[Tcl-bugs] [ tcl-Feature Requests-1428575 ] Revise Tcl_(Set|Get)ChannelError(Interp) interface

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Feature Requests item #1428575, was opened at 2006-02-09 14:39
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Category: 25. Channel System
Group: None
Status: Open
Resolution: None
Priority: 5
Submitted By: Don Porter (dgp)
Assigned to: Andreas Kupries (andreas_kupries)
Summary: Revise Tcl_(Set|Get)ChannelError(Interp) interface

Initial Comment:

TIP 219 confronted the difficult
problem that the Tcl_Channel extension
interface doesn't permit easy reporting
of error information in the usual Tcl
manner (code + msg + return options dictionary),
but has a history of returning only
a POSIX-style error code, a choice laid
down long ago, apparently inspired by
unix examples.

The TIP 219 solution was to provide
a pair of "bypass" areas where error
information could be stored and retrieved.
One area is attached to the Tcl_Interp;
the other within the ChannelState struct.
Each area has Get and Set accessors defined.

In either case, the "error information"
is passed into the "Set" accessor is
a single Tcl_Obj value.  The value is
documented to be a list made up of option
value pairs followed by a string message.
This format can be recognized as the
return options dictionary converted to a list
with the interp result lappended.

No public Tcl routine returns error information
in this format.  This forces the caller
of Tcl_SetChannelError(Interp) to do this
marshalling for themselves.  The one known
caller of these routines, Tcl's own
reflected channel support, indeed is forced
to define two routines MarshallError() and
UnmarshallError() to tend to this data formatting
task.

Seems to me all this complexity can be avoided
by just having the Set/Get accessors accept
two arguments, one for the interp result,
and one for the return options dictionary.

(I'm assuming the result code is not required,
since these interfaces are intended only for
the situation when the code is TCL_ERROR)

So the request is to revise these interfaces
to take two Tcl_Obj arguments instead of one.

In addition, the "safety precaution" described
in TIP 219, and implemented as FixLevelCode()
is, I believe, completely unnecessary.  The
-level and -code entries of the return options
dictionary only have effect for a result code
of TCL_RETURN.  Since these interfaces are 
intended only for result codes of TCL_ERROR,
there's no need to worry about them, assuming
both sides of handling the bypass areas enforce
the TCL_ERROR-only interpretation.

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