> > * We establish a new macro GNULIB_MBTOWC_SINGLE which means "assume that 
> > at most one thread invokes mbtowc-like functions". The mbtowc 
> > replacement uses this instead of using USE_UNLOCKED_IO. Tar can #define 
> > this new macro.
> 
> I like this idea. The packages can then define or not define each such
> macro individually.

Implemented through the following series of patches.


2021-03-07  Bruno Haible  <bruno@clisp.org>

	Rename GNULIB_WCHAR_SINGLE to GNULIB_WCHAR_SINGLE_LOCALE.
	* lib/lc-charset-dispatch.c: Test GNULIB_WCHAR_SINGLE_LOCALE instead of
	GNULIB_WCHAR_SINGLE.
	* lib/wcwidth.c: Likewise.
	* tests/test-wcwidth.c: Likewise.
	* doc/multithread.texi: Document GNULIB_WCHAR_SINGLE_LOCALE instead of
	GNULIB_WCHAR_SINGLE.
	* NEWS: Document the change.

2021-03-06  Bruno Haible  <bruno@clisp.org>

	mbrtowc: Allow locking optimization independently of 'unlocked-io'.
	* lib/mbtowc-lock.h: Test GNULIB_MBRTOWC_SINGLE_THREAD instead of
	USE_UNLOCKED_IO.
	* doc/multithread.texi: Document GNULIB_MBRTOWC_SINGLE_THREAD.

2021-03-06  Bruno Haible  <bruno@clisp.org>

	regex: Allow locking optimization independently of 'unlocked-io'.
	* lib/regex_internal.h: Test GNULIB_REGEX_SINGLE_THREAD instead of
	USE_UNLOCKED_IO.
	* doc/multithread.texi: Document GNULIB_REGEX_SINGLE_THREAD.

2021-03-06  Bruno Haible  <bruno@clisp.org>

	exclude: Allow stdio optimization independently of 'unlocked-io'.
	* lib/exclude.c: Test GNULIB_EXCLUDE_SINGLE_THREAD instead of
	USE_UNLOCKED_IO.
	* modules/exclude (Depends-on): Add unlocked-io-internal.
	* doc/multithread.texi: Document GNULIB_EXCLUDE_SINGLE_THREAD.

2021-03-06  Bruno Haible  <bruno@clisp.org>

	readutmp: Optimize stdio accesses.
	* lib/readutmp.c: Include unlocked-io.h unconditionally.
	* modules/readutmp (Depends-on): Add unlocked-io-internal.

2021-03-06  Bruno Haible  <bruno@clisp.org>

	mountlist: Optimize stdio accesses.
	* lib/mountlist.c: Include unlocked-io.h unconditionally.
	* modules/mountlist (Depends-on): Add unlocked-io-internal.

2021-03-06  Bruno Haible  <bruno@clisp.org>

	getusershell: Optimize stdio accesses when possible.
	* lib/getusershell.c: Test GNULIB_GETUSERSHELL_SINGLE_THREAD instead of
	USE_UNLOCKED_IO.
	* modules/getusershell (Depends-on): Add unlocked-io-internal.
	* doc/multithread.texi: Document GNULIB_GETUSERSHELL_SINGLE_THREAD.

2021-03-06  Bruno Haible  <bruno@clisp.org>

	unlocked-io-internal: New module.
	* m4/unlocked-io.m4 (gl_FUNC_GLIBC_UNLOCKED_IO): Don't define
	USE_UNLOCKED_IO here.
	* modules/unlocked-io-internal: New file, based on modules/unlocked-io.
	* modules/unlocked-io (Description): Clarify.
	(Files, Depends-on): Just use the unlocked-io-internal module.
	(configure.ac): Define GNULIB_STDIO_SINGLE_THREAD and USE_UNLOCKED_IO
	here.
	* doc/multithread.texi: Clarify when the 'unlocked-io' module can be
	used.

2021-03-06  Bruno Haible  <bruno@clisp.org>

	posixtm: Remove unused includes.
	* lib/posixtm.c: Don't include <stdio.h>, <sys/types.h>, unlocked-io.h.