dkim: Consider only 3 TXT records when fetching keys

Today, when looking up DKIM keys via DNS, we go through all the TXT
records that are published. This is already more lenient than what's
allowed by many implementations (as documented in the code).

Potentially, someone could craft a domain with a lot of TXT records to
make us waste time processing those keys.

To help reduce that possibility, this patch limits the lookup to 3
records. This is still more than normal and may be limited further to 1
in the future.

internal/dkim/dns_test.go

@@ -66,6 +66,34 @@ func TestSkipBadRecords(t *testing.T) {
 	}
 }
 
+func TestMaxKeyRecords(t *testing.T) {
+	ctx := WithLookupTXTFunc(t.Context(), testLookupTXT)
+	defer clear(results)
+
+	// Only the first maxKeyRecords records are used, even if the rest are
+	// valid keys.
+	key := "v=DKIM1; p=" + exampleRSAKeyB64
+	results["selector._domainkey.example.com"] = []string{
+		key, key, key, key, key,
+	}
+	pks, err := findPublicKeys(ctx, "example.com", "selector")
+	if err != nil || len(pks) != maxKeyRecords {
+		t.Errorf("findPublicKeys expected %d keys / nil, got %d / %v",
+			maxKeyRecords, len(pks), err)
+	}
+
+	// Records after the first maxKeyRecords are ignored, even if the first
+	// ones are not valid keys.
+	results["selector._domainkey.example.com"] = []string{
+		"not a tag", "not a tag", "not a tag", key,
+	}
+	pks, err = findPublicKeys(ctx, "example.com", "selector")
+	if err != nil || len(pks) != 0 {
+		t.Errorf("findPublicKeys expected 0 keys / nil, got %d / %v",
+			len(pks), err)
+	}
+}
+
 func TestParsePublicKey(t *testing.T) {
 	cases := []struct {
 		in  string