chg: fallback to original hg if stdio fds are missing
If stdio fds are missing (ex. fd 0 is not present), chg might open
fds that take the numbers 0, and attachio would send the wrong fds
to the client, which might cause unwanted behaviors. Avoid that by
detecting the missing fds and falling back to the original hg.
Differential Revision: https://phab.mercurial-scm.org/D9058
$ cat <<EOF >> $HGRCPATH
> [extensions]
> hooklib =
>
> [phases]
> publish = False
> EOF
$ hg init a
$ hg --cwd a debugbuilddag '.:parent.*parent'
$ hg --cwd a log -G
o changeset: 2:fa942426a6fd
| tag: tip
| parent: 0:1ea73414a91b
| user: debugbuilddag
| date: Thu Jan 01 00:00:02 1970 +0000
| summary: r2
|
| o changeset: 1:66f7d451a68b
|/ user: debugbuilddag
| date: Thu Jan 01 00:00:01 1970 +0000
| summary: r1
|
o changeset: 0:1ea73414a91b
tag: parent
user: debugbuilddag
date: Thu Jan 01 00:00:00 1970 +0000
summary: r0
$ hg init b
$ cat <<EOF >> b/.hg/hgrc
> [hooks]
> pretxnclose.reject_new_heads = \
> python:hgext.hooklib.reject_new_heads.hook
> EOF
$ hg --cwd b pull ../a
pulling from ../a
requesting all changes
adding changesets
adding manifests
adding file changes
error: pretxnclose.reject_new_heads hook failed: Changes on branch 'default' resulted in multiple heads
transaction abort!
rollback completed
abort: Changes on branch 'default' resulted in multiple heads
[255]
$ hg --cwd b pull ../a -r 1ea73414a91b
pulling from ../a
adding changesets
adding manifests
adding file changes
added 1 changesets with 0 changes to 0 files
new changesets 1ea73414a91b (1 drafts)
(run 'hg update' to get a working copy)