Add ${.sh.pid} as an alternative to $BASHPID (#109)

This variable is like Bash's $BASHPID, but in virtual subshells
it will retain its previous value as virtual subshells don't fork.
Both $BASHPID and ${.sh.pid} are different from $$ as the latter
is only set to the parent shell's process ID (i.e. it isn't set
to the process ID of the current subshell).

src/cmd/ksh93/include/defs.h:
- Add 'current_pid' for storing the current process ID at a valid
  memory address.
- Change 'ppid' from 'int32_t' to 'pid_t', as the return value from
  'getppid' is of the 'pid_t' data type.

src/cmd/ksh93/data/variables.c,
src/cmd/ksh93/include/variables.h,
src/cmd/ksh93/sh/init.c,
src/cmd/ksh93/sh/xec.c:
 - Add the ${.sh.pid} variable as an alternative to $BASHPID.
   The process ID is stored in a struct before ${.sh.pid} is set
   as environment variables are pointers that must point to a
   valid memory address. ${.sh.pid} is updated by the _sh_fork()
   function, which is called when ksh forks a new process with
   sh_fork() or sh_ntfork().

src/cmd/ksh93/tests/variables.sh:
- Add ${.sh.pid} to the list of special variables and add three
  regression tests for ${.sh.pid}.

src/cmd/ksh93/tests/subshell.sh:
- Update the PATH forking regression test to use ${.sh.pid} and
  remove the TODO note.

NEWS

@@ -8,6 +8,12 @@ Any uppercase BUG_* names are modernish shell bug IDs.
 - Fixed a crash that occurred intermittently if 'set -b'/'set -o notify' is
   active and $PS1 contains a command substitution running an external command.
 
+- Added the '${.sh.pid}' variable as an alternative to Bash's '$BASHPID'.
+  This variable is set to the current shell's PID, unlike '$$' (which is
+  set to the parent shell's PID). In virtual subshells '${.sh.pid}' is not
+  changed from its previous value, while in forked subshells '${.sh.pid}'
+  is set to the subshell's process ID.
+
 2020-08-05:
 
 - Fixed a bug in functions that caused ksh to crash when an array with an

src/cmd/ksh93/data/variables.c

@@ -99,6 +99,7 @@ const struct shtable2 shtab_variables[] =
 	".sh.stats",	0,				(char*)0,
 	".sh.math",	0,				(char*)0,
 	".sh.pool",	0,				(char*)0,
+	".sh.pid",	NV_INTEGER|NV_NOFREE,		(char*)0,
 	"SHLVL",	NV_INTEGER|NV_NOFREE|NV_EXPORT,	(char*)0,
 #if SHOPT_MULTIBYTE
 	"CSWIDTH",	0,				(char*)0,

src/cmd/ksh93/include/defs.h

@@ -122,7 +122,8 @@ struct shared
 	gid_t		groupid;
 	gid_t		egroupid;
 	pid_t		pid;
-	int32_t		ppid;
+	pid_t		ppid;
+	pid_t		current_pid;  /* pid of the current ksh instance */
 	unsigned char	sigruntime[2];
 	Namval_t	*bltin_nodes;
 	Namval_t	*bltin_cmds;

src/cmd/ksh93/include/variables.h

@@ -90,6 +90,7 @@
 #define SH_STATS	(shgd->bltin_nodes+60)
 #define SH_MATHNOD	(shgd->bltin_nodes+61)
 #define SH_JOBPOOL	(shgd->bltin_nodes+62)
-#define SHLVL		(shgd->bltin_nodes+63)
+#define SH_PIDNOD	(shgd->bltin_nodes+63)
+#define SHLVL		(shgd->bltin_nodes+64)
 
