Merge pull request #630 from shipperizer/ds/application

#630

## Description

Creation of a `juju_application` data source

The data source can be used in cases where deployment topology is split between different teams but there is still a lot of inter-operation between services 

As any data source this prevents hardcoding of values, in this case application names, without validation 


## Type of change

- Add new resource

## Environment

- Juju controller version: `3.5+`

- Terraform version: `1.8.2+`

## QA steps

Manual QA steps should be done to test this PR.

```tf
terraform {
 required_providers {
 juju = {
 source = "juju/juju"
 version = "> 0.14.0"
 }
 }

 required_version = ">= 1.5.0"
}

variable "revision" {
 type = string
}
########################

resource "juju_application" "hydra" {
 model = "test"
 trust = true

 charm {
 name = "hydra"
 channel = "latest/edge"
 base = "ubuntu@22.04"
 revision = var.revision
 }
}


### HYDRA dependent ###
data "juju_application" "hydra" {
 model = juju_application.hydra.model
 name = juju_application.hydra.name
}


```

docs/data-sources/application.md

@@ -0,0 +1,21 @@
+---
+# generated by https://github.com/hashicorp/terraform-plugin-docs
+page_title: "juju_application Data Source - terraform-provider-juju"
+subcategory: ""
+description: |-
+  A data source that represents a single Juju application deployment from a charm.
+---
+
+# juju_application (Data Source)
+
+A data source that represents a single Juju application deployment from a charm.
+
+
+
+<!-- schema generated by tfplugindocs -->
+## Schema
+
+### Required
+
+- `model` (String) The name of the model where the application is deployed.
+- `name` (String) Name of the application.

docs/index.md

@@ -26,6 +26,7 @@ Today this provider allows you to manage the following via resources:
 
 and refer to the following via data sources:
 
