Possible bug in Lazarus 4.4?
lazarus / 21-General

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Alex.Machado · 2025-12-08 05:05

Hi there,

after building and installing IntraWeb packages in Lazarus 4.4 I
started receiving an exception when creating a new IntraWeb project
(it used to work fine in Laz 3.4, 3.6 and 3.8 ).

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i [Debugger
Exception Notification]Project Lazarus raised exception class
'EListError' with message:List index (9) out of bounds  At address
1000D8488 
The exception happens here (Line 11 below):

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i function
TCustomFormEditor.DescendFromDesignerBaseClass(AClass:
TComponentClass): integer;var  i: Integer;begin  Result:=-1;  for i:=0
to FDesignerBaseClasses.Count-1 do  begin    if
AClass.InheritsFrom(TClass(FDesignerBaseClasses[i])) then    begin   
 if (Result<0)          or (TClass(FDesignerBaseClassesCanCreateForm[i
]).InheritsFrom(TClass(FDesignerBaseClasses[Result]))) then       
Result:=i;    end;  end;end; 
This code contains a big code smell, IMO:

It access elements in 2 separate lists:

FDesignerBaseClasses
FDesignerBaseClassesCanCreateForm

But only one is guaranteed to be within bounds via the loop variable,
FDesignerBaseClasses list.
The other list, FDesignerBaseClassesCanCreateForm, is not. In the case
presented here, it definitely contain a different number of elements.

I checked the code of version 3.8 and it's correct:

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i function
TCustomFormEditor.DescendFromDesignerBaseClass(AClass:
TComponentClass): integer;begin 
Result:=FDesignerBaseClasses.Count-1;  while (Result>=0)  and (not
AClass.InheritsFrom(TClass(FDesignerBaseClasses[Result]))) do   
dec(Result);end;
This is the related call stack:

Code: [Select]#0 FPC_RAISEEXCEPTION at :0
#1 CLASSES$_$TFPLIST_$__$$_ERROR$ANSISTRING$INT64+97 at :0
#2 CLASSES$_$TFPLIST_$__$$_CHECKINDEX$LONGINT+40 at :0
#3 CLASSES$_$TFPLIST_$__$$_GET$LONGINT$$POINTER+20 at :0
#4 TCustomFormEditor.DescendFromDesignerBaseClass(TCustomFormEditor($0
0000000091A5A60), TComponentClass($000000000CDDED10)) at
customformeditor.pp:1902
#5 GetDsgnComponentBaseClassname(TClass($000000000CDDED10)) at
sourcefilemanager.pas:8472
#6 LoadLFM(TUnitInfo($000000000C0CB050),
TCodeBuffer($000000000ADC4D40), [], []) at sourcefilemanager.pas:6493
#7 NewFile(TProjectFileDescriptor($000000000916B850),
'z:\IWProject\unit1.pas', '', [nfIsPartOfProject, nfOpenInEditor,
nfSave, nfCreateDefaultSrc], TObject($0000000006CFC0B0)) at
sourcefilemanager.pas:2532
#8 TMainIDE.DoNewFile(TMainIDE($0000000006497C40),
TProjectFileDescriptor($000000000916B850), 'z:\IWProject\unit1.pas',
'', [nfIsPartOfProject, nfOpenInEditor, nfSave, nfCreateDefaultSrc],
nil) at main.pp:5823
#9
TLazIDEInterface.DoNewEditorFile(TLazIDEInterface($0000000006497C40),
TProjectFileDescriptor($000000000916B850), 'z:\IWProject\unit1.pas',
'', [nfIsPartOfProject, nfOpenInEditor, nfSave, nfCreateDefaultSrc])
at lazideintf.pas:824

As you can see, my code calls DoNewEditorFile() when creating a new
IntraWeb project and a long sequence of events will trigger the
exception.

Is this a known thing? What can be done in this case?

