With Tcl 8.4's vfs, all filesystem access except TclpCreateProcess (and a few related helper functions) is hooked into a lookup table.  This means that all filesystem operations visible at the Tcl or C level can operate on a vfs, with the exception of 'exec' and 'open |cmd'.

It would be nice to be able to provide a more general mechanism (*) to such command pipelines, especially given that stdout/stderr/stdin are already nicely abstracted as Tcl_Channels.

The stumbling block appears to be that all the 'pipe' related code doesn't actually use Tcl_Channels.  Instead it uses a new 'TclFile' structure which is a lightweight wrapper around a native file-descriptor (and is slightly different on Windows vs Unix).  Then there are a whole suite of new functions TclpOpenFile, TclpCloseFile, TclpMakeFile, TclpReleaseFile which are there just to operate on those structures.  There is significant code-overlap between these TclFile-specific functions and the more general Tcl_Channel code.

Does anyone know any reason why all those 'TclFile' structures couldn't simply be removed and replaced with Tcl_Channels?  If this were done, all of the 'exec' baggage and associated APIs could be made rather simpler and potentially exposed to the Tcl level through the [tclvfs] extension.

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''*'' Obvious benefits are:

   * easier to write things like BLT's 'bgexec' and generally interact with pipelines from within Tcl
   * allow implementation of pipeline components from within Tcl (e.g. pipe output from one application through a Tcl proc and into another application).
   * force ''all'' native filesystem access through the 'Tcl_FS' lookup tables, and thereby make it very easy to compile a version of Tcl which can't access the native filesystem (by removing the 'nativeFilesystem' lookup table).

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[AK]: I don't know the true reason, but speculate that the implementation of command pipelines came before channels and there just left as they initially implemented. A problem might be how to implement the OS unnamed pipes between the processes in the pipeline without burdening tcl with doing this.