Fix hash table memory leak when restoring PATH

There is a bug in path_alias() that may cause a memory leak when
clearing the hash table while setting/restoring PATH.

This applies a fix from Siteshwar Vashist:
https://www.mail-archive.com/ast-developers@lists.research.att.com/msg01945.html

Note that, contrary to Siteshwar's analysis linked above, this bug
has nothing directly to do with subshells, forked or otherwise; it
can also be reproduced by temporarily setting PATH for a command,
for example, 'PATH=/dev/null true', and then doing a PATH search.

Modified analysis:
ksh maintains the value of PATH as a linked list. When a local
scope for PATH is created (e.g. in a virtual subshell or when doing
something like PATH=/foo/bar command ...), ksh duplicates PATH by
increasing the refcount for every element in the linked list by
calling the path_dup() and path_alias() functions. However, when
the state of PATH is restored, this refcount is not decreased. Next
time when PATH is reset to a new value, ksh calls the path_delete()
function to delete the linked list that stored the older path. But
the path_delete() function does not free elements whose refcount is
greater than 1, causing a memory leak.

src/cmd/ksh93/sh/path.c: path_alias():
- Decrease refcount and free old item if needed.
  (The 'old' variable was already introduced in 9906535, but
  its value was never used there; this fixes that as well.)

src/cmd/ksh93/tests/leaks.sh:
- Add regression test. With the bug, setting/restoring PATH
  (which clears the hash table) and doing a PATH search 16 times
  causes about 1.5 KiB of memory to be leaked.

NEWS

@@ -9,6 +9,9 @@ Any uppercase BUG_* names are modernish shell bug IDs.
 
 - Fixed a crash when listing indexed arrays.
 
+- Fixed a memory leak when restoring PATH when temporarily setting PATH
+  for a command (e.g. PATH=/foo/bar command ...) or in a virtual subshell.
+
 2020-07-07:
 
 - Four of the date formats accepted by 'printf %()T' have had their

src/cmd/ksh93/sh/path.c

@@ -1800,7 +1800,9 @@ void path_alias(register Namval_t *np,register Pathcomp_t *pp)
 		Pathcomp_t *old;
 		nv_offattr(np,NV_NOPRINT);
 		nv_stack(np,&talias_init);
-		old = np->nvalue.pathcomp;
+		old = (Pathcomp_t*)np->nvalue.cp;
+		if (old && (--old->refcount <= 0))
+			free((void*)old);
 		np->nvalue.cp = (char*)pp;
 		pp->refcount++;
 		nv_setattr(np,NV_TAGGED|NV_NOFREE);

src/cmd/ksh93/tests/leaks.sh

@@ -121,5 +121,20 @@ after=$(getmem)
 (( after > before )) && err_exit 'unset of associative array causes memory leak' \
 	"(leaked $((after - before)) $unit)"
 
+# ======
+# Memory leak when resetting PATH and clearing hash table
+# ...steady memory state:
+command -v ls >/dev/null	# add something to hash table
+PATH=/dev/null true		# set/restore PATH & clear hash table
+# ...test for leak:
+before=$(getmem)
+for	((i=0; i<16; i++))
+do	PATH=/dev/null true	# set/restore PATH & clear hash table
+	command -v ls		# do PATH search, add to hash table
+done >/dev/null
+after=$(getmem)
+(( after > before )) && err_exit 'memory leak on PATH reset before subshell PATH search' \
+	"(leaked $((after - before)) $unit)"
+
 # ======
 exit $((Errors<125?Errors:125))