Thanks in advance

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Alex.Machado · 2025-12-08 06:21

May I give you a suggestion? If you think
FDesignerBaseClassesCanCreateForm must have the same number of
elements as FDesignerBaseClasses, add this to the code:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i  
Assert(FDesignerBaseClasses.Count =
FDesignerBaseClassesCanCreateForm.Count); 
Better yet using the ToString() method of each class to give a perfect
error message:

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i  
Assert(FDesignerBaseClasses.Count =
FDesignerBaseClassesCanCreateForm.Count, 'FDesignerBaseClasses: ' +
FDesignerBaseClasses.ToString + ', FDesignerBaseClassesCanCreateForm:
' + FDesignerBaseClassesCanCreateForm.ToString); 
Where ToString will evidently return the list content in a human
readable format.

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bytebites · 2025-12-08 09:01

Trunk-version has this code in ide/customformeditor.pp
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i function
TCustomFormEditor.DescendFromDesignerBaseClass(AClass:
TComponentClass): integer;var  i: Integer;begin  Result:=-1;  for i:=0
to FDesignerBaseClasses.Count-1 do  begin    if
AClass.InheritsFrom(TClass(FDesignerBaseClasses[i])) then    begin   
 if (Result<0)          or
(AClass.InheritsFrom(TClass(FDesignerBaseClasses[Result]))) then     
 Result:=i;    end;  end;end;

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Zvoni · 2025-12-08 09:08

Implicitly relying on short-circuiting the "or"?
Ouch!
I don't like that at all!
Especially as there is a "ticking timebomb" for the first iteration
(Result:=-1)

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Thaddy · 2025-12-08 09:23

It is enough to remove the or and evaluate both separately:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i   if
(Result<0 then continue;// iterate next in loop  if
(AClass.InheritsFrom(TClass(FDesignerBaseClasses[Result]))) then     
 Result:=i;// shouldn't that be exit(i) ????The second condition will
be skipped by continue.
The loop continues until there is a match or finished with -1.(no
match)
Someone plz check if my second hunch is correct, assuming continue is
used as I suggested:Code: Pascal  [Select][+][-]window.onload =
function(){var x1 = document.getElementById("main_content_section");
if (x1) { var x = document.getElementsByClassName("geshi");for (var i
= 0; i exit(i)On my current machine I do not have Lazarus available to
test.
As I read it, the logic is that you can exit the loop when result is
not -1.

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bytebites · 2025-12-08 09:37

Classic code snippet player

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Thaddy · 2025-12-08 09:43

Well, norrmally I don't do that... No lazarus present atm.
The logic is not good anyway says my brain compiler.You can ignore it
for now. (always continues)

Another snipper player  Code: Pascal  [Select][+][-]window.onload =
function(){var x1 = document.getElementById("main_content_section");
if (x1) { var x = document.getElementsByClassName("geshi");for (var i
= 0; i function TCustomFormEditor.DescendFromDesignerBaseClass(AClass:
TComponentClass): integer;var        i: Integer;begin       
Result:=-1;        for i:=0 to FDesignerBaseClasses.Count-1 do       
       if AClass.InheritsFrom(TClass(FDesignerBaseClasses[i])) then  
                    if
(AClass.InheritsFrom(TClass(FDesignerBaseClasses[i]))) then          
                            exit(i);end;???? Line 8 --> changed result
to i.

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Zvoni · 2025-12-08 09:51

This code is making no sense to me
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i function
TCustomFormEditor.DescendFromDesignerBaseClass(AClass:
TComponentClass): integer;var  i: Integer;begin  Result:=-1;  for i:=0
to FDesignerBaseClasses.Count-1 do  begin    if
AClass.InheritsFrom(TClass(FDesignerBaseClasses[i])) then    begin   
 if (Result<0)          or
(AClass.InheritsFrom(TClass(FDesignerBaseClasses[Result]))) then     
 Result:=i;    end;  end;end;
Execution only enters Line 9 to Line 13 if Line 8 returns True.
Meaning: We have already established that
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i
TClass(FDesignerBaseClasses[i])is an Ancestor of "AClass".

So, why are we checking again against
AClass.InheritsFrom(TClass(FDesignerBaseClasses[Result]))
??
*confused*
This would only make sense if "AClass" has multiple
Ancestors/Inheritance

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BrunoK · 2025-12-08 12:14

I interpret this method as
  Return the highest index in FDesignerBaseClasses[ i ] that is a
parent class of aClass. That is either a parent class or the aClass
index if in the list.