 #endif /* SH_VALNOD */

src/cmd/ksh93/sh.1

@@ -1664,6 +1664,16 @@ discipline function is invoked.
 .B .sh.subshell
 The current depth for subshells and command substitution.
 .TP
+.B .sh.pid
+Set to the process ID of the current shell.
+This is distinct from
+.B $$
+as in forked subshells this is set to the process ID of the
+subshell instead of the parent shell's process ID.
+In virtual subshells
+.B .sh.pid
+retains its previous value.
+.TP
 .B .sh.value
 Set to the value of the variable at the time that the
 .B set

src/cmd/ksh93/sh/init.c

@@ -1220,7 +1220,7 @@ Shell_t *sh_init(register int argc,register char *argv[], Shinit_f userinit)
 		beenhere = 1;
 		shp = &sh;
 		shgd = newof(0,struct shared,1,0);
-		shgd->pid = getpid();
+		shgd->current_pid = shgd->pid = getpid();
 		shgd->ppid = getppid();
 		shgd->userid=getuid();
 		shgd->euserid=geteuid();
@@ -1832,6 +1832,7 @@ static Init_t *nv_init(Shell_t *shp)
 	nv_stack(LANGNOD, &ip->LANG_init);
 #endif /* _hdr_locale */
 	(PPIDNOD)->nvalue.lp = (&shp->gd->ppid);
+	(SH_PIDNOD)->nvalue.lp = (&shp->gd->current_pid);
 	(TMOUTNOD)->nvalue.lp = (&shp->st.tmout);
 	(MCHKNOD)->nvalue.lp = (&sh_mailchk);
 	(OPTINDNOD)->nvalue.lp = (&shp->st.optindex);

src/cmd/ksh93/sh/xec.c

@@ -2917,6 +2917,9 @@ pid_t _sh_fork(Shell_t *shp,register pid_t parent,int flags,int *jobid)
 	sh_onstate(SH_NOLOG);
 	if (shp->fn_reset)
 		shp->fn_depth = shp->fn_reset = 0;
+	/* Set ${.sh.pid} to the child process ID */
+	shp->gd->current_pid = getpid();
+	SH_PIDNOD->nvalue.lp = &shp->gd->current_pid;
 #if SHOPT_ACCT
 	sh_accsusp();
 #endif	/* SHOPT_ACCT */

src/cmd/ksh93/tests/subshell.sh

@@ -747,14 +747,13 @@ v=$($SHELL $testvars) && [[ "$v" == "a= b= c=0" ]] || err_exit 'variables set in
 
 # ======
 # Setting PATH in virtual subshell should trigger a fork; restoring PATH after leaving virtual subshell should not.
-# TODO: it would be really nice to have a ${.sh.pid} for this sort of test (like $BASHPID on bash)...
 SHELL=$SHELL "$SHELL" -c '
 	(
-		"$SHELL" -c "echo DEBUG \$PPID"
+		echo "DEBUG ${.sh.pid}"
 		readonly PATH
-		"$SHELL" -c "echo DEBUG \$PPID"
+		echo "DEBUG ${.sh.pid}"
 	)
-	"$SHELL" -c "echo DEBUG \$PPID"
+	echo "DEBUG ${.sh.pid}"
 	: extra command to disable "-c" exec optimization
 ' | awk '/^DEBUG/ { pid[NR] = $2; }  END { exit !(pid[1] == pid[2] && pid[2] == pid[3]); }' \
 || err_exit "setting PATH to readonly in subshell triggers an erroneous fork"

src/cmd/ksh93/tests/variables.sh

@@ -915,6 +915,7 @@ set -- \
 	".sh.stats" \
 	".sh.math" \
 	".sh.pool" \
+	".sh.pid" \
 	"SHLVL" \
 	"CSWIDTH"
 
@@ -991,5 +992,20 @@ $SHELL -c '
 e=$?
 ((e == 1)) || err_exit "Failure in making one or more special variables readonly in a subshell (exit status $e)"
 
+# ======
+# ${.sh.pid} should be the forked subshell's PID
+(
+	ulimit -t unlimited
+	[[ ${.sh.pid} == $$ ]]
+) && err_exit "\${.sh.pid} is the same as \$$ (both are $$)"
+
+# ${.sh.pid} should be the PID of the running job
+echo ${.sh.pid} > "$tmp/jobpid" &
+wait
+[[ $(cat "$tmp/jobpid") == ${.sh.pid} ]] && err_exit "\${.sh.pid} is not set to a job's PID (expected $!, got $(cat "$tmp/jobpid"))"
+
+# ${.sh.pid} should be the same as $$ in the parent shell
+[[ $$ == ${.sh.pid} ]] || err_exit "\${.sh.pid} and \$$ differ in the parent shell (expected $$, got ${.sh.pid})"
+
 # ======
 exit $((Errors<125?Errors:125))