Re-backport atomic job locking from 93v- (re: 52067c3d, 595a0a56)

[This commit was previously reverted because it seemed to cause the
build to fail on Cygwin. But I just re-tested it, and it's fine. It
may be that my Cygwin installation at the time was defective.]

Something similar was previously done in 07cc71b8 from a Debian
patch, and eventually reverted; it redefined the ast atomic
functions asoincint() and asodecint() to be gcc-specific. This
imports the upstream version from the ksh 93v- beta instead.

This commit is based on an OpenSUSE patch:
https://build.opensuse.org/package/view_file/shells/ksh/ksh93-joblock.dif

src/cmd/ksh93/include/jobs.h:
- Replace job locking mechanism with the 93v- version which uses
  the atomic libast functions asoincint(), asogetint() and
  asodecint(). See: src/lib/libast/man/aso.3

src/cmd/ksh93/sh/jobs.c: job_subsave():
- Revert gcc optimiser bug workaround from c258a04f.
  It should now be unnecessary.

src/cmd/ksh93/Mamfile

@@ -377,6 +377,9 @@ make install
 					make include/jobs.h implicit
 						prev ${PACKAGE_ast_INCLUDE}/vmalloc.h implicit
 						prev include/terminal.h implicit
+						make ${PACKAGE_ast_INCLUDE}/aso.h implicit
+							prev ${PACKAGE_ast_INCLUDE}/ast_common.h implicit
+						done ${PACKAGE_ast_INCLUDE}/aso.h implicit
 						prev FEATURE/options implicit
 						prev ${PACKAGE_ast_INCLUDE}/sfio.h implicit
 						prev ${PACKAGE_ast_INCLUDE}/ast.h implicit

src/cmd/ksh93/include/jobs.h

@@ -30,6 +30,7 @@
 #   include	<signal.h>
 #endif /* !SIGINT */
 #include	"FEATURE/options"
+#include	<aso.h>
 
 #undef JOBS
 #if defined(SIGCLD) && !defined(SIGCHLD)
@@ -120,16 +121,13 @@ extern struct jobs job;
 #define vmbusy()	0
 #endif
 
-#define job_lock()	(job.in_critical++)
+#define job_lock()	asoincint(&job.in_critical)
 #define job_unlock()	\
 	do { \
-		int	sig; \
-		if (!--job.in_critical && (sig = job.savesig)) \
-		{ \
-			if (!job.in_critical++ && !vmbusy()) \
-				job_reap(sig); \
-			job.in_critical--; \
-		} \
+		int	_sig; \
+		if (asogetint(&job.in_critical) == 1 && (_sig = job.savesig) && !vmbusy()) \
+		    job_reap(_sig); \
+		asodecint(&job.in_critical); \
 	} while(0)
 
 extern const char	e_jobusage[];

src/cmd/ksh93/include/version.h

@@ -17,7 +17,7 @@
 #include <releaseflags.h>
 
 #define SH_RELEASE_FORK	"93u+m"		/* only change if you develop a new ksh93 fork */
-#define SH_RELEASE_SVER	"1.0.0-rc.5"	/* semantic version number: https://semver.org */
+#define SH_RELEASE_SVER	"1.0.0-rc.6"	/* semantic version number: https://semver.org */
 #define SH_RELEASE_DATE	"2022-07-31"	/* must be in this format for $((.sh.version)) */
 #define SH_RELEASE_CPYR	"(c) 2020-2022 Contributors to ksh " SH_RELEASE_FORK
 

src/cmd/ksh93/sh/jobs.c

@@ -1878,14 +1878,8 @@ again:
 
 void *job_subsave(void)
 {
-	/*
-	 * We must make a lock first before doing anything else,
-	 * otherwise GCC will remove the job locking mechanism
-	 * as a result of compiler optimization.
-	 */
-	job_lock();
-
 	struct back_save *bp = new_of(struct back_save,0);
+	job_lock();
 	*bp = bck;
 	bp->prev = bck.prev;
 	bck.count = 0;