TXMLDocument.Active:＝ False导致FastMM4错误消息"FastMM在释放后检测到块已被修改". [英] TXMLDocument.Active := False causes FastMM4 errormessage "FastMM detected that a block has been modified after being freed"
问题描述
在FullDebugMode中使用带有FastMM4(版本4.92)的Delphi XE8会产生奇怪的效果.
we have a strange effect using Delphi XE8 with FastMM4 (Version 4.992) in FullDebugMode.
要重现效果，只需创建一个新的TForm应用程序，将FastMM4放在DPR文件的第一行， 在表单上放置一个按钮，然后在clickhandler中放置以下代码:
To reproduce the effect, just create a new TForm application, put FastMM4 in the first line of the DPR file, put a Button on the Form and put the following code in the 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！)
(You need to have FastMM 4 installed, FullDebugMode must be enabled in the FastMM4Options.inc file and the FullDebugMode.dll has to be in the output folder of your programm!)
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;
单击按钮时，FastMM4将显示带有以下报告的大消息框:
When you click the button, FastMM4 will show a large messagebox with the following report:
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$qqrpvt1][31407]
Current memory dump of 256 bytes starting at pointer address 7EA18B60:
98 72 5F 00 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80
80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80
80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 7F 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80
80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80
80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80
80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80
80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80
80 80 80 80 BD 4A 52 7A 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 00 00 00 00 00 00 00 00
˜ r _ . € € € € € € € € € € € € € € € € € € € € € € € € € € € €
€ € € € € € € € € € € € € € € € € € € € € € € € € € € € € € € €
€ € € € € € € € € € € € € € € € € € € € € € € € € € € € € € €
€ € € € € € € € € € € € € € € € € € € € € € € € € € € € € € € €
€ € € € € € € € € € € € € € € € € € € € € € € € € € € € € € € €
€ € € € € € € € € € € € € € € € € € € € € € € € € € € € € € € €
€ € € € € € € € € € € € € € € € € € € € € € € € € € € € € € € €
€ € € € ½ J R z € € € € € € € € € € € € € € € € . . . . . . . .
如果我们正确地解释了这一点，则FastMM会告诉我们TXMLDocument对象的内存 释放后已被修改.
If we interpret this correctly, then FastMM is telling us, that the Memory of the TXMLDocument Object has been modified, after it was freed.
某些代码"似乎已经在所有这些$ 80的中间写入了$ 7F到TXMLDocument的内存块中 已经被释放了.
"Some Code" seems to have written the $7F in the middle of all these $80 in the memoryblock of the TXMLDocument that has already been freed.
仅当两次访问ChildNodes(！)并且TXMLDocument的Active属性设置为false时，才会发生这种情况 在释放对象之前！
This does happen only if ChildNodes is accessed twice (!) and if the Active property of the TXMLDocument is set to false before the Object is freed!
问题:
有人可以解释这里发生了什么吗?
Can someone explain what is going on here?
-
是否将TXMLDocument.Active设置为false(通常被认为是错误的或错误的")? (是否会引起问题?)
is setting TXMLDocument.Active to false generally considered wrong or "bad"? (Is it known to cause Problems?)
我们在这里犯了其他错误吗?
are we making some other mistake here?
这在FastMM4中是个问题吗?
is this a problem in FastMM4?
这是TXMLDocument中的问题吗?
is this a problem in TXMLDocument?
其他观察结果:
如果在未将active设置为false的情况下释放了TXMLDocument，则没有问题.
If the TXMLDocument is freed without active being set to false, then there is no problem.
如果我们看一下TXMLDocument的析构函数，我们会发现之前还有一些其他代码 在此处将active设置为false:
If we look at the destructor of TXMLDocument we see that there is some additional code before active is set to false there:
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;
现在，如果我们修改自己的示例代码并调用
Now, if we modify our own examplecode and call
(FDocumentNode as IXMLNodeAccess).ClearDocumentRef;
在将active设置为false之前，问题就消失了！
before setting active to false, then the problem is gone!
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;
推荐答案
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其中.它们将一直存活到方法结束，然后被最终确定/清除.
When calling methods/functions that return interfaces, the compiler implicitly generates variables to put these results into. They live until the end of the method and are then being finalized/cleared.
在您的情况下，doc.DocumentElement.ChildNodes进行2个返回接口的方法调用.现在，当您销毁TXMLDocument实例时，这些隐式变量仍指向某些内存，并且由于在编译器生成的代码调用IntfClear时进行了_Release调用，因此它们将调用某些不再具有对象的方法-FastMM能够跟踪并报告.
In your case doc.DocumentElement.ChildNodes does 2 method calls that return interfaces. Now when you destroy the TXMLDocument instance these implicit variables still point to some memory and due to the _Release call being made when the compiler generated code calls IntfClear they call into some method that has no object anymore - FastMM is able to track and report that.
因此，上述通话被翻译为:
So this mentioned call is translated into this:
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
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没有再用于其他分配，这会导致异常的AV.
In many cases this error does not manifest without using FastMM unless the previously deallocated memory was not being reused for another allocation which then results in weird AVs.
经验法则:请勿破坏同一接口中某个接口引用的实例，因为这可能导致隐式创建的接口变量仍指向它们.
Rule of thumb: do not destroy instances that are referenced by some interface in the same scope as this might lead to implicitly created interface variables still pointing to them.
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