You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: change_exec_test.go
    path of file: ./repos/terraform/internal/stacks/stackruntime/internal/stackeval
    
    the code of the file until where you have to start completion: // Copyright (c) HashiCorp, Inc.
// SPDX-License-Identifier: BUSL-1.1

package stackeval

import (
	"context"
	"sync"
	"testing"

	"github.com/google/go-cmp/cmp"
	"github.com/hashicorp/terraform/internal/addrs"
	"github.com/hashicorp/terraform/internal/promising"
	"github.com/hashicorp/terraform/internal/stacks/stackaddrs"
	"github.com/hashicorp/terraform/internal/states"
	"github.com/hashicorp/terraform/internal/tfdiags"
	"github.com/zclconf/go-cty-debug/ctydebug"
	"github.com/zclconf/go-cty/cty"
)

func TestChangeExec(t *testing.T) {
	ctx := context.Background()

	type FakeMain struct {
		results *ChangeExecResults
	}
	instAAddr := stackaddrs.AbsComponentInstance{
		Stack: stackaddrs.RootStackInstance,
		Item: stackaddrs.ComponentInstance{
			Component: stackaddrs.Component{
				Name: "A",
			},
		},
	}
	instBAddr := stackaddrs.AbsComponentInstance{
		Stack: stackaddrs.RootStackInstance,
		Item: stackaddrs.ComponentInstance{
			Component: stackaddrs.Component{
				Name: "B",
			},
		},
	}
	// We don't actually register a task for instCAddr; this one's here
	// to test how we handle requesting the result from an unregistered task.
	instCAddr := stackaddrs.AbsComponentInstance{
		Stack: stackaddrs.RootStackInstance,
		Item: stackaddrs.ComponentInstance{
			Component: stackaddrs.Component{
				Name: "C",
			},
		},
	}
	valueAddr := addrs.OutputValue{Name: "v"}.Absolute(addrs.RootModuleInstance)

	_, err := promising.MainTask(ctx, func(ctx context.Context) (FakeMain, error) {
		changeResults, begin := ChangeExec(ctx, func(ctx context.Context, reg *ChangeExecRegistry[FakeMain]) {
			t.Logf("begin setup phase")
			reg.RegisterComponentInstanceChange(ctx, instAAddr, func(ctx context.Context, main FakeMain) (*ComponentInstanceApplyResult, tfdiags.Diagnostics) {
				
    
func TestChangeExec(t *testing.T) {
	ctx := context.Background()

	type FakeMain struct {
		results *ChangeExecResults
	}
	instAAddr := stackaddrs.AbsComponentInstance{
		Stack: stackaddrs.RootStackInstance,
		Item: stackaddrs.ComponentInstance{
			Component: stackaddrs.Component{
				Name: "A",
			},
		},
	}
	instBAddr := stackaddrs.AbsComponentInstance{
		Stack: stackaddrs.RootStackInstance,
		Item: stackaddrs.ComponentInstance{
			Component: stackaddrs.Component{
				Name: "B",
			},
		},
	}
	// We don't actually register a task for instCAddr; this one's here
	// to test how we handle requesting the result from an unregistered task.
	instCAddr := stackaddrs.AbsComponentInstance{
		Stack: stackaddrs.RootStackInstance,
		Item: stackaddrs.ComponentInstance{
			Component: stackaddrs.Component{
				Name: "C",
			},
		},
	}
	valueAddr := addrs.OutputValue{Name: "v"}.Absolute(addrs.RootModuleInstance)

