winpty injection

[Thomas Wolff <towo at towo dot net>]

The interworking problems of pty-based terminals with native Windows 
programs have long been discussed and raise recurring user reports and 
complaints. Mintty issues https://github.com/mintty/mintty/issues/56 
(Improve support for native console programs) and 
https://github.com/mintty/mintty/issues/376 (Use different encodings for 
native Windows commands output and remote output) have dozens of duplicates.
There is the fairly well-known wrapper winpty which solves most of the 
interworking problems. However, it needs to be involved explicitly and 
selectively for Windows command-line applications.

So I had the idea that cygwin could apply winpty wrapping automatically 
when it detects that a non-cygwin command-line program is being called. 
The attached patch demonstrates how this would work. It is a 
proof-of-concept, not ready for integration but provided for discussion 
of the approach (thus not sent to cygwin-patches).
If it is appreciated, there are a number of open issues:
* The patch offers two methods. The activated one (#ifdef 
inject_winpty_recursively) works but leaves a memory leak in the argv1 
array needed to expand the argv with the injected wrapper. The other 
one, which tries to inject winpty into argv and then continue the 
function linearly, would solve the memory leak but does not work (yet)...
* The winpty tool is not even a cygwin package yet. If the injection 
approach is approved, it should be packaged as a base package, or even 
its code could be migrated into cygwin.
* As some people may object such a magic (and it might in fact raise 
unexpected problems), it could also be made dependent on a setting in 
the CYGWIN environment variable.

Looking forward to feedback
Thomas

Attachment: 0000-winpty-injection-proof-of-concept.patch
Description: Text document