add a concurrency safe wrapper for Random

This commit moves Random to it's own file and introduces a wrapper
to make Random safe for concurrent use. The global random in Go
is safe for concurrent use but random instances created otherwise
are not. Adding a locked wrapper allows us to inject a random in to
per repository router and have it provide safe safe access as the
global.

4 files changed, 69 insertions, 13 deletions

diff --git a/internal/praefect/random.go b/internal/praefect/random.go
new file mode 100644
index 000000000..36321c2c3
--- /dev/null
+++ b/internal/praefect/random.go
@@ -0,0 +1,26 @@
+package praefect
+
+import (
+	"sync"
+)
+
+// Random is the interface of the Go random number generator.
+type Random interface {
+	// Intn returns a random integer in the range [0,n).
+	Intn(n int) int
+}
+
+// randomFunc is an adapter to turn conforming functions in to a Random.
+type randomFunc func(n int) int
+
+func (fn randomFunc) Intn(n int) int { return fn(n) }
+
+// NewLockedRandom wraps the passed in Random to make it safe for concurrent use.
+func NewLockedRandom(r Random) Random {
+	var m sync.Mutex
+	return randomFunc(func(n int) int {
+		m.Lock()
+		defer m.Unlock()
+		return r.Intn(n)
+	})
+}
diff --git a/internal/praefect/random_test.go b/internal/praefect/random_test.go
new file mode 100644
index 000000000..915cd977b
--- /dev/null
+++ b/internal/praefect/random_test.go
@@ -0,0 +1,41 @@
+package praefect
+
+import (
+	"sync"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// TestNewLockedRandom tests that n is correctly passed down to the random
+// implementation and that the return value is correctly passed back. To test
+// that access to the random is actually synchronized, we launch 50 goroutines
+// to call the random func concurrently to increment actual. If the calls to
+// random are not correctly synchronized, actual might be not match expected
+// at the end and the race detector should detect racy accesses even in the
+// cases where the values match.
+func TestNewLockedRandom(t *testing.T) {
+	expected := 50
+	actual := 0
+
+	random := NewLockedRandom(randomFunc(func(n int) int {
+		assert.Equal(t, 1, n)
+		actual++
+		return 2
+	}))
+
+	var wg sync.WaitGroup
+	wg.Add(expected)
+
+	for i := 0; i < expected; i++ {
+		go func() {
+			defer wg.Done()
+			assert.Equal(t, 2, random.Intn(1))
+		}()
+	}
+
+	wg.Wait()
+
+	require.Equal(t, expected, actual)
+}
diff --git a/internal/praefect/router_per_repository.go b/internal/praefect/router_per_repository.go
index fe9008a42..d2174c9bb 100644
--- a/internal/praefect/router_per_repository.go
+++ b/internal/praefect/router_per_repository.go
@@ -51,17 +51,6 @@ type PrimaryGetter interface {
 	GetPrimary(ctx context.Context, virtualStorage string, relativePath string) (string, error)
 }
 
-// Random is the interface of the Go random number generator.
-type Random interface {
-	// Intn returns a random integer in the range [0,n).
-	Intn(n int) int
-}
-
-// RandomFunc is an adapter to turn conforming functions in to a Random.
-type RandomFunc func(n int) int
-
-func (fn RandomFunc) Intn(n int) int { return fn(n) }
-
 // PerRepositoryRouter implements a router that routes requests respecting per repository primary nodes.
 type PerRepositoryRouter struct {
 	conns Connections
diff --git a/internal/praefect/router_per_repository_test.go b/internal/praefect/router_per_repository_test.go
index abe355e89..02be7775b 100644
--- a/internal/praefect/router_per_repository_test.go
+++ b/internal/praefect/router_per_repository_test.go
@@ -99,7 +99,7 @@ func TestPerRepositoryRouter_RouteStorageAccessor(t *testing.T) {
 						"valid-choice-2",
 					},
 				},
-				RandomFunc(func(n int) int {
+				randomFunc(func(n int) int {
 					require.Equal(t, tc.numCandidates, n)
 					return tc.pickCandidate
 				}),
@@ -198,7 +198,7 @@ func TestPerRepositoryRouter_RouteRepositoryAccessor(t *testing.T) {
 					return "primary", nil
 				}),
 				tc.healthyNodes,
-				RandomFunc(func(n int) int {
+				randomFunc(func(n int) int {
 					t.Helper()
 					require.Equal(t, tc.numCandidates, n)
 					return tc.pickCandidate