avoid string slice panic in version check

.changelog/23039.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+resource/aws_rds_cluster: Fix crash when configured `engine_version` string is shorter than the `EngineVersion` string returned from the AWS API
+```

internal/service/rds/cluster.go

@@ -1541,8 +1541,15 @@ func rdsClusterSetResourceDataEngineVersionFromCluster(d *schema.ResourceData, c
 }
 
 func compareActualEngineVersion(d *schema.ResourceData, oldVersion string, newVersion string) {
-	if oldVersion != newVersion && string(append([]byte(oldVersion), []byte(".")...)) != string([]byte(newVersion)[0:len(oldVersion)+1]) {
+	newVersionSubstr := newVersion
+
+	if len(newVersion) > len(oldVersion) {
+		newVersionSubstr = string([]byte(newVersion)[0 : len(oldVersion)+1])
+	}
+
+	if oldVersion != newVersion && string(append([]byte(oldVersion), []byte(".")...)) != newVersionSubstr {
 		d.Set("engine_version", newVersion)
 	}
+
 	d.Set("engine_version_actual", newVersion)
 }

internal/service/rds/engine_version_test.go

@@ -0,0 +1,76 @@
+package rds
+
+import (
+	"testing"
+)
+
+func TestCompareActualEngineVersion(t *testing.T) {
+	t.Parallel()
+
+	type testCase struct {
+		configuredVersion           string
+		actualVersion               string
+		expectedEngineVersion       string
+		expectedEngineVersionActual string
+	}
+	tests := map[string]testCase{
+		"point version upgrade": {
+			configuredVersion:           "8.0",
+			actualVersion:               "8.0.27",
+			expectedEngineVersion:       "",
+			expectedEngineVersionActual: "8.0.27",
+		},
+		"minor version upgrade": {
+			configuredVersion:           "8.0",
+			actualVersion:               "8.1.1",
+			expectedEngineVersion:       "8.1.1",
+			expectedEngineVersionActual: "8.1.1",
+		},
+		"major version upgrade": {
+			configuredVersion:           "8.1",
+			actualVersion:               "9.0.0",
+			expectedEngineVersion:       "9.0.0",
+			expectedEngineVersionActual: "9.0.0",
+		},
+		"aurora upgrade": {
+			configuredVersion:           "5.7.mysql_aurora.2.07",
+			actualVersion:               "5.7.serverless_mysql_aurora.2.08.3",
+			expectedEngineVersion:       "5.7.serverless_mysql_aurora.2.08.3",
+			expectedEngineVersionActual: "5.7.serverless_mysql_aurora.2.08.3",
+		},
+		"aurora upgrade - used to crash": {
+			configuredVersion:           "5.7.serverless_mysql_aurora.2.07",
+			actualVersion:               "5.7.mysql_aurora.2.08.3",
+			expectedEngineVersion:       "5.7.mysql_aurora.2.08.3",
+			expectedEngineVersionActual: "5.7.mysql_aurora.2.08.3",
+		},
+		"oracle": {
+			configuredVersion:           "12.1.0.2.v20",
+			actualVersion:               "19.0.0.0.ru-2021-04.rur-2021-04.r1",
+			expectedEngineVersion:       "19.0.0.0.ru-2021-04.rur-2021-04.r1",
+			expectedEngineVersionActual: "19.0.0.0.ru-2021-04.rur-2021-04.r1",
+		},
+		"oracle - used to crash": {
+			configuredVersion:           "19.0.0.0.ru-2021-04.rur-2021-04.r1",
+			actualVersion:               "12.1.0.2.v20",
+			expectedEngineVersion:       "12.1.0.2.v20",
+			expectedEngineVersionActual: "12.1.0.2.v20",
+		},
+	}
+
+	for name, test := range tests {
+		name, test := name, test
+		t.Run(name, func(t *testing.T) {
+			r := ResourceCluster()
+			d := r.Data(nil)
+			compareActualEngineVersion(d, test.configuredVersion, test.actualVersion)
+
+			if want, got := test.expectedEngineVersion, d.Get("engine_version"); got != want {
+				t.Errorf("unexpected engine_version; want: %q, got: %q", want, got)
+			}
+			if want, got := test.expectedEngineVersionActual, d.Get("engine_version_actual"); got != want {
+				t.Errorf("unexpected engine_version_actual; want: %q, got: %q", want, got)
+			}
+		})
+	}
+}