	_, err := promising.MainTask(ctx, func(ctx context.Context) (FakeMain, error) {
		changeResults, begin := ChangeExec(ctx, func(ctx context.Context, reg *ChangeExecRegistry[FakeMain]) {
			t.Logf("begin setup phase")
			reg.RegisterComponentInstanceChange(ctx, instAAddr, func(ctx context.Context, main FakeMain) (*ComponentInstanceApplyResult, tfdiags.Diagnostics) {
				t.Logf("producing result for A")
				return &ComponentInstanceApplyResult{
					FinalState: states.BuildState(func(ss *states.SyncState) {
						ss.SetOutputValue(valueAddr, cty.StringVal("a"), false)
					}),
				}, nil
			})
			reg.RegisterComponentInstanceChange(ctx, instBAddr, func(ctx context.Context, main FakeMain) (*ComponentInstanceApplyResult, tfdiags.Diagnostics) {
				t.Logf("B is waiting for A")
				aState, _, err := main.results.ComponentInstanceResult(ctx, instAAddr)
				if err != nil {
					return nil, nil
				}
				t.Logf("producing result for B")
				aOutputVal := aState.FinalState.OutputValue(valueAddr)
				if aOutputVal == nil {
					return nil, nil
				}
				return &ComponentInstanceApplyResult{
					FinalState: states.BuildState(func(ss *states.SyncState) {
						ss.SetOutputValue(
							valueAddr, cty.TupleVal([]cty.Value{aOutputVal.Value, cty.StringVal("b")}), false,
						)
					}),
				}, nil
			})
			t.Logf("end setup phase")
		})

		main := FakeMain{
			results: changeResults,
		}

		// We must call "begin" before this task returns, since internally
		// there's now a promise that our task is responsible for resolving.
		t.Logf("about to start execution phase")
		begin(ctx, main)

		// Now we'll pretend that we're doing normal stackeval stuff that
		// involves some interdependencies between the results. Specifically,
		// the "B" task depends on the result from the "A" task.
		var wg sync.WaitGroup
		wg.Add(3)
		var gotAResult, gotBResult, gotCResult *ComponentInstanceApplyResult
		var errA, errB, errC error
		promising.AsyncTask(ctx, promising.NoPromises, func(ctx context.Context, _ promising.PromiseContainer) {
			t.Logf("requesting result C")
			gotCResult, _, errC = main.results.ComponentInstanceResult(ctx, instCAddr)
			t.Logf("got result C")
			wg.Done()
		})
		promising.AsyncTask(ctx, promising.NoPromises, func(ctx context.Context, _ promising.PromiseContainer) {
			t.Logf("requesting result B")
			gotBResult, _, errB = main.results.ComponentInstanceResult(ctx, instBAddr)
			t.Logf("got result B")
			wg.Done()
		})
		promising.AsyncTask(ctx, promising.NoPromises, func(ctx context.Context, _ promising.PromiseContainer) {
			t.Logf("requesting result A")
			gotAResult, _, errA = main.results.ComponentInstanceResult(ctx, instAAddr)
			t.Logf("got result A")
			wg.Done()
		})
		wg.Wait()

		if errA != nil {
			t.Errorf("A failed: %s", errA)
		}
		if errB != nil {
			t.Errorf("B failed: %s", errB)
		}
		if diff := cmp.Diff(ErrChangeExecUnregistered{instCAddr}, errC); diff != "" {
			t.Errorf("wrong error for C\n%s", diff)
		}
		if errA != nil || errB != nil {
			t.FailNow()
		}
		if gotAResult == nil {
			t.Fatal("A state is nil")
		}
		if gotBResult == nil {
			t.Fatal("B state is nil")
		}
		if gotCResult != nil {
			t.Fatal("C state isn't nil, but should have been")
		}

		gotAOutputVal := gotAResult.FinalState.OutputValue(valueAddr)
		if gotAOutputVal == nil {
			t.Fatal("A state has no value")
		}
		gotBOutputVal := gotBResult.FinalState.OutputValue(valueAddr)
		if gotBOutputVal == nil {
			t.Fatal("B state has no value")
		}

		gotAVal := gotAOutputVal.Value
		wantAVal := cty.StringVal("a")
		gotBVal := gotBOutputVal.Value
		wantBVal := cty.TupleVal([]cty.Value{wantAVal, cty.StringVal("b")})
		if diff := cmp.Diff(wantAVal, gotAVal, ctydebug.CmpOptions); diff != "" {
			t.Errorf("wrong result for A\n%s", diff)
		}
		if diff := cmp.Diff(wantBVal, gotBVal, ctydebug.CmpOptions); diff != "" {
			t.Errorf("wrong result for B\n%s", diff)
		}

		return main, nil
	})
	if err != nil {
		t.Fatal(err)
	}

}