VRT_blob() to return vrt_null_blob for empty blobs

bin/varnishd/cache/cache_vrt.c

@@ -1163,6 +1163,9 @@ VRT_blob(VRT_CTX, const char *err, const void *src, size_t len, unsigned type)
 	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
 	CHECK_OBJ_NOTNULL(ctx->ws, WS_MAGIC);
 
+	if (src == NULL || len == 0)
+		return (vrt_null_blob);
+
 	p = (void *)WS_Alloc(ctx->ws, sizeof *p);
 	if (p == NULL) {
 		VRT_fail(ctx, "Workspace overflow (%s)", err);

include/vrt.h

@@ -57,6 +57,8 @@
  *	vrt_null_strands added
  *	vrt_null_blob added
  *	VRT_NULL_BLOB_TYPE added as the .type of vrt_null_blob
+ *	VRT_blob() changed to return vrt_null_blob for
+ *	    len == 0 or src == NULL arguments
  * 13.0 (2021-03-15)
  *	Move VRT_synth_page() to deprecated status
  *	Add VRT_synth_strands() and VRT_synth_blob()

vmod/vmod_blob.c

@@ -539,10 +539,6 @@ vmod_sub(VRT_CTX, VCL_BLOB b, VCL_BYTES n, VCL_BYTES off)
 		return (NULL);
 	}
 
-	if (n == 0)
-		return (vrt_null_blob);
-
-
 	return (VRT_blob(ctx, "blob.sub",
 	    (const char *)b->blob + off, n, b->type));
 }