[Tcl-bugs] [ tcl-Bugs-2558422 ] const Tcl_ObjTypes breaking stubs contract.

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Bugs item #2558422, was opened at 2009-02-02 22:01
Message generated for change (Comment added) made by dkf
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Category: 10. Objects
Group: development: 8.6b1.1
Status: Open
Resolution: Accepted
Priority: 9
Private: No
Submitted By: Pat Thoyts (patthoyts)
Assigned to: Jan Nijtmans (nijtmans)
Summary: const Tcl_ObjTypes breaking stubs contract.

Initial Comment:
The change to the core Tcl_ObjTypes of 15 Oct 2008 has the return type for Tcl_GetObjType and the storage attribute for the core type structures to const.  As noted in the commit comment for some (all?) compilers this will move the storage location in the binary to the TEXT segment so that the structure is truly constant.

However, this change critically affects the following extensions: tcom (Windows COM support);  tcljava, tclblend, nsjava (all java bridges, possibly all variations on a single code base); combat (CORBA bridge); e4graph.

All these extensions use the same technique of overriding the cmdName type to add code to the freeIntRepProc member. These extensions now crash when loaded into an 8.6 interpreter.

The reason these extensions do this is to provide for command-like references to things that need cleaning up automatically. I could go on with an explanation of the COM package's use of this but really the main point is that this commit has changed the stubs contract here. Extensions that were fine using stubs with 8.4 and 8.5 now crash with 8.6.

Can we have this reverted:

C:\opt\tcl\src\tcl.git>git bisect good
63c56af1f8be10728f09ff1d066ec899977d9e16 is first bad commit
commit 63c56af1f8be10728f09ff1d066ec899977d9e16
Author: nijtmans <nijtmans>
Date:   Wed Oct 15 06:17:03 2008 +0000

    Add "const" to many internal
    const tables, so those will be
    put by the C-compiler in the
    TEXT segment in stead of the
    DATA segment. This makes those
    table sharable in shared libraries.



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>Comment By: Donal K. Fellows (dkf)
Date: 2009-02-03 18:57

Message:
I've applied the simplest possible fix: putting the structure in writable
memory (with suitable warning messages in comments). I've also added a
proper declaration in tclInt.h (since the structure is theoretically
accessed from two files anyway, though the code in tclProc.c is actually
commented out at the moment).

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Comment By: Jan Nijtmans (nijtmans)
Date: 2009-02-03 15:29

Message:
I did some tests with tclblend, against Tcl 8.6, and came
up with a patch which makes everything compile against
Tcl 8.6. See:
  [tcljava-Patches-2561078] Compile/Run against Tcl 8.6

The way TclBlend should have done it was create a new
object type which is a copy of the "cmdName" object
type but with different functions. Then every command
which needs java functionality can use this new
object type in stead of the original "cmdName" type.
[tcljava-Patches-2561078] doesn't do that yet, but
in stead simply silences the warning.

Expect this to be fixed in a few days.

Regards,
       Jan Nijtmans

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Comment By: Pat Thoyts (patthoyts)
Date: 2009-02-03 15:14

Message:
That's fine with me. I agree that this overriding of core types seems
somewhat evil. On the other hand, what else is Tcl_GetObjType for if not to
provide for this eventuality. Having dug around in google code a bit it
looks like the technique was pioneered by the Feather extension and
subsequently picked up by the other extensions listed. In the future it may
be worth trying to TIP some kind of official mechanism for this where a
limited lifetime command needs to be managed without requiring an explicit
delete call. This will have to be a Tcl 9 change though I think.

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Comment By: Jan Nijtmans (nijtmans)
Date: 2009-02-03 09:09

Message:
I agree that breaking a stubs contract is crucial, so I will revert
the const-ness of the internal Tcl_ObjTypes. However, changing
the freeInteRepProc function at runtime is clearly a hack the
core should warn about, that's why I prefer to keep the
signature of Tcl_GetObjtype as is. If extensions want to use
this hack, they can easily fix that with a type cast.

----------------------------------------------------------------------

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