The index out of range seems curious because normally if Result is < 0
(not set yet), shortcut boolean evaluation should not, even if range
check on, use result = -1 value to index the FDesignerBaseClasses
list.

I suggest the OP Quote from: Alex.Machado on December 08, 2025,
06:05:45 am  tries the following code :
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i function
TCustomFormEditor.DescendFromDesignerBaseClass(AClass:
TComponentClass): integer;var  i: Integer;begin  Result:=-1;  for i:=0
to FDesignerBaseClasses.Count-1 do  begin    if
AClass.InheritsFrom(TClass(FDesignerBaseClasses[i])) then    begin   
 if (Result < 0) then        Result := i      else if
(AClass.InheritsFrom(TClass(FDesignerBaseClasses[Result]))) then     
 Result := i;    end;  end;end;Difficult to understand what the
developer intended when he modified the code.

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Zvoni · 2025-12-08 12:20

Quote from: BrunoK on December 08, 2025, 01:14:10 pmI interpret this
method as
  Return the highest index in FDesignerBaseClasses[ i ] that is a
parent class of aClass. That is either a parent class or the aClass
index if in the list.

The index out of range seems curious because normally if Result is < 0
(not set yet), shortcut boolean evaluation should not, even if range
check on, use result = -1 value to index the FDesignerBaseClasses
list.
Hmmm.... you mean like Running through the whole Inheritance-tree?

As regards to your Code:
Thaddy has a IMO nicer solution in Reply4 / First Code-Block
EDIT: Thaddy missed, that the "original" code also sets Result to i if
Result<0

So i don't understand the very first line "Result:=-1;"....
The Function-Result will never be "-1", since in the very first
iteration it checks "Result<0" (which shortcircuits the "or"-ed second
check), setting Result to 0 (First value of i)

What IS actually the purpose of this Function?

EDIT2: hmm.... i think i have understood the purpose of the
Function....
It tries to find the LAST Ancestor in that List
"FDesignerBaseClasses"...

Why not turn it around?
Proposal (untested)
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i    
function TCustomFormEditor.DescendFromDesignerBaseClass(AClass:
TComponentClass): integer;    var      i: Integer;    begin     
Result:=-1;      for i:=FDesignerBaseClasses.Count-1 Downto 0 do     
begin        if AClass.InheritsFrom(TClass(FDesignerBaseClasses[i]))
then Exit(i);              end;    end;

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Thaddy · 2025-12-08 12:55

@Zvoni
I believe my second example is correct (where I changed line 8  :
The result stays -1 unless a match is found.
I believe is that is how the loop should be interpreted.
It is also faster and it compiles.

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Zvoni · 2025-12-08 13:07

Quote from: Thaddy on December 08, 2025, 01:55:58 pm@Zvoni
I believe my second example is correct (where I changed line :
The result stays -1 unless a match is found.
I believe is that is how the loop should be interpreted.
It is also faster and it compiles.
Not sure.
Your code jumps out of the Function/returns on the first match in the
List.
The "original" (ignoring its weakness and/or convoluted way) and my
proposal return the "last" Entry which has a match.

EDIT: My comments and proposal are to Bytebites post Reply #2, quoting
the code from Trunk

But funnily enough...
QuoteI checked the code of version 3.8 and it's correct:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i    
function TCustomFormEditor.DescendFromDesignerBaseClass(AClass:
TComponentClass): integer;    begin     
Result:=FDesignerBaseClasses.Count-1;      while (Result>=0)      and
(not AClass.InheritsFrom(TClass(FDesignerBaseClasses[Result]))) do   
   dec(Result);    end;I would still be right, that it returns the
"Last match" (Highest Index) in the list, otherwise -1

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BrunoK · 2025-12-08 13:12

Quote from: Zvoni on December 08, 2025, 01:20:07 pmEDIT: Thaddy
missed, that the "original" code also sets Result to i if Result<0
Saw that at once ...

QuoteSo i don't understand the very first line "Result:=-1;"....
Obviously one would want to know that AClass was not in the list.

QuoteWhat IS actually the purpose of this Function?
I don't know in this context. I could imagine good reasons, like short
circuiting inheritance calls to virtual methods.

