Implement stringifcation of STRANDS via a workspace allocation.

When a VMOD returns a STRANDS, it is a requirement that the storage
be stable, but that is not really different from any other type of
return value.

Completes #2873

bin/varnishtest/tests/m00000.vtc

@@ -120,10 +120,3 @@ varnish v1 -errvcl {Expression has type STRING, expected REAL} {
 		set resp.http.who = std.random("foo", "bar");
 	}
 }
-
-varnish v1 -errvcl {Cannot convert type STRING(STRANDS) to STRING(STRING_LIST)} {
-	import debug;
-	sub vcl_deliver {
-		set resp.http.who = debug.return_strands(req.url + "bar");
-	}
-}

bin/varnishtest/tests/v00058.vtc

@@ -78,6 +78,13 @@ varnish v1 -arg "-i foobar" -vcl {
 				req.http.Foo + req.http.Unset + req.http.Bar);
 		debug.sethdr(resp.http.Hdr-6, req.http.Foo);
 		debug.sethdr(resp.http.Hdr-7, req.http.Unset);
+
+		set resp.http.Hdr-8 =
+		    debug.return_strands(
+			debug.return_strands(
+			    req.url + "<-->" + req.url
+			)
+		    );
 	}
 } -start
 
@@ -108,6 +115,7 @@ client c1 {
 	expect resp.http.Hdr-5 == "foobar"
 	expect resp.http.Hdr-6 == "foo"
 	expect resp.http.Hdr-7 == ""
+	expect resp.http.Hdr-8 == "/<-->/"
 } -run
 
 # out of workspace
@@ -140,8 +148,7 @@ varnish v1 -vcl+backend {
 				= debug.concatenate(req.http.Foo + req.http.Bar
 							+ req.http.Baz
 							+ req.http.Quux);
-		}
-		elsif (req.url == "/2") {
+		} elsif (req.url == "/2") {
 			# VRT_CollectStrands() invokes VCL failure.
 			set req.http.Result
 				= debug.collect(req.http.Foo + req.http.Bar

lib/libvcc/vcc_expr.c

@@ -186,8 +186,8 @@ vcc_expr_edit(struct vcc *tl, vcc_type_t fmt, const char *p, struct expr *e1,
 			    "  const char * strs_%u_s[%d];\n",
 			    tl->unique, tl->unique, e3->nstr);
 			VSB_printf(e->vsb,
-			    "\v+\nVRT_BundleStrands(%d, &strs_%u_a, strs_%u_s,"
-			    "\v+\n%s,\nvrt_magic_string_end)\v-\v-",
+			    "VRT_BundleStrands(%d, &strs_%u_a, strs_%u_s,"
+			    "\v+\n%s,\nvrt_magic_string_end\v-\n)",
 			    e3->nstr, tl->unique, tl->unique,
 			VSB_data(e3->vsb));
 			tl->unique++;
@@ -1313,6 +1313,12 @@ vcc_expr0(struct vcc *tl, struct expr **e, vcc_type_t fmt)
 		vcc_expr_cor(tl, e, fmt);
 	ERRCHK(tl);
 
+	if ((*e)->fmt == fmt)
+		return;
+
+	if ((*e)->fmt != STRINGS && fmt->stringform)
+		vcc_expr_tostring(tl, e, STRINGS);
+
 	if ((*e)->fmt->stringform) {
 		VSB_printf(tl->sb, "Cannot convert type %s(%s) to %s(%s)\n",
 		    vcc_utype((*e)->fmt), (*e)->fmt->name,
@@ -1321,9 +1327,6 @@ vcc_expr0(struct vcc *tl, struct expr **e, vcc_type_t fmt)
 		return;
 	}
 
-	if ((*e)->fmt != STRINGS && fmt->stringform)
-		vcc_expr_tostring(tl, e, STRINGS);
-
 	if ((*e)->fmt == STRINGS && fmt->stringform) {
 		if (fmt == STRING_LIST)
 			(*e)->fmt = STRING_LIST;

lib/libvcc/vcc_types.c

@@ -138,6 +138,7 @@ const struct type STRANDS[1] = {{
 	.magic =		TYPE_MAGIC,
 	.name =			"STRANDS",
 	.stringform =		1,
+	.tostring =		"VRT_CollectStrands(ctx,\v+\n\v1\v-\n)",
 }};
 
 const struct type STRINGS[1] = {{