Thanks to Giuliano mentioning debug libraries I have been able to
duplicate the problem and to find the source of the bug - but it is as
weird as it gets.
FYI: tests done on Linux Mint 18 with Lazarus 1.8.0 and fpc 3.0.4.
The evidence so far:
1. Gabor's program ends with an exception when using the FPC release
RTL and FCL, but not when using RTL and FCL libraries compiled for
debugging.
2. If I use GDB to step through the final steps of the program, the bug
disappears!
3. If I add a delay at the end (e.g. a call to sleep(1)), the bug
disappears.
4. Playing around with code optimisations and debugging flags seems to
have no effect.
5. Using the age old technique of commenting out bits of the code until
the bug disappears, I tracked the problem down.
In Gabor's program, the bug is removed by commenting out this line
(shown in context)
for i:=0 to SQR.GetCount-1 do
begin
case SQR[i].getItemType of isc_info_svc_line:begin
// s:=SQR[i].getAsString; {This is where the problem starts}
end;
end;
if (s<>'') then WriteLn(s);
end;
Going deeper into the fbintf package, I have further tracked the problem
down to this line of code
SetString(Result,FBufPtr+3,len,CP_ACP);
FBufPtr is a pointer and it is fairly simple bit of code, copying a
string from a buffer (received from Firebird) into an AnsiString.
Commenting out the line stops the bug. The bug also goes away by
changing it to
var s: string;
i: integer;
...
Result := '';
SetString(s,FBufPtr+3,len,CP_ACP);
for i := 1 to length(s) do
Result += s[i];
which can only be described as a WTF moment. The problem has to be due
to string disposal.
Finally, knowing all this, I go back to Gabor's program and add the line
(again shown in context):
repeat
SQR:=SM.Query(nil,SRB);
for i:=0 to SQR.GetCount-1 do
begin
case SQR[i].getItemType of isc_info_svc_line:begin
s:=SQR[i].getAsString;
end;
end;
if (s<>'') then WriteLn(s);
s := ''; {Line
added here}
end;
until (s='');
and guess what - bug goes away.
It looks like what is happening is that SetString is setting an
AnsiString in such as away as to cause a problem when the memory manager
cleans up - but only as a race condition and if the string is not
cleaned up explicitly.
Looks like an FPC bug report to me.
On 09/03/18 09:14, Gabor Boros via Lazarus wrote:
Hi All,
The result of the attached example (which use MWA's Firebird Pascal
API) for me is an exception:
Gstat execution time Fri Mar 9 09:29:18 2018
Database header page information:
Flags 0
Generation 173
System Change Number 0
Page size 8192
ODS version 12.0
Oldest transaction 161
Oldest active 162
Oldest snapshot 162
Next transaction 164
Sequence number 0
Next attachment ID 27
Implementation HW=AMD/Intel/x64 little-endian
OS=Linux CC=gcc
Shadow count 0
Page buffers 0
Next header page 0
Database dialect 3
Creation date Feb 2, 2018 14:07:24
Attributes force write
Variable header data:
*END*
Gstat completion time Fri Mar 9 09:29:18 2018
Heap dump by heaptrc unit
355 memory blocks allocated : 948252/948432
355 memory blocks freed : 948252/948432
0 unfreed memory blocks : 0
True heap size : 131072
True free heap : 131072
An unhandled exception occurred at $00007FF97F0A3147:
EAccessViolation:
$00007FF97F0A3147
If comment out cthreads from the uses the exception is:
An unhandled exception occurred at $00007F21B71F5147:
EAccessViolation:
$00007F21B71F5147
$00007F21B71F751B
$00007F21B71F769C
$00007F21B746BFFF
If run with gdb (and cthreads):
Heap dump by heaptrc unit
355 memory blocks allocated : 948252/948432
355 memory blocks freed : 948252/948432
0 unfreed memory blocks : 0
True heap size : 131072
True free heap : 131072
[Thread 0x7ffff5991700 (LWP 2874) exited]
[Thread 0x7ffff6192700 (LWP 2873) exited]
[Thread 0x7ffff7fe3740 (LWP 2869) exited]
Cannot find user-level thread for LWP 2875: generic error
Any idea how to detect what/where is the source of the exception?
I use Linux 64bit, FPC 3.0.4 and Lazarus fixes_1_8.
Gabor
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