don't error on eval of missing instances

internal/terraform/context_plan2_test.go

@@ -5955,3 +5955,60 @@ output "staying" {
 		t.Fatalf("unexpected changes: %s", diff)
 	}
 }
+
+func TestContext2Plan_multiInstanceSelfRef(t *testing.T) {
+	// The postcondition here references self, but because instances are
+	// processed concurrently some instances may not be registered yet during
+	// evaluation. This should still evaluate without error, because we know our
+	// self value exists.
+	m := testModuleInline(t, map[string]string{
+		"main.tf": `
+resource "test_resource" "test" {
+}
+
+data "test_data_source" "foo" {
+  count = 100
+  lifecycle {
+    postcondition {
+      condition = self.attr == null
+      error_message = "error"
+    }
+  }
+  depends_on = [test_resource.test]
+}
+`,
+	})
+
+	p := new(testing_provider.MockProvider)
+	p.GetProviderSchemaResponse = getProviderSchemaResponseFromProviderSchema(&providerSchema{
+		DataSources: map[string]*configschema.Block{
+			"test_data_source": {
+				Attributes: map[string]*configschema.Attribute{
+					"attr": {
+						Type:     cty.String,
+						Computed: true,
+					},
+				},
+			},
+		},
+		ResourceTypes: map[string]*configschema.Block{
+			"test_resource": {
+				Attributes: map[string]*configschema.Attribute{
+					"attr": {
+						Type:     cty.String,
+						Computed: true,
+					},
+				},
+			},
+		},
+	})
+
+	ctx := testContext2(t, &ContextOpts{
+		Providers: map[addrs.Provider]providers.Factory{
+			addrs.NewDefaultProvider("test"): testProviderFuncFixed(p),
+		},
+	})
+
+	_, diags := ctx.Plan(m, states.NewState(), SimplePlanOpts(plans.NormalMode, testInputValuesUnset(m.Module.Variables)))
+	assertNoErrors(t, diags)
+}

internal/terraform/evaluate.go

@@ -681,15 +681,14 @@ func (d *evaluationStateData) GetResource(addr addrs.Resource, rng tfdiags.Sourc
 		// and need to be replaced by the planned value here.
 		if is.Current.Status == states.ObjectPlanned {
 			if change == nil {
-				// If the object is in planned status then we should not get
-				// here, since we should have found a pending value in the plan
-				// above instead.
-				diags = diags.Append(&hcl.Diagnostic{
-					Severity: hcl.DiagError,
-					Summary:  "Missing pending object in plan",
-					Detail:   fmt.Sprintf("Instance %s is marked as having a change pending but that change is not recorded in the plan. This is a bug in Terraform; please report it.", instAddr),
-					Subject:  &config.DeclRange,
-				})
+				// FIXME: This is usually an unfortunate case where we need to
+				// lookup an individual instance referenced via "self" for
+				// postconditions which we know exists, but because evaluation
+				// must always get the resource in aggregate some instance
+				// changes may not yet be registered.
+				instances[key] = cty.DynamicVal
+				// log the problem for debugging, since it may be a legitimate error we can't catch
+				log.Printf("[WARN] instance %s is marked as having a change pending but that change is not recorded in the plan", instAddr)
 				continue
 			}
 			instances[key] = change.After