rpatterns: Make memcursor concurrent safe

Summary: This MR enables the memory cursor to be concurrent safe in cases where the testing involves concurrency and prevents a panic when multiple go routines try to write to the store at the same time causing a panic with the message "concurrent map writes"

rpatterns/cursor.go

@@ -3,6 +3,7 @@ package rpatterns
 import (
 	"context"
 	"strconv"
+	"sync"
 
 	"github.com/luno/reflex"
 )
@@ -75,14 +76,20 @@ func MemCursorStore(opts ...MemOpt) reflex.CursorStore {
 }
 
 type memCursorStore struct {
+	mu      sync.Mutex
 	cursors map[string]string
 }
 
 func (m *memCursorStore) GetCursor(_ context.Context, consumerName string) (string, error) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
 	return m.cursors[consumerName], nil
 }
 
 func (m *memCursorStore) SetCursor(_ context.Context, consumerName string, cursor string) error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
 	if m.cursors == nil {
 		m.cursors = make(map[string]string)
 	}

rpatterns/cursor_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"math/rand"
+	"sync"
 	"testing"
 
 	"github.com/luno/jettison/jtest"
@@ -160,3 +161,37 @@ func TestMemoryCStore(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, c2, actual)
 }
+
+func TestConcurrentWrites(t *testing.T) {
+	var (
+		writerReadyGroup     sync.WaitGroup
+		writerCompletedGroup sync.WaitGroup
+	)
+
+	store := rpatterns.MemCursorStore()
+	ctx := context.Background()
+
+	writerCount := 100
+	writerReadyGroup.Add(writerCount)
+	for i := 0; i < writerCount; i++ {
+		writerCompletedGroup.Add(1)
+		go func(writerReadyGroup *sync.WaitGroup, writerCompletedGroup *sync.WaitGroup) {
+			writerReadyGroup.Done()
+			writerReadyGroup.Wait()
+			for i := 0; i < 100; i++ {
+				// Write the thing
+				val := fmt.Sprintf("%v", i)
+				err := store.SetCursor(ctx, "single-key", val)
+				jtest.RequireNil(t, err)
+
+				// Flush is a noop but covering it will ensure that it stays concurrent safe
+				err = store.Flush(ctx)
+				jtest.RequireNil(t, err)
+			}
+			writerCompletedGroup.Done()
+		}(&writerReadyGroup, &writerCompletedGroup)
+	}
+
+	writerReadyGroup.Wait()
+	writerCompletedGroup.Wait()
+}