De-duplicate email addresses in notify-mailer. (#4015)

Resolves #4003

cmd/notify-mailer/main.go

@@ -89,6 +89,21 @@ func (m *mailer) printStatus(to string, cur, total int, start time.Time) {
 		to, cur, total, completion, elapsed)
 }
 
+// uniq returns a slice of strings consisting of the input slice with all
+// duplicates removed. It preserves the ordering of the input slice.
+func uniq(input []string) []string {
+	var output []string
+	uniqMap := map[string]bool{}
+	for _, s := range input {
+		// Only append to the output items that have not been seen.
+		if _, ok := uniqMap[s]; !ok {
+			output = append(output, s)
+		}
+		uniqMap[s] = true
+	}
+	return output
+}
+
 func (m *mailer) run() error {
 	if err := m.ok(); err != nil {
 		return err
@@ -99,6 +114,12 @@ func (m *mailer) run() error {
 		return err
 	}
 
+	lenBefore := len(destinations)
+
+	destinations = uniq(destinations)
+	m.log.Infof("Before de-duping: %d email addresses. After de-duping: %d email addresses",
+		lenBefore, len(destinations))
+
 	err = m.mailer.Connect()
 	if err != nil {
 		return err
@@ -232,6 +253,12 @@ Similarly, the -end flag specifies which registration ID of the -toFile to end
 processing at. In combination these can be used to process only a fixed number
 of recipients at a time, and to resume mailing after early termination.
 
+Notify-mailer will de-duplicate email addresses, but only within the range given
+by the -start and -end arguments. For instance, if you split up an email job into
+five batches using -start and -end, there's a possibility that a given email address
+may receive up to five emails, if that email address is registered across multiple
+accounts.
+
 During mailing the -sleep argument is used to space out individual messages.
 This can be used to ensure that the mailing happens at a steady pace with ample
 opportunity for the operator to terminate early in the event of error. The

cmd/notify-mailer/main_test.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"io/ioutil"
+	"reflect"
 	"testing"
 	"time"
 
@@ -15,6 +16,15 @@ import (
 	"github.com/letsencrypt/boulder/test"
 )
 
+func TestUniq(t *testing.T) {
+	input := []string{"c", "d", "c", "c", "e", "d", "e", "e"}
+	expected := []string{"c", "d", "e"}
+	output := uniq(input)
+	if !reflect.DeepEqual(output, expected) {
+		t.Errorf("Expected %s, got %s", expected, output)
+	}
+}
+
 func TestCheckpointIntervalOK(t *testing.T) {
 	// Test a number of intervals know to be OK, ensure that no error is
 	// produced when calling `ok()`.