPEG_FORMATNOTSUPPORTED!';       // Not all jpegs are supported. In this case we fall back to WIC or the standard LoadGraph loader (WIC).   
    end;
  FINALLY
    Img.Free;
    Stream.Free;
  END;
end;



function TJpegDecode.ToBitmap: TBitmap;
begin
  if @self=nil
  then result := nil
  else
   begin
    result := TBitmap.Create;
    try
     if not ToBitmap(result)   // This will raise an EOutOfResources for large files!
     then FreeAndNil(result);
    except
      FreeAndNil(Result);
      raise;
    end;
   end;
end;

A memory block has been leaked. The size is: 36

This block was allocated by thread 0xD0C, and the stack trace (return addresses) at the time was: 407246 40830F 408ADE 43231B [Unknown function at __dbk_fcall_wrapper] 407246 40A532 53C353 [Unknown function at TMethodImplementationIntercept] 6E006F [Unknown function at TMethodImplementationIntercept] 7765648F [RtlNtStatusToDosError] 77656494 [RtlNtStatusToDosError] 767A7BEA [Unknown function at IsNLSDefinedString]

The block is currently used for an object of class: EOutOfResources The allocation number is: 4181

Current memory dump of 256 bytes starting at pointer address 7EEEA6C0: 74 7F............ t D . ü $ ú ......

A memory block has been leaked. The size is: 132
This block was allocated by thread 0xD0C, and the stack trace (return addresses) at the time was: 407246 40A2E7 40A518 53C341 [Unknown function at TMethodImplementationIntercept] 6E006F [Unknown function at TMethodImplementationIntercept] 7765648F [RtlNtStatusToDosError] 77656494 [RtlNtStatusToDosError] 767A7BEA [Unknown function at IsNLSDefinedString] 7677F0BA [VirtualQueryEx] 7677F177 [VirtualQuery] 898FD9 [GetFrameBasedStackTrace]

The block is currently used for an object of class: UnicodeString

The allocation number is: 4180

Current memory dump of 256 bytes starting at pointer address 7EFA24F0: B0 04 02 00 01 00 00 00......... . . . . . . . : . . . Not . . enough storage . . i . s . . a . v . a . i . l . a . b . l . e . . t . o . . p . r . o . c . e . s . s . . t . h . i . s . . c . o . m . m . a . n . d ............

This application has leaked memory. The small block leaks are (excluding expected leaks registered by pointer):

21 - 36 bytes: EOutOfResources x 1 117 - 132 bytes: UnicodeString x 1

为什么会在那里泄漏内存？

Answer 1

正如@kami 在评论中提到的，EHeapException 有一个默认为 False 的内部 AllowFree 标志，防止 EHeapException 的实例被异常处理程序释放。

EOutOfResources 派生自 EOutOfMemory，后者又派生自 EHeapException.

SysUtils 单元有 2 个类型为 EOutOfMemory 和 EInvalidPointer 的单例对象。每当 RTL 直接引发这两种特定异常类型时，它每次都会引发那些类的 相同实例 。所以他们有一个 AllowFree 标志来防止异常处理程序释放单例。当 SysUtils 单元完成时，单例被释放。

这实际上是记录在案的行为：

http://docwiki.embarcadero.com/Libraries/en/System.SysUtils.EHeapException

Note: Memory for these exceptions is pre-allocated whenever an application starts and remains allocated as long as the application is running. Never raise EHeapException or its descendants directly.

http://docwiki.embarcadero.com/Libraries/en/System.SysUtils.EOutOfMemory

Memory for the EOutOfMemory exception is pre-allocated whenever an application starts and remains allocated as long as the application is running.

Note: Never raise EOutOfMemory directly. Instead, call the global OutOfMemoryError procedure.

然而，尽管 EOutOfResources 派生自 EHeapException，但它从未以单例方式使用，因此它的 AllowFree 标志确实不应该为 False。所以在我看来这里有几个错误：

EOutOfResources 并不是真正的堆错误，不应该从 EHeapException 开始。它实际上是一个常见的异常，例如 Vcl.Graphics 单元为其一些 GDI 错误引发 EOutOfResources，这与堆无关。
EOutOfResources 将其 AllowFree 标志设置为 False，而它应该是 True。标志是 private，所以它不能被覆盖，除非 SysUtils 单元，它只在完成期间对 2 个单例进行覆盖。所以基本上，所有 EHeapException派生的异常都会在运行时泄漏。
当 AllowFree 为 False 时，单例以及所有其他后代实例不会传递给 RTL 的 RegisterExpectedMemoryLeak() 函数，因此可以从泄漏报告中省略它们。

此泄漏问题自 Delphi 5 以来就存在，并且已报告给 Embarcadero：

RSP-17193: EOutOfResources memory leak

RSP-19737: EOutOfResources exception causes memory leak

为什么我会在 EOutOfResources 中泄漏内存？

Why do I leak memory in EOutOfResources?

delphi

jpeg

delphi-10.3-rio