command/show: Disable plan state lineage checks

When showing a saved plan, we do not need to check the state lineage
against current state, because the plan cannot be applied. This is
relevant when plan and apply specify a `-state` argument to choose a
non-default state file. In this case, the stored prior state in the plan
will not match the default state file, so a lineage check will always
error.

internal/backend/backend.go

@@ -275,6 +275,13 @@ type Operation struct {
 	// the variables set in the plan are used instead, and they must be valid.
 	AllowUnsetVariables bool
 
+	// When loading a plan file for a read-only operation, we may want to
+	// disable the state lineage checks which are only relevant for operations
+	// which can modify state. An example where this is important is showing
+	// a plan which was prepared against a non-default state file, because the
+	// lineage checks are always against the default state.
+	DisablePlanFileStateLineageChecks bool
+
 	// View implements the logic for all UI interactions.
 	View views.Operation
 

internal/backend/local/backend_local.go

@@ -284,7 +284,7 @@ func (b *Local) localRunForPlanFile(op *backend.Operation, pf *planfile.Reader,
 		))
 		return nil, snap, diags
 	}
-	if currentStateMeta != nil {
+	if !op.DisablePlanFileStateLineageChecks && currentStateMeta != nil {
 		// If the caller sets this, we require that the stored prior state
 		// has the same metadata, which is an extra safety check that nothing
 		// has changed since the plan was created. (All of the "real-world"

internal/command/show.go

@@ -94,6 +94,7 @@ func (c *ShowCommand) Run(args []string) int {
 	opReq.PlanFile = planFile
 	opReq.ConfigLoader, err = c.initConfigLoader()
 	opReq.AllowUnsetVariables = true
+	opReq.DisablePlanFileStateLineageChecks = true
 	if err != nil {
 		diags = diags.Append(err)
 		c.showDiagnostics(diags)

internal/command/show_test.go

@@ -15,7 +15,9 @@ import (
 	"github.com/hashicorp/terraform/internal/plans"
 	"github.com/hashicorp/terraform/internal/providers"
 	"github.com/hashicorp/terraform/internal/states"
+	"github.com/hashicorp/terraform/internal/states/statemgr"
 	"github.com/hashicorp/terraform/internal/terraform"
+	"github.com/hashicorp/terraform/version"
 	"github.com/mitchellh/cli"
 	"github.com/zclconf/go-cty/cty"
 )
@@ -575,6 +577,52 @@ func TestShow_json_output_state(t *testing.T) {
 	}
 }
 
+func TestShow_planWithNonDefaultStateLineage(t *testing.T) {
+	// Create a temporary working directory that is empty
+	td := tempDir(t)
+	testCopyDir(t, testFixturePath("show"), td)
+	defer os.RemoveAll(td)
+	defer testChdir(t, td)()
+
+	// Write default state file with a testing lineage ("fake-for-testing")
+	testStateFileDefault(t, testState())
+
+	// Create a plan with a different lineage, which we should still be able
+	// to show
+	_, snap := testModuleWithSnapshot(t, "show")
+	state := testState()
+	plan := testPlan(t)
+	stateMeta := statemgr.SnapshotMeta{
+		Lineage:          "fake-for-plan",
+		Serial:           1,
+		TerraformVersion: version.SemVer,
+	}
+	planPath := testPlanFileMatchState(t, snap, state, plan, stateMeta)
+
+	ui := cli.NewMockUi()
+	view, done := testView(t)
+	c := &ShowCommand{
+		Meta: Meta{
+			testingOverrides: metaOverridesForProvider(testProvider()),
+			Ui:               ui,
+			View:             view,
+		},
+	}
+
+	args := []string{
+		planPath,
+	}
+	if code := c.Run(args); code != 0 {
+		t.Fatalf("bad: \n%s", ui.ErrorWriter.String())
+	}
+
+	want := `No changes. Your infrastructure matches the configuration.`
+	got := done(t).Stdout()
+	if !strings.Contains(got, want) {
+		t.Errorf("missing expected output\nwant: %s\ngot:\n%s", want, got)
+	}
+}
+
 // showFixtureSchema returns a schema suitable for processing the configuration
 // in testdata/show. This schema should be assigned to a mock provider
 // named "test".