feat: A plan file will report the state storage provider among its required providers, if PSS is in use.

See the code comment added in this commit. This addition does not impact an apply command as the missing provider will be detected before this code is executed. However I'm making this change so that the method is still accurate is being able to return a complete list of providers needed by the plan.

Author: SarahFrench

internal/plans/plan.go

@@ -181,9 +181,30 @@ func (p *Plan) ProviderAddrs() []addrs.AbsProviderConfig {
 	}
 
 	m := map[string]addrs.AbsProviderConfig{}
+
+	// Get all provider requirements from resources.
 	for _, rc := range p.Changes.Resources {
 		m[rc.ProviderAddr.String()] = rc.ProviderAddr
 	}
+
+	// Get the provider required for pluggable state storage, if that's in use.
+	//
+	// This check should be redundant as:
+	// 1) Any provider used for state storage would be in required_providers, which is checked separately elsewhere.
+	// 2) An apply operation that uses the planfile only checks the providers needed for the plan _after_ the operations backend
+	//    for the operation is set up, and that process will detect if the provider needed for state storage is missing.
+	//
+	// However, for completeness when describing the providers needed by a plan, it is included here.
+	if !p.StateStore.Empty() {
+		address := addrs.AbsProviderConfig{
+			Module:   addrs.RootModule, // A state_store block is only ever in the root module
+			Provider: *p.StateStore.Provider.Source,
+			// Alias: aliases are not permitted when using a provider for PSS.
+		}
+
+		m[p.StateStore.Provider.Source.String()] = address
+	}
+
 	if len(m) == 0 {
 		return nil
 	}

internal/plans/plan_test.go

@@ -15,9 +15,33 @@ import (
 )
 
 func TestProviderAddrs(t *testing.T) {
+	// Inputs for plan
+	provider := &Provider{}
+	err := provider.SetSource("registry.terraform.io/hashicorp/pluggable")
+	if err != nil {
+		panic(err)
+	}
+	err = provider.SetVersion("9.9.9")
+	if err != nil {
+		panic(err)
+	}
+	config, err := NewDynamicValue(cty.ObjectVal(map[string]cty.Value{
+		"foo": cty.StringVal("bar"),
+	}), cty.Object(map[string]cty.Type{
+		"foo": cty.String,
+	}))
+	if err != nil {
+		panic(err)
+	}
 
 	// Prepare plan
 	plan := &Plan{
+		StateStore: StateStore{
+			Type:      "pluggable_foobar",
+			Provider:  provider,
+			Config:    config,
+			Workspace: "default",
+		},
 		VariableValues: map[string]DynamicValue{},
 		Changes: &ChangesSrc{
 			Resources: []*ResourceInstanceChangeSrc{
@@ -70,6 +94,11 @@ func TestProviderAddrs(t *testing.T) {
 			Module:   addrs.RootModule,
 			Provider: addrs.NewDefaultProvider("test"),
 		},
+		// Provider used for pluggable state storage
+		{
+			Module:   addrs.RootModule,
+			Provider: addrs.NewDefaultProvider("pluggable"),
+		},
 	}
 
 	for _, problem := range deep.Equal(got, want) {