feat(application): Adding custom plan validator for ResourceKey

ResourceKey schema is validated using a custom PlanValidator named StringIsResourceKeyValidator to to ensure that the string is an int or OCI image information as a URL.
This will allow for failure during the planning phase if the values are not as expected.

Signed-off-by: gatici <[email protected]>

internal/juju/resources.go

@@ -124,10 +124,7 @@ func (osFilesystem) Stat(name string) (os.FileInfo, error) {
 // Used to detect resources which are given with revision number
 func isInt(s string) bool {
 	_, err := strconv.Atoi(s)
-	if err != nil {
-		return false
-	}
-	return true
+	return err == nil
 }
 
 // Upload sends the provided resource blob up to Juju.

internal/provider/resource_application.go

@@ -55,7 +55,7 @@ There are a few scenarios that need to be considered:
 
 * A resource can be added or changed at any time. If the charm has resources and none are specified in the plan, Juju will use the resource defined in the charm's specified channel. Juju does not allow resources to be removed from an application.
 
-* If a charm is refreshed, by changing the charm revision or channel, the resource is also refreshed to the current defined channel listed for the charm if the resource is specified by revision. This is normal behavior for juju but not typical behavior for terraform.
+* If a charm is refreshed, by changing the charm revision or channel, the resource is also refreshed to the current defined channel listed for the charm if the resource is specified by revision. Please note that this is normal behavior for Juju but not typical behavior for Terraform.
 
 * Resources specified by URL to an OCI image repository will never be refreshed (upgraded) by juju during a charm refresh unless explicitly changed in the plan.
 `
@@ -272,8 +272,11 @@ func (r *applicationResource) Schema(_ context.Context, _ resource.SchemaRequest
 				},
 			},
 			ResourceKey: schema.MapAttribute{
-				Optional:            true,
-				ElementType:         types.StringType,
+				Optional:    true,
+				ElementType: types.StringType,
+				Validators: []validator.Map{
+					StringIsResourceKeyValidator{},
+				},
 				MarkdownDescription: resourceKeyMarkdownDescription,
 			},
 		},

internal/provider/validator_resourcekey.go

@@ -0,0 +1,50 @@
+// Copyright 2024 Canonical Ltd.
+// Licensed under the AGPLv3, see LICENCE file for details.
+
+package provider
+
+import (
+	"context"
+	"fmt"
+	"strconv"
+
+	"github.com/hashicorp/terraform-plugin-framework/schema/validator"
+)
+
+type StringIsResourceKeyValidator struct{}
+
+// Description returns a plain text description of the validator's behavior, suitable for a practitioner to understand its impact.
+func (v StringIsResourceKeyValidator) Description(context.Context) string {
+	return "string must conform to a charm resource: a resource revision number from CharmHub or a custom OCI image resource"
+}
+
+// MarkdownDescription returns a markdown formatted description of the validator's behavior, suitable for a practitioner to understand its impact.
+func (v StringIsResourceKeyValidator) MarkdownDescription(context.Context) string {
+	return resourceKeyMarkdownDescription
+}
+
+// ValidateMap Validate runs the main validation logic of the validator, reading configuration data out of `req` and updating `resp` with diagnostics.
+func (v StringIsResourceKeyValidator) ValidateMap(ctx context.Context, req validator.MapRequest, resp *validator.MapResponse) {
+	// If the value is unknown or null, there is nothing to validate.
+	if req.ConfigValue.IsUnknown() || req.ConfigValue.IsNull() {
+		return
+	}
+
+	var resourceKey map[string]string
+	resp.Diagnostics.Append(req.ConfigValue.ElementsAs(ctx, &resourceKey, false)...)
+	if resp.Diagnostics.HasError() {
+		return
+	}
+	for name, value := range resourceKey {
+		if isInt(value) {
+			providedRev, err := strconv.Atoi(value)
+			if err != nil || providedRev <= 0 {
+				resp.Diagnostics.AddAttributeError(
+					req.Path,
+					"Invalid Resource Revision",
+					fmt.Sprintf("value of %q is expected to be a valid revision number: %s", name, err),
+				)
+			}
+		}
+	}
+}

internal/provider/validator_resourcekey_test.go

@@ -0,0 +1,12 @@
+// Copyright 2024 Canonical Ltd.
+// Licensed under the AGPLv3, see LICENCE file for details.
+
+package provider_test
+
+import (
+	"testing"
+)
+
+func TestResourceKeyValidatorValid(t *testing.T) {
+
+}