Fix out of storage condition when failing to allocate transient synth

Do not assert if the transient stevedore fails to provide storage for
the synth body.

Log an SLT_Error record when failing on out of storage.

Document that synth objects are created on transient storage.

Fix an objcore leak that would come if we failed to create the object.

Fixes: #1740

bin/varnishd/cache/cache_req_fsm.c

@@ -223,6 +223,8 @@ cnt_synth(struct worker *wrk, struct req *req)
 	struct http *h;
 	double now;
 	struct vsb *synth_body;
+	ssize_t sz, szl;
+	uint8_t *ptr;
 
 	CHECK_OBJ_NOTNULL(wrk, WORKER_MAGIC);
 	CHECK_OBJ_NOTNULL(req, REQ_MAGIC);
@@ -269,22 +271,25 @@ cnt_synth(struct worker *wrk, struct req *req)
 	req->objcore = HSH_Private(wrk);
 	CHECK_OBJ_NOTNULL(req->objcore, OBJCORE_MAGIC);
 	if (STV_NewObject(req->objcore, wrk, TRANSIENT_STORAGE, 1024)) {
-		ssize_t sz, szl;
-		uint8_t *ptr;
-
 		szl = VSB_len(synth_body);
 		assert(szl >= 0);
-		if (szl > 0) {
-			sz = szl;
-			AN(ObjGetSpace(wrk, req->objcore, &sz, &ptr));
-			assert(sz >= szl);
+	} else
+		szl = -1;
+	if (szl > 0) {
+		sz = szl;
+		if (ObjGetSpace(wrk, req->objcore, &sz, &ptr) && sz >= szl) {
 			memcpy(ptr, VSB_data(synth_body), szl);
 			ObjExtend(wrk, req->objcore, szl);
-		}
-
-		cnt_vdp(req, NULL);
-		(void)HSH_DerefObjCore(wrk, &req->objcore);
+		} else
+			szl = -1;
 	}
+	if (szl < 0) {
+		VSLb(req->vsl, SLT_Error, "Could not get storage");
+		req->doclose = SC_OVERLOAD;
+	} else
+		cnt_vdp(req, NULL);
+
+	(void)HSH_DerefObjCore(wrk, &req->objcore);
 	VSB_delete(synth_body);
 
 	VSLb_ts_req(req, "Resp", W_TIM_real(wrk));

doc/sphinx/users-guide/storage-backends.rst

@@ -132,8 +132,9 @@ previously banned objects to reappear.
 Transient Storage
 -----------------
 
-If you name any of your storage backend "Transient" it will be
-used for transient (short lived) objects. By default Varnish
+If you name any of your storage backend "Transient" it will be used
+for transient (short lived) objects. This includes the temporary
+objects created when returning a synthetic object. By default Varnish
 would use an unlimited malloc backend for this.
 
 .. XXX: Is this another paramater? In that case handled in the same manner as above? benc