QuoteEDIT2: hmm.... i think i have understood the purpose of the
Function....
It tries to find the LAST Ancestor in that List
"FDesignerBaseClasses"...

Why not turn it around?
Before the new version it was like you suggest but the developer saw
things differently..

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Zvoni · 2025-12-08 13:29

Quote from: BrunoK on December 08, 2025, 02:12:59 pmBefore the new
version it was like you suggest but the developer saw things
differently..
Still not making any sense to me.

This is apparently the function in current trunk (See ByteBites post)
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i    
function TCustomFormEditor.DescendFromDesignerBaseClass(AClass:
TComponentClass): integer;    var      i: Integer;    begin     
Result:=-1;      for i:=0 to FDesignerBaseClasses.Count-1 do     
begin        if AClass.InheritsFrom(TClass(FDesignerBaseClasses[i]))
then        begin          if (Result<0)              or
(AClass.InheritsFrom(TClass(FDesignerBaseClasses[Result]))) then     
     Result:=i;        end;      end;    end;
For all intents and purposes let's say "FDesignerBaseClasses" has 12
Entries (From wherever those entries come from)
Result is "-1" from the get go
for i=0, 1, 2 Line 8 returns false (No Match), but on i = 3 there is a
Match
Since Result is -1 at that point, left side of the "or" short-circuits
(right side is ignored), and sets Result to 3
for i=4,5,6 there is no match again, but on i=7 there is a match
Since Result is now >0 (from iteration i=3) right side of the "or"
gets executed, but it checks if there is a match for "Result" (which
is "3" in our example) as Index of the List.
WHICH HAS ALREADY BEEN ESTABLISHED AS TRUE on the fourth iteration
(i=3), and sets the Result to the "current" i (which is "7").

Irrespective of any intermediary Results: The Function returns the
highest Index with a Match (Or -1 if no match at all).

Where is the difference to my Proposal or the Code in Lazarus 3.8?

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Thaddy · 2025-12-08 14:55

@Zvoni
Either that reversed example you gave or my second example was right.
The original code is wrong: it has no collector,
It must be either first or last or -1.
I still think my second example is right. The other tries make no
sense.
BTW my second example works and is now tested,
It ends up at the same index as the reverse you gave.
The other answers have a severe logic problem.
@Bytebites: you did not rebuild Lazarus, did you?

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Thaddy · 2025-12-08 15:14

Quote from: BrunoK on December 08, 2025, 02:12:59 pmQuote from: Zvoni
on December 08, 2025, 01:20:07 pmEDIT: Thaddy missed, that the
"original" code also sets Result to i if Result<0
Saw that at once ...


You saw that wrong..... My intention was to fix the logic and I did.
Simply recompile Lazarus for proof. The OR was incorrect and what
broke it.

If you do not recompile you will never see it is correct.
The reverse logic from Zvoni also compiles and renders the same
result.
The second error was passing implicitly -1 to the call, because -1 was
used as index where it should have been the index, not [result], but
i.

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BrunoK · 2025-12-08 15:53

Quote from: Thaddy on December 08, 2025, 04:14:42 pmQuote from: BrunoK
on December 08, 2025, 02:12:59 pmQuote from: Zvoni on December 08,
2025, 01:20:07 pmEDIT: Thaddy missed, that the "original" code also
sets Result to i if Result<0
Saw that at once ...


You saw that wrong..... My intention was to fix the logic and I did.
Simply recompile Lazarus for proof. The OR was incorrect and what
broke it.

You know, Thaddy, you edit all your posta to hide the erroneous code
you suggest. It's a constant behaviour of yours.

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Alex.Machado · 2025-12-08 19:29

Thank you all for your responses.

Quote from: Zvoni on December 08, 2025, 01:20:07 pm
What IS actually the purpose of this Function?


According to the history in github repo, the (faulty) method present
in 4.4 was introduced in order to:

Quoteide: fixed getting deepest designer base class
So I believe this is the purpose.

BTW, I rebuilt Lazarus 4.4 with the new function in trunk (as
mentioned by @Bytebites) and it works, or at least doesn't give me any
exceptions.

Cheers,