copy rrs (#4416)

* Revert "make copies of RRs before returning them (#4409)"

This reverts commit 8b2ff6c3.

* Document copying responses

See #4409 and the comments. This documents that issue, but doesn't
change the in-tree plugins just yet.
Signed-off-by: Miek Gieben <miek@miek.nl>

* Update plugin.md
Co-authored-by: Chris O'Haver <cohaver@infoblox.com>
Co-authored-by: Chris O'Haver <cohaver@infoblox.com>

plugin.md

@@ -131,6 +131,15 @@ The `fallthrough` directive should optionally accept a list of zones. Only queri
 in one of those zones should be allowed to fallthrough. See `plugin/pkg/fallthrough` for the
 implementation.
 
+## Mutating a Response
+
+Using a custom `ResponseWriter`, a plugin can mutate a response when another plugin further down the chain writes the response to the client.
+If a plugin mutates a response it MUST make a copy of the entire response before doing so. A
+response is a pointer to a `dns.Msg` and as such you will be manipulating the original response,
+which could have been generated from a data store. E.g. the *file* plugin creates a response that
+the *rewrite* plugin then rewrites; not copying the data, means it's **also** mutating the data of
+the *file*'s data store. A response can be copied by using the `Copy()` method.
+
 ## General Guidelines
 
 Some general guidelines:

plugin/file/lookup.go

@@ -171,12 +171,7 @@ func (z *Zone) Lookup(ctx context.Context, state request.Request, qname string)
 			return z.externalLookup(ctx, state, elem, rrs)
 		}
 
-		treeRRs := elem.Type(qtype)
-		// make a copy of the element RRs to prevent response writers from mutating the tree
-		rrs := make([]dns.RR, len(treeRRs))
-		for i, rr := range treeRRs {
-			rrs[i] = dns.Copy(rr)
-		}
+		rrs := elem.Type(qtype)
 
 		// NODATA
 		if len(rrs) == 0 {