Fix crash/freeze upon interrupting command substitution with pipe

On some systems (at least Linux and macOS):

1. Run on a command line: t=$(sleep 10|while :; do :; done)
2. Press Ctrl+C in the first 10 seconds.
3. Execute any other command substitution. The shell crashes.

Analysis: Something in the job_wait() call in the sh_subshell()
restore routine may be interrupted by a signal such as SIGINT on
Linux and macOS. Exactly what that interruptible thing is remains
to be determined. In any case, since job_wait() was invoked after
sh_popcontext(), interrupting it caused the sh_subshell() restore
routine to be aborted, resulting in an inconsistent state of the
shell. The fix is to sh_popcontext() at a later stage instead.

src/cmd/ksh93/sh/subshell.c: sh_subshell():
- Rename struct checkpt buff to checkpoint because it's clearer.
- Move the sh_popcontext() call to near the end, just after
  decreasing the subshell level counters and restoring the global
  subshell data struct to its parent. This seems like a logical
  place for it and could allow other things to be interrupted, too.
- Get rid of the if(shp->subshell) because it is known that the
  value is > 0 at this point.
- The short exit routine run if the subshell forked now needs a new
  sh_popcontext() call, because this is handled before restoring
  the virtual subshell state.
- While we're here, do a little more detransitioning from all those
  pointless shp pointers.

Fixes: https://github.com/ksh93/ksh/issues/397

NEWS

@@ -3,6 +3,11 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2021-12-27:
+
+- Fixed a crash or freeze that would occur on Linux when using Ctrl+C to
+  interrupt a command substitution containing a pipe in an interactive shell.
+
 2021-12-26:
 
 - Listing aliases or tracked aliases in a script no longer corrupts

src/cmd/ksh93/include/version.h

@@ -21,7 +21,7 @@
 
 #define SH_RELEASE_FORK	"93u+m"		/* only change if you develop a new ksh93 fork */
 #define SH_RELEASE_SVER	"1.0.0-beta.2"	/* semantic version number: https://semver.org */
-#define SH_RELEASE_DATE	"2021-12-26"	/* must be in this format for $((.sh.version)) */
+#define SH_RELEASE_DATE	"2021-12-27"	/* must be in this format for $((.sh.version)) */
 #define SH_RELEASE_CPYR	"(c) 2020-2021 Contributors to ksh " SH_RELEASE_FORK
 
 /* Scripts sometimes field-split ${.sh.version}, so don't change amount of whitespace. */

src/cmd/ksh93/sh/subshell.c

@@ -492,7 +492,7 @@ Sfio_t *sh_subshell(Shell_t *shp,Shnode_t *t, volatile int flags, int comsub)
 	int *saveexitval = job.exitval;
 	char **savsig;
 	Sfio_t *iop=0;
-	struct checkpt buff;
+	struct checkpt checkpoint;
 	struct sh_scoped savst;
 	struct dolnod   *argsav=0;
 	int argcnt;
@@ -509,7 +509,7 @@ Sfio_t *sh_subshell(Shell_t *shp,Shnode_t *t, volatile int flags, int comsub)
 	}
 	shp->curenv = ++subenv;
 	savst = shp->st;
-	sh_pushcontext(shp,&buff,SH_JMPSUB);
+	sh_pushcontext(&sh,&checkpoint,SH_JMPSUB);
 	shp->subshell++;		/* increase level of virtual subshells */
 	shgd->realsubshell++;		/* increase ${.sh.subshell} */
 	sp->prev = subshell_data;
@@ -605,7 +605,7 @@ Sfio_t *sh_subshell(Shell_t *shp,Shnode_t *t, volatile int flags, int comsub)
 		shp->cpid = 0;
 		sh_sigreset(0);
 	}
-	jmpval = sigsetjmp(buff.buff,0);
+	jmpval = sigsetjmp(checkpoint.buff,0);
 	if(jmpval==0)
 	{
 		if(comsub)
@@ -681,10 +681,10 @@ Sfio_t *sh_subshell(Shell_t *shp,Shnode_t *t, volatile int flags, int comsub)
 		sh_trap(trap,0);
 		free(trap);
 	}
-	sh_popcontext(shp,&buff);
+	if(sh.subshell==0)	/* we must have forked with sh_subfork(); this is the child process */
-	if(shp->subshell==0)	/* must be child process */
 	{
 		subshell_data = sp->prev;
+		sh_popcontext(&sh,&checkpoint);
 		if(jmpval==SH_JMPSCRIPT)
 			siglongjmp(*shp->jmplist,jmpval);
 		shp->exitval &= SH_EXITMASK;
@@ -886,16 +886,14 @@ Sfio_t *sh_subshell(Shell_t *shp,Shnode_t *t, volatile int flags, int comsub)
 	}
 	shp->subshare = sp->subshare;
 	shp->subdup = sp->subdup;
-	if(shp->subshell)
+	sh.subshell--;			/* decrease level of virtual subshells */
-	{
-		shp->subshell--;		/* decrease level of virtual subshells */
 	shgd->realsubshell--;		/* decrease ${.sh.subshell} */
-	}
 	subshell_data = sp->prev;
+	sh_popcontext(&sh,&checkpoint);
 	if(!argsav  ||  argsav->dolrefcnt==argcnt)
 		sh_argfree(shp,argsav,0);
-	if(shp->topfd != buff.topfd)
+	if(sh.topfd != checkpoint.topfd)
-		sh_iorestore(shp,buff.topfd|IOSUBSHELL,jmpval);
+		sh_iorestore(&sh,checkpoint.topfd|IOSUBSHELL,jmpval);
 	if(sp->sig)
 	{
 		if(sp->prev)