+* Applications
 * Machines
 * Models
 * Offers

internal/provider/data_source_application.go

@@ -0,0 +1,142 @@
+// Copyright 2024 Canonical Ltd.
+// Licensed under the Apache License, Version 2.0, see LICENCE file for details.
+
+package provider
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/hashicorp/terraform-plugin-framework/datasource"
+	"github.com/hashicorp/terraform-plugin-framework/datasource/schema"
+	"github.com/hashicorp/terraform-plugin-framework/types"
+	"github.com/hashicorp/terraform-plugin-log/tflog"
+
+	"github.com/juju/terraform-provider-juju/internal/juju"
+)
+
+// Ensure provider defined types fully satisfy framework interfaces.
+var _ datasource.DataSourceWithConfigure = &applicationDataSource{}
+
+// NewApplicationDataSource returns a new data source for a Juju application.
+func NewApplicationDataSource() datasource.DataSourceWithConfigure {
+	return &applicationDataSource{}
+}
+
+type applicationDataSource struct {
+	client *juju.Client
+
+	// context for the logging subsystem.
+	subCtx context.Context
+}
+
+type applicationDataSourceModel struct {
+	ApplicationName types.String `tfsdk:"name"`
+	ModelName       types.String `tfsdk:"model"`
+}
+
+// Metadata returns the full data source name as used in terraform plans.
+func (d *applicationDataSource) Metadata(_ context.Context, req datasource.MetadataRequest, resp *datasource.MetadataResponse) {
+	resp.TypeName = req.ProviderTypeName + "_application"
+}
+
+// Schema returns the schema for the application data source.
+func (d *applicationDataSource) Schema(_ context.Context, _ datasource.SchemaRequest, resp *datasource.SchemaResponse) {
+	resp.Schema = schema.Schema{
+		Description: "A data source that represents a single Juju application deployment from a charm.",
+		Attributes: map[string]schema.Attribute{
+			"name": schema.StringAttribute{
+				Description: "Name of the application.",
+				Required:    true,
+			},
+			"model": schema.StringAttribute{
+				Description: "The name of the model where the application is deployed.",
+				Required:    true,
+			},
+		},
+	}
+}
+
+// Configure enables provider-level data or clients to be set in the
+// provider-defined DataSource type. It is separately executed for each
+// ReadDataSource RPC.
+func (d *applicationDataSource) Configure(ctx context.Context, req datasource.ConfigureRequest, resp *datasource.ConfigureResponse) {
+	// Prevent panic if the provider has not been configured.
+	if req.ProviderData == nil {
+		return
+	}
+
+	client, ok := req.ProviderData.(*juju.Client)
+	if !ok {
+		resp.Diagnostics.AddError(
+			"Unexpected Data Source Configure Type",
+			fmt.Sprintf("Expected *juju.Client, got: %T. Please report this issue to the provider developers.", req.ProviderData),
+		)
+		return
+	}
+
+	d.client = client
+	d.subCtx = tflog.NewSubsystem(ctx, LogDataSourceApplication)
+}
+
+// Read is called when the provider must read resource values in order
+// to update state. Planned state values should be read from the
+// ReadRequest and new state values set on the ReadResponse.
+// Take the juju api input from the ID, it may not exist in the plan.
+// Only set optional values if they exist.
+func (d *applicationDataSource) Read(ctx context.Context, req datasource.ReadRequest, resp *datasource.ReadResponse) {
+	// Prevent panic if the provider has not been configured.
+	if d.client == nil {
+		addDSClientNotConfiguredError(&resp.Diagnostics, "application")
+		return
+	}
+	var data applicationDataSourceModel
+
+	// Read Terraform prior data into the application
+	resp.Diagnostics.Append(req.Config.Get(ctx, &data)...)
+	if resp.Diagnostics.HasError() {
+		return
+	}
+
+	appName := data.ApplicationName.ValueString()
+	modelName := data.ModelName.ValueString()
+	d.trace("Read", map[string]interface{}{
+		"Model": modelName,
+		"Name":  appName,
+	})
+
+	response, err := d.client.Applications.ReadApplication(&juju.ReadApplicationInput{
+		ModelName: modelName,
+		AppName:   appName,
+	})
+	if err != nil {
+		resp.Diagnostics.Append(handleApplicationNotFoundError(ctx, err, &resp.State)...)
+		return
+	}
+	if response == nil {
+		return
+	}
+	d.trace("read application", map[string]interface{}{"resource": appName, "response": response})
+
+	data.ApplicationName = types.StringValue(appName)
+	data.ModelName = types.StringValue(modelName)
+
+	d.trace("Found", applicationDataSourceModelForLogging(ctx, &data))
+	resp.Diagnostics.Append(resp.State.Set(ctx, &data)...)
+}
+
+func (d *applicationDataSource) trace(msg string, additionalFields ...map[string]interface{}) {
+	if d.subCtx == nil {
+		return
+	}
+
+	tflog.SubsystemTrace(d.subCtx, LogDataSourceApplication, msg, additionalFields...)
+}
+
+func applicationDataSourceModelForLogging(_ context.Context, app *applicationDataSourceModel) map[string]interface{} {
+	value := map[string]interface{}{
+		"application-name": app.ApplicationName.ValueString(),
+		"model":            app.ModelName.ValueString(),
+	}
+	return value
+}

