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POSIX proposal to add chdir() operation to posix_spawn()

There has been a recent (very recent, like within the past few hours) proposal
to add the ability to do a chdir() (or fchdir()) to posix_spawn

There are more changes than what are included here, but the rest are mostly
all just boilerplate changes to support this addition.

If anyone has any comments, I'm happy to pass them along, or you can directly
(this is issue 1208 if anyone wants to comment directly.)


[I have made some whitespace and line wrapping changes to this text]

    posix_spawn_file_actions_addchdir( )
    posix_spawn_file_actions_addchdir, posix_spawn_file_actions_addfchdir - add
	chdir or fchdir action to spawn file actions object (ADVANCED REALTIME)
    #include <spawn.h>
    int posix_spawn_file_actions_addchdir(posix_spawn_file_actions_t *restrict file_actions,
	const char *restrict path);
    int posix_spawn_file_actions_addfchdir(posix_spawn_file_actions_t *file_actions,
	 int filedes);
    The posix_spawn_file_actions_addchdir( ) function shall add a chdir action to the
    object referenced by file_actions that shall cause the working directory to be set to
    path (as if chdir(path) had been called) when a new process is spawned using this
    file actions object. A relative path shall be interpreted in relation to the working
    directory determined by any prior actions. The string described by path shall be
    copied by the posix_spawn_file_actions_addchdir( ) function.

    The posix_spawn_file_actions_addfchdir( ) function shall add a fchdir action to the
    object referenced by file_actions that shall cause the working directory to be set to
    filedes (as if fchdir(filedes) had been called) when a new process is spawned using
    this file actions object.

    A spawn file actions object is as defined in posix_spawn_file_actions_addclose( ).

    File actions are performed in a new process created by posix_spawn( ) or
    posix_spawnp( ) in the same order that they were added to the file actions object. 
   Thus, the execution of an addopen action that was created by a call to
   posix_spawn_file_actions_addopen( ) that specifies a relative path will be affected
   by the execution of a chdir or fchdir action that was created by a previous call to
   posix_spawn_file_actions_addchdir( ) or posix_spawn_file_actions_addfchdir( ).
   Likewise, a relative path passed to posix_spawn( ) will be affected by the last chdir
   or fchdir action in the file action list.

    Upon successful completion, these functions shall return zero; otherwise, an error
    number shall be returned to indicate the error.

    The posix_spawn_file_actions_addfchdir( ) function shall fail if:
    [EBADF] The value specified by fildes is negative.
    These functions shall fail if:
    [ENOMEM] Insufficient memory exists to add to the spawn file actions object.
    It shall not be considered an error for the path or fildes argument passed to these
    functions to specify a path or file descriptor for which the specified operation could
    not be performed at the time of the call. Any such error will be detected when the
    associated file actions object is later used during a posix_spawn( ) or
    posix_spawnp( ) operation.
    These functions may fail if:
    [EINVAL] The value specified by file_actions is invalid.


    The posix_spawn_file_actions_addchdir( ) and posix_spawn_file_actions_addfchdir( )
    functions are part of the Spawn option and need not be provided on all implementations.

    Changing the working directory of a child process can be useful when invoking
    utilities such as pax. Furthermore, the ability to add fchdir actions to posix_spawn( )
    gives the caller as much control over relative pathnames processed in the context
    of the child as it would otherwise have using openat( ), since all file actions are
    processed in sequence in the context of the child at a point where the child process
    is still single-threaded. Without chdir or fchdir actions, changing the working
    directory of the child would require a shim utility (some implementations provide

                 env -C /new/path program args...

    as an extension, but the standard does not require this extension), or else temporarily
    changing the working directory in the parent process prior to calling posix_spawn( )
    (but this requires locking in a multi-threaded process, to ensure that no other thread
    is impacted by the temporary change to global state).

    Refer to the RATIONALE section in posix_spawn_file_actions_addclose( ).


    chdir( ), fchdir( ), posix_spawn( ), posix_spawn_file_actions_addclose( ),
    posix_spawn_file_actions_destroy( ).
    XBD <spawn.h>

    First released in Issue 8. Derived from Solaris posix_spawn_file_actions_addchdir_np.

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