Add lint for db_instance's engine attribute

docs/rules/README.md

@@ -11,6 +11,7 @@ These rules warn of possible errors that can occur at `terraform apply`. Rules m
 |aws_alb_invalid_security_group|✔|
 |aws_alb_invalid_subnet|✔|
 |aws_db_instance_invalid_db_subnet_group|✔|
+|[aws_db_instance_invalid_engine](aws_db_instance_invalid_engine.md)||
 |aws_db_instance_invalid_option_group|✔|
 |aws_db_instance_invalid_parameter_group|✔|
 |aws_db_instance_invalid_type||

docs/rules/aws_db_instance_invalid_engine.md

@@ -0,0 +1,38 @@
+# aws_db_instance_invalid_engine
+
+Disallow using invalid engine name.
+
+## Example
+
+```hcl
+resource "aws_db_instance" "default" {
+  allocated_storage    = 20
+  engine               = "mysql57" // invalid engine name!
+  engine_version       = "5.7"
+  instance_class       = "db.t2.micro"
+  name                 = "mydb"
+  username             = "foo"
+  password             = "bar"
+  db_subnet_group_name = "my_database_subnet_group"
+  parameter_group_name = "default.mysql5.6"
+}
+```
+
+```
+$ tflint
+1 issue(s) found:
+
+Warning: "mysql57" is invalid engine. (aws_db_invalid_engine)
+
+  on template.tf line 3:
+   3:   engine               = "mysql57" // invalid engine name!
+
+```
+
+## Why
+
+Apply will fail. (Plan will succeed with the invalid value though)
+
+## How To Fix
+
+Select valid engine name according to the [document](https://docs.aws.amazon.com/cli/latest/reference/rds/describe-db-engine-versions.html#options)

rules/aws_db_instance_invalid_engine.go

@@ -0,0 +1,80 @@
+package rules
+
+import (
+	"fmt"
+
+	hcl "github.com/hashicorp/hcl/v2"
+	"github.com/terraform-linters/tflint-plugin-sdk/tflint"
+	"github.com/terraform-linters/tflint-ruleset-aws/project"
+)
+
+// AwsDBInstanceInvalidEngineRule checks whether "aws_db_instance" has invalid engine.
+type AwsDBInstanceInvalidEngineRule struct {
+	resource      string
+	attributeName string
+	engines       map[string]bool
+}
+
+// NewAwsDBInstanceInvalidEngineRule returns new rule with default attributes
+func NewAwsDBInstanceInvalidEngineRule() *AwsDBInstanceInvalidEngineRule {
+	return &AwsDBInstanceInvalidEngineRule{
+		resource:      "aws_db_instance",
+		attributeName: "engine",
+		engines: map[string]bool{
+			// https://docs.aws.amazon.com/cli/latest/reference/rds/describe-db-engine-versions.html#options
+			"aurora":            true,
+			"aurora-mysql":      true,
+			"aurora-postgresql": true,
+			"mariadb":           true,
+			"mysql":             true,
+			"oracle-ee":         true,
+			"oracle-se2":        true,
+			"oracle-se1":        true,
+			"oracle-se":         true,
+			"postgres":          true,
+			"sqlserver-ee":      true,
+			"sqlserver-se":      true,
+			"sqlserver-ex":      true,
+			"sqlserver-web":     true,
+		},
+	}
+}
+
+// Name returns the rule name
+func (r *AwsDBInstanceInvalidEngineRule) Name() string {
+	return "aws_db_instance_invalid_engine"
+}
+
+// Enabled returns whether the rule is enabled by default
+func (r *AwsDBInstanceInvalidEngineRule) Enabled() bool {
+	return true
+}
+
+// Severity returns the rule severity
+func (r *AwsDBInstanceInvalidEngineRule) Severity() string {
+	return tflint.ERROR
+}
+
+// Link returns the rule reference link
+func (r *AwsDBInstanceInvalidEngineRule) Link() string {
+	return project.ReferenceLink(r.Name())
+}
+
+// Check checks whether "aws_db_instance" has invalid engine.
+func (r *AwsDBInstanceInvalidEngineRule) Check(runner tflint.Runner) error {
+	return runner.WalkResourceAttributes(r.resource, r.attributeName, func(attribute *hcl.Attribute) error {
+		var engine string
+		err := runner.EvaluateExpr(attribute.Expr, &engine, nil)
+
+		return runner.EnsureNoError(err, func() error {
+			if !r.engines[engine] {
+				runner.EmitIssueOnExpr(
+					r,
+					fmt.Sprintf("\"%s\" is invalid engine.", engine),
+					attribute.Expr,
+				)
+			}
+			return nil
+		})
+	})
+}

rules/aws_db_instance_invalid_engine_test.go

@@ -0,0 +1,55 @@
+package rules
+
+import (
+	"testing"
+
+	hcl "github.com/hashicorp/hcl/v2"
+	"github.com/terraform-linters/tflint-plugin-sdk/helper"
+)
+
+func Test_AwsDBInstanceInvalidEngine(t *testing.T) {
+	cases := []struct {
+		Name     string
+		Content  string
+		Expected helper.Issues
+	}{
+		{
+			Name: "aurora-postgres is invalid",
+			Content: `
+resource "aws_db_instance" "aurora_postgresql" {
+    engine = "aurora-postgres"
+}`,
+			Expected: helper.Issues{
+				{
+					Rule:    NewAwsDBInstanceInvalidEngineRule(),
+					Message: "\"aurora-postgres\" is invalid engine.",
+					Range: hcl.Range{
+						Filename: "resource.tf",
+						Start:    hcl.Pos{Line: 3, Column: 14},
+						End:      hcl.Pos{Line: 3, Column: 31},
+					},
+				},
+			},
+		},
+		{
+			Name: "aurora-postgresql is valid",
+			Content: `
+resource "aws_db_instance" "aurora_postgresql" {
+    engine = "aurora-postgresql"
+}`,
+			Expected: helper.Issues{},
+		},
+	}
+
+	rule := NewAwsDBInstanceInvalidEngineRule()
+
+	for _, tc := range cases {
+		runner := helper.TestRunner(t, map[string]string{"resource.tf": tc.Content})
+
+		if err := rule.Check(runner); err != nil {
+			t.Fatalf("Unexpected error occurred: %s", err)
+		}
+
+		helper.AssertIssues(t, tc.Expected, runner.Issues)
+	}
+}

rules/provider.go

@@ -8,6 +8,7 @@ import (
 // Rules is a list of all rules
 var Rules = append([]tflint.Rule{
 	NewAwsDBInstanceDefaultParameterGroupRule(),
+	NewAwsDBInstanceInvalidEngineRule(),
 	NewAwsDBInstanceInvalidTypeRule(),
 	NewAwsDBInstancePreviousTypeRule(),
 	NewAwsDynamoDBTableInvalidStreamViewTypeRule(),