MM-21328: Fix KVCompareAndSet when new==old (#13612)

`KVCompareAndSet(key, sameValue, sameValue)` can fail spuriously on MySQL if the underlying `UPDATE` requires no actual changes. As per the [MySQL documentation](https://dev.mysql.com/doc/refman/8.0/en/mysql-affected-rows.html), we can't rely on rows affected in this case:

> For UPDATE statements, the affected-rows value by default is the number of rows actually changed. If you specify the CLIENT_FOUND_ROWS flag to mysql_real_connect() when connecting to mysqld, the affected-rows value is the number of rows “found”; that is, matched by the WHERE clause.

It's not tenable to change `CLIENT_FOUND_ROWS` for the all connection, so handle this case in the code instead by running a `SELECT` after the fact. Note that `KVCompareAndSet` has no guarantee of atomicity in this case, but neither would `CompareAndSwap` on which this is method was inspired.

Finally, note that no changes are required for Postgres, which has sane semantics as the default.

Fixes: https://mattermost.atlassian.net/browse/MM-21328

Co-authored-by: mattermod <mattermod@users.noreply.github.com>

store/storetest/plugin_store.go

@@ -14,6 +14,7 @@ import (
 )
 
 func TestPluginStore(t *testing.T, ss store.Store) {
+	t.Run("CompareAndSet", func(t *testing.T) { testPluginCompareAndSet(t, ss) })
 	t.Run("PluginSaveGet", func(t *testing.T) { testPluginSaveGet(t, ss) })
 	t.Run("PluginSaveGetExpiry", func(t *testing.T) { testPluginSaveGetExpiry(t, ss) })
 	t.Run("PluginDelete", func(t *testing.T) { testPluginDelete(t, ss) })
@@ -21,6 +22,74 @@ func TestPluginStore(t *testing.T, ss store.Store) {
 	t.Run("PluginDeleteExpired", func(t *testing.T) { testPluginDeleteExpired(t, ss) })
 }
 
+func testPluginCompareAndSet(t *testing.T, ss store.Store) {
+	kv := &model.PluginKeyValue{
+		PluginId: model.NewId(),
+		Key:      model.NewId(),
+		Value:    []byte(model.NewId()),
+		ExpireAt: 0,
+	}
+	defer func() {
+		_ = ss.Plugin().Delete(kv.PluginId, kv.Key)
+	}()
+
+	t.Run("set non-existent key should succeed given nil old value", func(t *testing.T) {
+		ok, err := ss.Plugin().CompareAndSet(kv, nil)
+		require.Nil(t, err)
+		assert.True(t, ok)
+	})
+
+	t.Run("set existing key with new value should succeed given same old value", func(t *testing.T) {
+		_, err := ss.Plugin().SaveOrUpdate(kv)
+		require.Nil(t, err)
+
+		kvNew := &model.PluginKeyValue{
+			PluginId: kv.PluginId,
+			Key:      kv.Key,
+			Value:    []byte(model.NewId()),
+			ExpireAt: 0,
+		}
+
+		ok, err := ss.Plugin().CompareAndSet(kvNew, kv.Value)
+		require.Nil(t, err)
+		assert.True(t, ok)
+	})
+
+	t.Run("set existing key with new value should fail given different old value", func(t *testing.T) {
+		_, err := ss.Plugin().SaveOrUpdate(kv)
+		require.Nil(t, err)
+
+		kvNew := &model.PluginKeyValue{
+			PluginId: kv.PluginId,
+			Key:      kv.Key,
+			Value:    []byte(model.NewId()),
+			ExpireAt: 0,
+		}
+
+		ok, err := ss.Plugin().CompareAndSet(kvNew, []byte(model.NewId()))
+		require.Nil(t, err)
+		assert.False(t, ok)
+	})
+
+	t.Run("set existing key with same value should succeed given same old value", func(t *testing.T) {
+		_, err := ss.Plugin().SaveOrUpdate(kv)
+		require.Nil(t, err)
+
+		ok, err := ss.Plugin().CompareAndSet(kv, kv.Value)
+		require.Nil(t, err)
+		assert.True(t, ok)
+	})
+
+	t.Run("set existing key with same value should fail given different old value", func(t *testing.T) {
+		_, err := ss.Plugin().SaveOrUpdate(kv)
+		require.Nil(t, err)
+
+		ok, err := ss.Plugin().CompareAndSet(kv, []byte(model.NewId()))
+		require.Nil(t, err)
+		assert.False(t, ok)
+	})
+}
+
 func testPluginSaveGet(t *testing.T, ss store.Store) {
 	kv := &model.PluginKeyValue{
 		PluginId: model.NewId(),