Apache Module: globlal variables revert to original state
lazarus / 38-Networking and Web Programming

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Tor · 2012-06-22 18:15

Hi,

I hope this is the right section of the Forum to ask this:

I started to write an apache module with freepascal, basically using 
http://wiki.freepascal.org/FPC_and_Apache_Modules.
I use some global variables to hold some information, but I noticed
that after 100 (or was it 101?)  accesses they revert to their
original state, which they had after the initialization of the module.

I would like to know
1. why?
and more importantly:
2. How do I prevent it?

It would be nice if someone could answer that for me (I didn't find
anything with google).
If needed I can provide a short example, which produces this effect.

Thanks in advance,
Tor

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Leledumbo · 2012-06-23 03:43

From here, global variables aren't thread safe. Try protecting any
write attempt to it in a critical section.

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Tor · 2012-06-23 08:55

Thanks for the information, but... it doesn't help.

Here my test code (based on the hello_world module) with
CriticalSection:

Code: [Select]library mod_hello;

{$ifdef fpc}
  {$mode objfpc}{$H+}
{$endif}

{$IFDEF WIN32}
  {$DEFINE WINDOWS}
{$ENDIF}

{$define Apache2_2}

uses SysUtils, httpd, apr;

var
 test_module: module; public name 'hello_module';
 str:String;
 b:boolean;
 mutex:TRTLCriticalSection;



const
  MODULE_NAME = 'mod_hello.so';

exports
 test_module name 'hello_module';


{*******************************************************************
*  Handles apache requests
*******************************************************************}
function DefaultHandler(r: Prequest_rec): Integer; cdecl;
var
  RequestedHandler:String;
begin
  RequestedHandler := r^.handler;

  { We decline to handle a request if hello-handler is not the value
of r->handler }
  if not SameText(RequestedHandler, 'testapache-handler') then
  begin
    Result := DECLINED;
    Exit;
  end;
  ap_set_content_type(r, 'text/html');
  Result := OK;


  if (r^.header_only <> 0) then
  begin
    Exit;
  end;

  if (b) then
   begin
     EnterCriticalsection(mutex);
     str:='bar';
     LeaveCriticalsection(mutex);
   end else
   begin
     EnterCriticalsection(mutex);
     b:=true;
     str:='FOOOOOOOOOOOOOOOOOOO!';
     LeaveCriticalsection(mutex);
   end;

   ap_rputs(PChar(str),r);

end;

{*******************************************************************
*  Registers the hooks
*******************************************************************}


procedure RegisterHooks(p: Papr_pool_t); cdecl;

begin

  ap_hook_handler(@DefaultHandler, nil, nil, APR_HOOK_MIDDLE);
end;

{*******************************************************************
*  Library initialization code
*******************************************************************}
var i:Integer;
begin
    b:=False;
    InitCriticalSection(mutex);
    STANDARD20_MODULE_STUFF(test_module);

    with test_module do
    begin
      name := MODULE_NAME;
      register_hooks := @RegisterHooks;
    end;


end. 

If I use this module I first see the foo and after a refresh the bar.
But if I refresh around 100 times or wait a while and then refresh, it
says foo again.

EDIT: I found out, that Apache has the (for me) nasty habit of giving
each thread its own copy of the global variables. There are some
methods to have shared global variables, but none of them are
currently availabe in FreePascal.
=> So I just write my own server with synapse....