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Removes the custom errors in src/recite/erro.rs, and replaces them with
std::io::Errors throughout (recite(), incant_, macros).
Fixed a bug with the way forking to the background is handled, where
registering the signal handler in main for all processes would break
couplets (i.e. pipes). Instead, this sets up a new signal handler each
time a process is forked into the background. It uses a Vec<i32> to keep
track of all the background processes.
Notes:
First off, there is some defunct code in the main repl loop, which is an
example of killing zombie processes after each prompt. This should be
removed, but I kept it in, just in case I go back to it for some reason.
To be honest, I have no clue why this code works. In theory, I should
have to remove the pid from the pids: Vec<i32> if waitpid returns a
positive integer. However, when I tried this, it completely broke the
program. ¯\_(ツ)_/¯
Also, it's worth noting that registering a signal handler with
signal_hook::low_level::register, is somewhat costly, according to their
docs. Given that this only occurs for background processes that are
forked, however, I think it is acceptable.
Finally, we never unregister the signal handler, so I'm not sure if
that's still hanging out in memory somewhere or no.
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First off, moved the giant match statements out of recite(), and into
macros in src/recite/ps.rs. There still needs to be two, since any verse
using the 'couplet' meter will need to redirect its STDOUT. Now the
recite() function returns a Result<(), Mishap>, which can be invoked
when calling the incant_ functions.
Custom errors were added in the form of 'Mishap''s. They are intended to
be returned from the incant_ functions, in the event that something goes
wrong with the Command::spawn() or Child::wait(). They each take a
String, which should be the verb or stanza that was entered by the user.
The incant_ functions separate the functionality of each type of meter
from the recite() function. They return a Result<i32, Mishap>, where
i32 is the exit code of the program that ran, and Mishap is a possible
error.
Before, the shell was cheating at forking a process to the background.
It would actually spawn a thread to wait for that process to finish.
Now, the program simply registers a handler for SIGCHLD, and uses libc's
waitpid() function to reap the child process, and print some output to
the user, indicating that it's finished.
Notes:
This was a huge patch which did some desperately needed cleanup of the
recite() function. Moving forward, will need to add more documentation,
and will probably scrap the custom errors, since this implementation is
a little half-baked. It's worth looking into in the future, but we can
probably live with io::Error's for the time being.
Fixing forking was a pretty big deal, though. In Linux, and other
u**x-like operating systems, parent processes need to reap their child
processes, otherwise they become zombies. Previously, the dwvsh did this
by spawning a separate thread to wait for child processes that were
forked to the background. Now, we are registering a handle for SIGCHLD,
which is a signal that gets sent to the parent when one of their
children finishes, or is killed. Using waitpid(2), we can determine
which process ended, and do something about it. In the case of a
processes that was forked into the background, when it finished,
waitpid(2) will return its PID. For foreground processes, it returns -1.
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