If executing a command not at the end of a pipeline, and
not optimised in the calling code, we can fork before
"prefork" substitutions, at the same point as
for background commands.
Move fork before our preliminary scan of arguments to find
a command: in the cases of early fork
we don't need this information before forking.
Ensure we _exit if forked in execcmd_exec().
Rationalise use of forks and pipes.
Ensure we _exit instead of returning from execcmd_exec() if we
have forked. Before the optimisation code after the fork always ran
to the check at the end, but that code is overkill for the logic
between the early fork and the existing one.
Remove old workaround to fork in caller of execcmd for current shell
constructs as no longer needed with early fork below.
Close input of newly created pipe on fork (destined for RHS of pipe
which we never execute): this replaces a workaround from
zsh-workers/32171, commit 9887fc3d7b.
Set last1 on early fork as needed by some instances of shell
constructs on LHS of pipeline to know they are exiting.
This is too hard to make robust. Problems include
- Reading input in different places causes lines to appear and disappear
- There is no easy way to sequence the asynchronous operations
following bg.
- Response to job control of user commands on some OSes (MacOS,
for example) is unpredicatble.
Don't close associated file descriptors in the closem()
tidy up function as they should remain visible to external
processes. Override if about to exit.
Unit test for the failing case: note this relies on the
existence of /proc/self/fd or equivalent.
Only do this if killpg(dead_pid, 0) returns -1, indicating
the pgprp doesn't exist any more, else there is a race
if other proceses have started using it.
In execcmd the case of running the last command in a pipeline
asynchronously for the purpose of & and &! is easy to work out,
and we can avoid side effects and unnecessary execution time in
the parent shell by forking earlier.
If process group leader exits, allow a newly forked process to become
process leader. If a foreground job, reattach the shell to the
terminal until that happens.
Unblock signals when reading output for command subsitution so that
we can do this reattaching immediately.
