This is a large change to how io_buffers are filled. The essential problem
comes about with code like (example):
echo ( /bin/pwd )
The output of /bin/pwd must go to fish, not the tty. To arrange for this,
fish does the following:
1. Invoke pipe() to create a pipe.
2. Add an io_bufferfill_t redirection that owns the write end of the pipe.
3. After fork (or equiv), call dup2() to replace pwd's stdout with this pipe.
Now when /bin/pwd writes, it will send output to the read end of the pipe.
But who reads it?
Prior to this fix, fish would do the following in a loop:
1. select() on the pipe with a 10 msec timeout
2. waitpid(WNOHANG) on the pwd proc
This polling is ugly and confusing and is what is replaced here.
With this new change, fish now reads from the pipe via a background thread:
1. Spawn a background pthread, which select()s on the pipe's read end with
a long (100 msec) timeout.
2. In the foreground, waitpid() (allowing hanging) on the pwd proc.
The big win here is a major simplification of job_t::continue_job() since
it no longer has to worry about filling buffers. This will make things
easier for concurrent execution.
It may not be obvious why the background thread still needs a poll (100 msec).
The answer is for cases where the write end of the fd escapes, in particular
background processes invoked inside command substitutions. psub is perhaps
the only important case of this (other shells typically just hang here).
This makes some significant architectual improvements to io_pipe_t and
io_buffer_t.
Prior to this fix, io_buffer_t subclassed io_pipe_t. io_buffer_t is now
replaced with a class io_bufferfill_t, which does not subclass pipe.
io_pipe_t no longer remembers both fds. Instead it has an autoclose_fd_t,
so that the file descriptor ownership is clear.
This switches IO redirections after fork() to use the dup2_list_t,
instead of io_chain_t. This results in simpler code with much simpler
error handling.
Prior to this fix, we would write to a fifo via cat >$filename & .
However in some cases (and soon in all cases) we open the file before
the fork, not after. This results in a deadlock because the file open
cannot succeed until a write begins.
Switch to using tee to write to the file. Because tee opens the file itself,
fish is no longer responsible and the deadlock is resolved.
This represents a "resolved" io_chain_t, where all of the different io_data_t
types have been reduced to a sequence of dup2() and close(). This will
eliminate a lot of the logic duplication around posix_spawn vs fork, and pave
the way for in-process redirections.
This is a large change to how io_buffers are filled. The essential problem
comes about with code like (example):
echo ( /bin/pwd )
The output of /bin/pwd must go to fish, not the tty. To arrange for this,
fish does the following:
1. Invoke pipe() to create a pipe.
2. Add an io_bufferfill_t redirection that owns the write end of the pipe.
3. After fork (or equiv), call dup2() to replace pwd's stdout with this pipe.
Now when /bin/pwd writes, it will send output to the read end of the pipe.
But who reads it?
Prior to this fix, fish would do the following in a loop:
1. select() on the pipe with a 10 msec timeout
2. waitpid(WNOHANG) on the pwd proc
This polling is ugly and confusing and is what is replaced here.
With this new change, fish now reads from the pipe via a background thread:
1. Spawn a background pthread, which select()s on the pipe's read end with
a long (100 msec) timeout.
2. In the foreground, waitpid() (allowing hanging) on the pwd proc.
The big win here is a major simplification of job_t::continue_job() since
it no longer has to worry about filling buffers. This will make things
easier for concurrent execution.
It may not be obvious why the background thread still needs a poll (100 msec).
The answer is for cases where the write end of the fd escapes, in particular
background processes invoked inside command substitutions. psub is perhaps
the only important case of this (other shells typically just hang here).
This makes some significant architectual improvements to io_pipe_t and
io_buffer_t.
Prior to this fix, io_buffer_t subclassed io_pipe_t. io_buffer_t is now
replaced with a class io_bufferfill_t, which does not subclass pipe.
io_pipe_t no longer remembers both fds. Instead it has an autoclose_fd_t,
so that the file descriptor ownership is clear.
This switches IO redirections after fork() to use the dup2_list_t,
instead of io_chain_t. This results in simpler code with much simpler
error handling.
Prior to this fix, we would write to a fifo via cat >$filename & .
However in some cases (and soon in all cases) we open the file before
the fork, not after. This results in a deadlock because the file open
cannot succeed until a write begins.
Switch to using tee to write to the file. Because tee opens the file itself,
fish is no longer responsible and the deadlock is resolved.
This represents a "resolved" io_chain_t, where all of the different io_data_t
types have been reduced to a sequence of dup2() and close(). This will
eliminate a lot of the logic duplication around posix_spawn vs fork, and pave
the way for in-process redirections.
By exclusively waiting by pgrp, we can fail to reap processes that
change their own pgrp then either crash or close their fds. If we wind
up in a situation where `waitpid(2)` returns 0 or ECHLD even though we
did not specify `WNOHANG` but we still have unreaped child processes,
wait on them by pid.
Closes#5596.
* brew.fish: Add `update-reset` subcommand
This command resets all tap's remotes to the latest available upstream. Ideal for debugging before reporting bugs or just housekeeping.
Add missing newlines.
* Add `brew.fish` changes to CHANGELOG.md
We were checking for the $TMUX variable to determine if we were
running under tmux. However when running the tests, the terminal becomes
expect, even though the TMUX variable is still set, so we spew tmux-isms
at expect. Check the value of $TERM for 'screen'.
Since fish began resolving symlinks it broke the running-from-build-dir
detection in fish.cpp if the build directory were a symlink (which is
common on some platforms where the default user HOME directory is a
symlink in the first place, e.g. FreeBSD).
