fix: add validation to cron (#159)

Author: kylecarbs

go.mod

@@ -46,6 +46,7 @@ require (
 	github.com/mitchellh/reflectwalk v1.0.2 // indirect
 	github.com/oklog/run v1.0.0 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
+	github.com/robfig/cron/v3 v3.0.1 // indirect
 	github.com/rogpeppe/go-internal v1.8.0 // indirect
 	github.com/vmihailenco/msgpack v4.0.4+incompatible // indirect
 	github.com/vmihailenco/msgpack/v4 v4.3.12 // indirect

go.sum

@@ -143,6 +143,8 @@ github.com/pkg/errors v0.8.1/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINE
 github.com/pkg/errors v0.9.1/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/robfig/cron/v3 v3.0.1 h1:WdRxkvbJztn8LMz/QEvLN5sBU+xKpSqwwUO1Pjr4qDs=
+github.com/robfig/cron/v3 v3.0.1/go.mod h1:eQICP3HwyT7UooqI/z+Ov+PtYAWygg1TEWWzGIFLtro=
 github.com/rogpeppe/go-internal v1.6.1/go.mod h1:xXDCJY+GAPziupqXw64V24skbSoqbTEfhy4qGm1nDQc=
 github.com/rogpeppe/go-internal v1.8.0 h1:FCbCCtXNOY3UtUuHUYaghJg4y7Fd14rXifAYUAtL9R8=
 github.com/rogpeppe/go-internal v1.8.0/go.mod h1:WmiCO8CzOY8rg0OYDC4/i/2WRWAB6poM+XZ2dLUbcbE=

provider/script.go

@@ -2,13 +2,17 @@ package provider
 
 import (
 	"context"
+	"fmt"
 
 	"github.com/google/uuid"
 	"github.com/hashicorp/terraform-plugin-sdk/v2/diag"
 	"github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema"
 	"github.com/hashicorp/terraform-plugin-sdk/v2/helper/validation"
+	"github.com/robfig/cron/v3"
 )
 
+var ScriptCRONParser = cron.NewParser(cron.Second | cron.Minute | cron.Hour | cron.Dom | cron.Month | cron.DowOptional | cron.Descriptor)
+
 func scriptResource() *schema.Resource {
 	return &schema.Resource{
 		Description: "Use this resource to run a script from an agent.",
@@ -56,6 +60,17 @@ func scriptResource() *schema.Resource {
 				Type:        schema.TypeString,
 				Optional:    true,
 				Description: "The cron schedule to run the script on. This is a cron expression.",
+				ValidateFunc: func(i interface{}, s string) ([]string, []error) {
+					v, ok := i.(string)
+					if !ok {
+						return []string{}, []error{fmt.Errorf("got type %T instead of string", i)}
+					}
+					_, err := ScriptCRONParser.Parse(v)
+					if err != nil {
+						return []string{}, []error{fmt.Errorf("%s is not a valid cron expression: %w", v, err)}
+					}
+					return nil, nil
+				},
 			},
 			"start_blocks_login": {
 				Type:        schema.TypeBool,