Unsetting gdbm tied variable basically works.

Allows variables in nested scope.

However, explicitly untying a variable doesn't properly expose
the scope above.

ChangeLog

+2015-01-26  Peter Stephenson  <p.stephenson@samsung.com>
+
+	* 34402: Src/Modules/db_gdbm.c: make unsetting a tied gdbm
+	variable work and hence allow tied variables in nested scope.
+	Untying still doesn't uncover scope properly.
+
 2015-01-25  Barton E. Schaefer  <schaefer@zsh.org>
 
 	* 34399: Src/utils.c: fix polltty thinko from 34365

Src/Modules/db_gdbm.c

@@ -43,6 +43,9 @@ static char *backtype = "db/gdbm";
 
 static const struct gsu_scalar gdbm_gsu =
 { gdbmgetfn, gdbmsetfn, gdbmunsetfn };
+/**/
+static const struct gsu_hash gdbm_hash_gsu =
+{ hashgetfn, hashsetfn, gdbmhashunsetfn };
 
 static struct builtin bintab[] = {
     BUILTIN("ztie", 0, bin_ztie, 1, -1, 0, "d:f:", NULL),
@@ -84,12 +87,14 @@ bin_ztie(char *nam, char **args, Options ops, UNUSED(int func))
 	return 1;
     }
 
-    if (!(tied_param = createspecialhash(pmname, &getgdbmnode, &scangdbmkeys, 0))) {
+    if (!(tied_param = createspecialhash(pmname, &getgdbmnode, &scangdbmkeys,
+					 PM_REMOVABLE))) {
         zwarnnam(nam, "cannot create the requested parameter %s", pmname);
 	gdbm_close(dbf);
 	return 1;
     }
 
+    tied_param->gsu.h = &gdbm_hash_gsu;
     tied_param->u.hash->tmpdata = (void *)dbf;
 
     return 0;
@@ -225,6 +230,25 @@ scangdbmkeys(HashTable ht, ScanFunc func, int flags)
 
 }
 
+/**/
+static void
+gdbmhashunsetfn(Param pm, UNUSED(int exp))
+{
+    GDBM_FILE dbf = (GDBM_FILE)(pm->u.hash->tmpdata);
+
+    if (!dbf) /* paranoia */
+	return;
+
+    gdbm_close(dbf);
+    pm->u.hash->tmpdata = NULL;
+
+    /* hash table is now normal, so proceed normally... */
+    pm->node.flags &= ~PM_SPECIAL;
+    pm->gsu.h = &stdhash_gsu;
+    pm->gsu.h->setfn(pm, NULL);
+    pm->node.flags |= PM_UNSET;
+}
+
 #else
 # error no gdbm
 #endif /* have gdbm */