internal/provider/data_source_application_test.go

@@ -0,0 +1,104 @@
+// Copyright 2024 Canonical Ltd.
+// Licensed under the Apache License, Version 2.0, see LICENCE file for details.
+
+package provider
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/hashicorp/terraform-plugin-testing/helper/acctest"
+	"github.com/hashicorp/terraform-plugin-testing/helper/resource"
+)
+
+func TestAcc_DataSourceApplicationLXD_Edge(t *testing.T) {
+	if testingCloud != LXDCloudTesting {
+		t.Skip(t.Name() + " only runs with LXD")
+	}
+	modelName := acctest.RandomWithPrefix("tf-datasource-application-test-model")
+	applicationName := acctest.RandStringFromCharSet(10, acctest.CharSetAlpha)
+
+	resource.ParallelTest(t, resource.TestCase{
+		PreCheck:                 func() { testAccPreCheck(t) },
+		ProtoV6ProviderFactories: frameworkProviderFactories,
+		Steps: []resource.TestStep{
+			{
+				Config: testAccDataSourceApplicationLXD(modelName, applicationName),
+				Check: resource.ComposeTestCheckFunc(
+					resource.TestCheckResourceAttr("data.juju_application.this", "model", modelName),
+					resource.TestCheckResourceAttr("data.juju_application.this", "name", applicationName),
+				),
+			},
+		},
+	})
+}
+
+func TestAcc_DataSourceApplicationK8s_Edge(t *testing.T) {
+	if testingCloud != MicroK8sTesting {
+		t.Skip(t.Name() + " only runs with MicroK8s")
+	}
+	modelName := acctest.RandomWithPrefix("tf-datasource-application-test-model")
+	applicationName := acctest.RandStringFromCharSet(10, acctest.CharSetAlpha)
+
+	resource.ParallelTest(t, resource.TestCase{
+		PreCheck:                 func() { testAccPreCheck(t) },
+		ProtoV6ProviderFactories: frameworkProviderFactories,
+		Steps: []resource.TestStep{
+			{
+				Config: testAccDataSourceApplicationK8s(modelName, applicationName),
+				Check: resource.ComposeTestCheckFunc(
+					resource.TestCheckResourceAttr("data.juju_application.this", "model", modelName),
+					resource.TestCheckResourceAttr("data.juju_application.this", "name", applicationName),
+				),
+			},
+		},
+	})
+}
+
+func testAccDataSourceApplicationLXD(modelName, applicationName string) string {
+	return fmt.Sprintf(`
+resource "juju_model" "model" {
+  name = %q
+}
+
+resource "juju_application" "this" {
+  name = %q
+  model = juju_model.model.name
+  trust = true
+
+  charm {
+    name     = "ubuntu"
+	channel  = "latest/stable"
+  }
+}
+
+data "juju_application" "this" {
+  model = juju_model.model.name
+  name = juju_application.this.name
+
+}`, modelName, applicationName)
+}
+
+func testAccDataSourceApplicationK8s(modelName, applicationName string) string {
+	return fmt.Sprintf(`
+resource "juju_model" "model" {
+  name = %q
+}
+
+resource "juju_application" "this" {
+  name = %q
+  model = juju_model.model.name
+  trust = true
+
+  charm {
+    name     = "zinc-k8s"
+	channel  = "latest/stable"
+  }
+}
+
+data "juju_application" "this" {
+  model = juju_model.model.name
+  name = juju_application.this.name
+
+}`, modelName, applicationName)
+}

internal/provider/helpers.go

@@ -16,10 +16,11 @@ import (
 //
 //	@module=juju.resource-application
 const (
-	LogDataSourceMachine = "datasource-machine"
-	LogDataSourceModel   = "datasource-model"
-	LogDataSourceOffer   = "datasource-offer"
-	LogDataSourceSecret  = "datasource-secret"
+	LogDataSourceApplication = "datasource-application"
+	LogDataSourceMachine     = "datasource-machine"
+	LogDataSourceModel       = "datasource-model"
+	LogDataSourceOffer       = "datasource-offer"
+	LogDataSourceSecret      = "datasource-secret"
 
 	LogResourceApplication     = "resource-application"
 	LogResourceAccessModel     = "resource-access-model"

internal/provider/provider.go

@@ -389,6 +389,7 @@ func (p *jujuProvider) Resources(_ context.Context) []func() resource.Resource {
 // the Metadata method. All data sources must have unique names.
 func (p *jujuProvider) DataSources(_ context.Context) []func() datasource.DataSource {
 	return []func() datasource.DataSource{
+		func() datasource.DataSource { return NewApplicationDataSource() },
 		func() datasource.DataSource { return NewMachineDataSource() },
 		func() datasource.DataSource { return NewModelDataSource() },
 		func() datasource.DataSource { return NewOfferDataSource() },

templates/index.md.tmpl

@@ -26,6 +26,7 @@ Today this provider allows you to manage the following via resources:
 
 and refer to the following via data sources:
 
+* Applications
 * Machines
 * Models
 * Offers