Actions to perform after forking a child process.
To spawn a child executable on Unix, the parent forks a copy of itself, then has the child copy set up the environment for the new program and execute it.
However, we cannot run any OCaml code in the forked child process. This is because `fork` only duplicates its own domain. To the child, it appears that all other domains have stopped responding and if it tries to e.g. perform a GC then the child process will hang.
Therefore, the fork call and all child actions need to be written in C. This module provides some support code for doing that. Individual backends will wrap these actions with higher-level APIs and can also add their own platform-specific actions
type c_action = Obj.tAn action to be performed in a child process after forking. This must be a tuple whose first field is a fork_fn.
An action that calls run k in the parent process to create the C action. run passes the action to k, which forks the child and runs it. When k returns, run can free any resources used.
val execve : string -> argv:string array -> env:string array -> tSee execve(2).
val chdir : string -> tchdir path changes directory to path.
val fchdir : Eio_unix__.Rcfd.t -> tfchdir fd changes directory to fd.
inherit_fds mapping marks file descriptors as not close-on-exec and renumbers them.
For each (fd, src, flags) in mapping, we use dup2 to duplicate src as fd. If there are cycles in mapping, a temporary FD is used to break the cycle. A mapping from an FD to itself simply clears the close-on-exec flag.
After this, the new FDs may also be set as blocking or non-blocking, depending on flags.