TXMLDocument.Active:= False导致FastMM4错误消息"FastMM在被释放后检测到块已被修改"

Question

TXMLDocument.Active:= False导致FastMM4错误消息"FastMM在被释放后检测到块已被修改"

use*_*897 2 delphi fastmm txmldocument memory-corruption

在FullDebugMode中使用带有FastMM4(版本4.992)的Delphi XE8会产生奇怪的效果.

要重现效果,只需要创建一个新的TForm的应用程序,把FastMM4在DPR文件的第一行,放一个按钮在窗体上,并把下面的代码在clickHandler:

(您需要安装FastMM 4,必须在FastMM4Options.inc文件中启用FullDebugMode,并且FullDebugMode.dll必须位于程序的输出文件夹中!)

procedure TForm3.Button4Click(Sender: TObject);
var
    dm: tdatamodule;
    doc: txmldocument;
begin
    //this causes the messagebox to be shown directly after clicking the button. 
    //without it the box is shown when the program is exited.
    FastMM4.FullDebugModeScanMemoryPoolBeforeEveryOperation := true;        

    //prepare a xml document
    dm := tdatamodule.create(nil);
    doc := txmldocument.create(dm);
    doc.LoadFromXml('<?xml version="1.0" encoding="ISO-8859-1" standalone="yes"?><root/>');

    //doc.Active := true;   //no need to set active to true. doc is already active at this point!

    //the following does matter. ChildNodes needs to be accessed twice!
    //doc.DocumentElement.ChildNodes.FindNode('first');
    //doc.DocumentElement.ChildNodes.FindNode('second');

    //alternative access:
    doc.DocumentElement.ChildNodes.count;                                       //first access to ChildNodes
    doc.DocumentElement.ChildNodes.count;                                       //second access to ChildNodes

    //this does matter. if doc stays active then there is no problem!
    doc.Active := false;                                                        //active needs to be set to false!

    //doc.free; //doesn't matter if doc is freed manually. doc will be freed when datamodule is freed. problem occurs either way.
    dm.free;
end;

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单击该按钮时,FastMM4将显示一个包含以下报告的大型消息框:

FastMM has detected an error during a free block scan operation. FastMM detected that a block has been modified after being freed. 

Modified byte offsets (and lengths): 80(1)

The previous block size was: 228

This block was previously allocated by thread 0xC74, and the stack trace (return addresses) at the time was:
406C46 [System.pas][System][@GetMem$qqri][4565]
407A73 [System.pas][System][TObject.NewInstance$qqrv][15975]
5DF25D [Xml.XMLDoc.pas][Xml.XMLDoc][Xmldoc.TXMLDocument.NewInstance$qqrv][2368]
408202 [System.pas][System][@ClassCreate$qqrpvzc][17290]
5DF116 [Xml.XMLDoc.pas][Xml.XMLDoc][Xmldoc.TXMLDocument.$bctr$qqrp25System.Classes.TComponent][2344]
5E2094 [Unit3.pas][Unit3][TForm3.Button4Click$qqrp14System.TObject][47]
407E6F [System.pas][System][@IsClass$qqrxp14System.TObjectp17System.TMetaClass][16465]
51EB39 [Vcl.Controls.pas][Vcl.Controls][Controls.TControl.Click$qqrv][7361]
535CF3 [Vcl.StdCtrls.pas][Vcl.StdCtrls][Stdctrls.TCustomButton.Click$qqrv][5327]
536801 [Vcl.StdCtrls.pas][Vcl.StdCtrls][Stdctrls.TCustomButton.CNCommand$qqrr26Winapi.Messages.TWMCommand][5788]
51E5C8 [Vcl.Controls.pas][Vcl.Controls][Controls.TControl.WndProc$qqrr24Winapi.Messages.TMessage][7245]

The block was previously used for an object of class: TXMLDocument

The allocation number was: 650

The block was previously freed by thread 0xC74, and the stack trace (return addresses) at the time was:
406C62 [System.pas][System][@FreeMem$qqrpv][4613]
407A91 [System.pas][System][TObject.FreeInstance$qqrv][15984]
40824D [System.pas][System][@ClassDestroy$qqrxp14System.TObject][17333]
5DF241 [Xml.XMLDoc.pas][Xml.XMLDoc][Xmldoc.TXMLDocument.$bdtr$qqrv][2363]
4C3661 [System.Classes.pas][System.Classes][Classes.TComponent.DestroyComponents$qqrv][15648]
4C3100 [System.Classes.pas][System.Classes][Classes.TComponent.$bdtr$qqrv][15445]
4C5543 [System.Classes.pas][System.Classes][Classes.TDataModule.$bdtr$qqrv][16694]
407B97 [System.pas][System][TObject.Free$qqrv][16052]
5E20F2 [Unit3.pas][Unit3][TForm3.Button4Click$qqrp14System.TObject][63]
51EB39 [Vcl.Controls.pas][Vcl.Controls][Controls.TControl.Click$qqrv][7361]
535CF3 [Vcl.StdCtrls.pas][Vcl.StdCtrls][Stdctrls.TCustomButton.Click$qqrv][5327]

