The changes are intrusive to other implementations (POSIX and Windows 32-bit).
So, I revert the changes of replacing translate_sys2libc_fd_int.

I don't argue about the technical necessity for the change. I agree the fact it works (without such changes).

Just to clarify this change for readers not accustomed to Windows internals: This function was used to translate the file descriptor as passed to gpg (which is a HANDLE) to the libc file descriptor as used by stdio. Obviously we won't anymore work with stdio file descriptors in the future but use the Windows32 API (ReadFile et al). libc fds 0,1,2 are handled in a special way on Windows.

Fixed in master.

The use cases are:

• oPassphraseFD for gpgsm, gpg
• oStatusFD for gpg-auth, gpg-wks-client, gpg-card, gpg-pair-tool, gpgtar, gpgconf, gpgsm, gpg, gpgv
• oLoggerFD for gpgsm, gpg, gpgv
• oAttributeFD for gpg
• oCommandFD for gpg
• oOverrideSessionKeyFD for gpg

With the fix of T6523, make check goes all well (on Wine emulation and on Windows, for i686 and for x86_64).

Fixed in master.

I modified ffi.c, to have renamed process-spawn-io function doing I/O by C.

Calling assuan_release before kbx_client_data_release is the best (and we join the thread).

tests/openpgp/import.scm hangs with 4096*4.

Test with Wine (i686) emulation, I encountered another hang at: Checking armored_key_8192

It looks like having the datastream_thread may be not worth.
One possibility is to implement synchronous read from pipe in kbx_client_data_wait, instead of datastream_thread.

See also commit rG6fcc263c18 from 2020 where I switched to D-lines.

Let's schedule that for 2.6

You are right, it is a bad habit not to check this. Thanks for your patch.