Hopefully fix flaky Consul fencing test (#23280)

* Hopefully fix flaky fencing test when run in Enterprise

* Fix typo

vault/external_tests/consul_fencing_binary/consul_fencing_test.go

@@ -68,6 +68,18 @@ func TestConsulFencing_PartitionedLeaderCantWrite(t *testing.T) {
 	})
 	require.NoError(t, err)
 
+	// We need to wait for the KV mount to be ready on all servers - it runs an
+	// "Upgrade" process on mount and will error until that's done. In practice
+	// that's so fast that in CE I've never seen it fail yet, but in Enterprise
+	// there is the added complexity of the upgrade running on the primary while
+	// standby nodes wait for it to complete. So in Enterprise the first PATCH
+	// sent to a standby often comes in before the standby has "seen" that the
+	// primary has completed the upgrade and fails causing the whole test to
+	// error. We can prevent that by taking a short loop here to wait for a read
+	// to succeed on the standby/non-leader (since KV-v2 does upgrade check on
+	// read too).
+	waitForKVv2Upgrade(t, ctx, notLeader.APIClient(), "/test")
+
 	// Start two background workers that will cause writes to Consul in the
 	// background. KV v2 relies on a single active node for correctness.
 	// Specifically its patch operation does a read-modify-write under a
@@ -263,6 +275,46 @@ func TestConsulFencing_PartitionedLeaderCantWrite(t *testing.T) {
 	require.Equal(t, expect1, sets[1], "Client 1 writes lost")
 }
 
+func waitForKVv2Upgrade(t *testing.T, ctx context.Context, client *api.Client, path string) {
+	t.Helper()
+
+	ctx, cancel := context.WithTimeout(ctx, 10*time.Second)
+	defer cancel()
+
+	kv := client.KVv2(path)
+	attempts := 0
+	wait := 20 * time.Millisecond
+	for {
+		// Attempt to perform a write on the KVv2 mount. It will fail until the
+		// backend is done upgrading. If this is a performance standby in Ent then
+		// it will not complete until the primary is done upgrading AND the standby
+		// has noticed that!
+		_, err := kv.Put(ctx, "test-upgrade-done", map[string]interface{}{
+			"ok": 1,
+		})
+		if err == nil {
+			return
+		}
+		t.Logf("waitForKVv2Upgrade: write failed: %s", err)
+		select {
+		case <-ctx.Done():
+			t.Fatalf("context cancelled waiting for KVv2 (%s) upgrade to complete: %s",
+				path, ctx.Err())
+			return
+		case <-time.After(wait):
+		}
+		attempts++
+		// We don't quite want exponential backoff because it really should be fast,
+		// but just reduce log spam on failures if it's taking a while.
+		if attempts > 4 {
+			wait = 250 * time.Millisecond
+		}
+		if attempts > 10 {
+			wait = time.Second
+		}
+	}
+}
+
 func makeSet(n int) []int {
 	a := make([]int, n)
 	for i := 0; i < n; i++ {