posix_spawn-internal: improvements on native Windows

[Bruno Haible]

Here are some improvements for the posix_spawn* implementation on
native Windows.

The first patch is a small fix (only visible if, among the file
action, there is an _addopen of a file in O_APPEND mode, followed by
a _adddup2 of that file descriptor).

The next 4 patches optimize away most DuplicateHandle calls. In
particular, the last among these patches applies some optimization
rules:

         (1)    dup2 (oldfd, newfd) ... close (newfd)
             -> nop                 ... nop

         (2)    dup2 (oldfd, newfd) ... open (newfd, ...)
             -> nop                 ... open (newfd, ...)

         (3)    dup2 (oldfd, newfd) ... dup2 (otherfd, newfd)
             -> nop                 ... dup2 (otherfd, newfd)

         (4)    dup2 (oldfd, newfd)       ... close (oldfd)
             -> dup2_close (oldfd, newfd) ... nop

         (5)    dup2 (oldfd, newfd)       ... open (oldfd, ...)
             -> dup2_close (oldfd, newfd) ... open (oldfd, ...)

         (6)    dup2 (oldfd, newfd)       ... dup2 (otherfd, oldfd)
             -> dup2_close (oldfd, newfd) ... dup2 (otherfd, oldfd)

where dup2_close shuffles around file descriptors without doing any
Windows API call.

The rules (1), (2), (3), (5), (6) are rarely relevant in practice;
however, rule (4) is widely applicable.


2022-09-09  Bruno Haible  <bruno@clisp.org>

        posix_spawn-internal: Optimize DuplicateHandle calls on native Windows.
        * lib/windows-spawn.h (DELAYED_DUP2_OLDFD, DELAYED_DUP2_NEWFD): New
        macros.
        (struct IHANDLE): Add a linked_fd field.
        * lib/spawni.c (SPAWN_INTERNAL_OPTIMIZE_DUPLICATEHANDLE): New macro.
        (do_delayed_dup2, do_remaining_delayed_dup2): New functions.
        (close_inheritable_handles): Don't close handles in DELAYED_DUP2_NEWFD
        entries.
        (do_close): Add a third parameter. Optimize delayed dup2 calls.
        (do_open): Use do_close.
        (do_dup2): Likewise. Prepare for optimizing the DuplicateHandle call.
        (__spawni): Do the remaining delayed dup2 invocations after the loop
        over the actions.

        posix_spawn-internal: Refactor.
        * lib/windows-spawn.h (struct IHANDLE): New type.
        (struct inheritable_handles): Combine handles and flags into a single
        array.
        * lib/windows-spawn.c (init_inheritable_handles, compose_handles_block,
        spawnpvech): Update.
        * lib/spawni.c (grow_inheritable_handles, shrink_inheritable_handles,
        do_open, do_dup2, do_close): Update.

        posix_spawn-internal: Optimize DuplicateHandle calls on native Windows.
        * lib/spawni.c (open_handle): Return an inheritable HANDLE.
        (do_open): Don't call DuplicateHandle. Remove curr_process parameter.
        (__spawni): Update.

        posix_spawn-internal: Optimize DuplicateHandle calls on native Windows.
        * lib/windows-spawn.h (KEEP_OPEN_IN_PARENT): New macro.
        * lib/windows-spawn.c (init_inheritable_handles): When a handle is
        already inheritable, don't bother duplicating it; instead, just mark it
        as KEEP_OPEN_IN_PARENT.
        * lib/spawni.c (shrink_inheritable_handles, close_inheritable_handles,
        do_open, do_dup2, do_close): Don't close handles that are marked as
        KEEP_OPEN_IN_PARENT.

2022-09-09  Bruno Haible  <bruno@clisp.org>

        posix_spawn-internal: Don't lose flags while duplicating an fd.
        * lib/spawni.c (do_dup2): Fix the flags of the new fd.

0001-posix_spawn-internal-Don-t-lose-flags-while-duplicat.patch
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0002-posix_spawn-internal-Optimize-DuplicateHandle-calls-.patch
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0003-posix_spawn-internal-Optimize-DuplicateHandle-calls-.patch
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0004-posix_spawn-internal-Refactor.patch
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0005-posix_spawn-internal-Optimize-DuplicateHandle-calls-.patch
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