The current thread ID is 0xC74, and the stack trace (return addresses) leading to this error is:
416526 [fastmm4.pas][FastMM4][InternalScanMemoryPool$qqruiui][10018]
416601 [fastmm4.pas][FastMM4][ScanMemoryPoolForCorruptions$qqrv][10092]
4161DF [fastmm4.pas][FastMM4][DebugFreeMem$qqrpv][9761]
406C62 [System.pas][System][@FreeMem$qqrpv][4613]
407A91 [System.pas][System][TObject.FreeInstance$qqrv][15984]
40824D [System.pas][System][@ClassDestroy$qqrxp14System.TObject][17333]
407B8A [System.pas][System][TObject.$bdtr$qqrv][16044]
40D5DD [System.pas][System][TInterfacedObject._Release$qqsv][37311]
40D4B7 [System.pas][System][@IntfClear$qqrr44System.%DelphiInterface$17System.IInterface%][36327]
40B189 [System.pas][System][@FinalizeArray$qqrpvt1ui][31704]
40B079 [System.pas][System][@FinalizeRecord$qqrpvt1][31407]

Current memory dump of 256 bytes starting at pointer address 7EA18B60:
98 72 5F 00 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80
80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80
80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 7F 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80
80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80
80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80
80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80
80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80
80 80 80 80 BD 4A 52 7A 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 00 00 00 00 00 00 00 00
˜  r  _  .  €  €  €  €  €  €  €  €  €  €  €  €  €  €  €  €  €  €  €  €  €  €  €  €  €  €  €  €
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如果我们正确地解释了这一点,那么FastMM告诉我们,TXMLDocument对象的内存在被释放后已被修改.

"Some Code"似乎已经在已经释放的TXMLDocument的内存块中的所有这80美元中间写了$ 7F.

只有在ChildNodes被访问两次(!)并且在释放Object之前将TXMLDocument的Active属性设置为false时,才会发生这种情况!

问题:

有人能解释一下这里发生了什么吗？

将TXMLDocument.Active设置为false通常被视为错误或"坏"？(它是否会引起问题？)
我们在这里犯了其他错误吗？
这是FastMM4中的一个问题吗？
这是TXMLDocument中的问题吗？

补充观察:

如果释放TXMLDocument而没有将active设置为false,则没有问题.

如果我们查看TXMLDocument的析构函数,我们会看到在active设置为false之前还有一些额外的代码:

destructor TXMLDocument.Destroy;
begin
  Destroying;
  if FOwnerIsComponent and Active and Assigned(FDocumentNode) and (FRefCount > 1) then      //additional code
    (FDocumentNode as IXMLNodeAccess).ClearDocumentRef;                                     //additional code
  SetActive(False);
  FreeAndNil(FXMLStrings);
  inherited;
end;

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现在,如果我们修改自己的examplecode并调用

(FDocumentNode as IXMLNodeAccess).ClearDocumentRef;

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在将active设置为false之前,问题就消失了!

代码看起来像这样:

type
  TMyXMLDocument = class(TXMLDocument);

procedure TForm3.Button4Click(Sender: TObject);
var
    dm: tdatamodule;
    doc: txmldocument;
begin
    FastMM4.FullDebugModeScanMemoryPoolBeforeEveryOperation := true;

    dm := tdatamodule.create(nil);
    doc := txmldocument.create(dm);
    doc.LoadFromXml('<?xml version="1.0" encoding="ISO-8859-1" standalone="yes"?><root/>');

    doc.DocumentElement.ChildNodes.count;
    doc.DocumentElement.ChildNodes.count;

    (TMyXMLDocument(doc).GetDocumentElement as IXMLNodeAccess).ClearDocumentRef;  //<-- with this  additional hack the problem is gone!

    doc.Active := false;                                                        //no more problem!

    dm.free;
end;

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Answer 1

Ste*_*nke 5

在调用返回接口的方法/函数时,编译器会隐式生成变量以将这些结果放入.它们一直存在到方法结束,然后被最终确定/清除.

在您的情况下doc.DocumentElement.ChildNodes,2个方法调用返回接口.现在当你破坏TXMLDocument实例时,这些隐式变量仍然指向一些内存,并且由于在编译器生成代码调用时调用了_Release,IntfClear它们调用了一些没有对象的方法 - FastMM能够跟踪和报告它.

所以这个提到的电话被翻译成:

var
  ...
  nodes1: IXMLNodeList;
  node1: IXMLNode;
  nodes2: IXMLNodeList;
  node2: IXMLNode;
begin
  ...

  node1 := doc.DocumentElement;
  nodes1 := node1.ChildNodes;
  nodes1.count;
  node2 := doc.DocumentElement;
  nodes2 := node2.ChildNodes;
  nodes2.count;

  dm.free;

  nodes1 := nil;
  node1 := nil;
  nodes2 := nil;
  node2 := nil; // <- boom

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在许多情况下,如果不使用FastMM,则不会显示此错误,除非先前释放的内存未被重用于另一个分配,然后导致奇怪的AV.

经验法则:不要销毁同一范围内某些接口引用的实例,因为这可能会导致隐式创建的接口变量仍然指向它们.

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