backend: Remove Operation.Parallelism field

The presence of this field was confusing because in practice the local
backend doesn't use it for anything and the remote backend was using it
only to return an error if it's set to anything other than the default,
under the assumption that it would always match ContextOpts.Parallelism.

The "command" package is the one actually responsible for handling this
option, and it does so by placing it into the partial ContextOpts which it
passes into the backend when preparing for a local operation. To make that
clearer, here we remove Operation.Parallelism and change the few uses of
it to refer to ContextOpts.Parallelism instead, so that everyone is
reading and writing this value from the same place.

internal/backend/backend.go

@@ -249,7 +249,6 @@ type Operation struct {
 	// behavior of the operation.
 	PlanMode     plans.Mode
 	AutoApprove  bool
-	Parallelism  int
 	Targets      []addrs.Targetable
 	ForceReplace []addrs.AbsResourceInstance
 	Variables    map[string]UnparsedVariableValue

internal/backend/remote/backend_apply.go

@@ -42,7 +42,7 @@ func (b *Remote) opApply(stopCtx, cancelCtx context.Context, op *backend.Operati
 		return nil, diags.Err()
 	}
 
-	if op.Parallelism != defaultParallelism {
+	if b.ContextOpts != nil && b.ContextOpts.Parallelism != defaultParallelism {
 		diags = diags.Append(tfdiags.Sourceless(
 			tfdiags.Error,
 			"Custom parallelism values are currently not supported",

internal/backend/remote/backend_apply_test.go

@@ -46,7 +46,6 @@ func testOperationApplyWithTimeout(t *testing.T, configDir string, timeout time.
 	return &backend.Operation{
 		ConfigDir:    configDir,
 		ConfigLoader: configLoader,
-		Parallelism:  defaultParallelism,
 		PlanRefresh:  true,
 		StateLocker:  clistate.NewLocker(timeout, stateLockerView),
 		Type:         backend.OperationTypeApply,
@@ -223,7 +222,10 @@ func TestRemote_applyWithParallelism(t *testing.T) {
 	op, configCleanup, done := testOperationApply(t, "./testdata/apply")
 	defer configCleanup()
 
-	op.Parallelism = 3
+	if b.ContextOpts == nil {
+		b.ContextOpts = &terraform.ContextOpts{}
+	}
+	b.ContextOpts.Parallelism = 3
 	op.Workspace = backend.DefaultStateName
 
 	run, err := b.Operation(context.Background(), op)

internal/backend/remote/backend_plan.go

@@ -38,7 +38,7 @@ func (b *Remote) opPlan(stopCtx, cancelCtx context.Context, op *backend.Operatio
 		return nil, diags.Err()
 	}
 
-	if op.Parallelism != defaultParallelism {
+	if b.ContextOpts != nil && b.ContextOpts.Parallelism != defaultParallelism {
 		diags = diags.Append(tfdiags.Sourceless(
 			tfdiags.Error,
 			"Custom parallelism values are currently not supported",

internal/backend/remote/backend_plan_test.go

@@ -44,7 +44,6 @@ func testOperationPlanWithTimeout(t *testing.T, configDir string, timeout time.D
 	return &backend.Operation{
 		ConfigDir:    configDir,
 		ConfigLoader: configLoader,
-		Parallelism:  defaultParallelism,
 		PlanRefresh:  true,
 		StateLocker:  clistate.NewLocker(timeout, stateLockerView),
 		Type:         backend.OperationTypePlan,
@@ -198,7 +197,10 @@ func TestRemote_planWithParallelism(t *testing.T) {
 	op, configCleanup, done := testOperationPlan(t, "./testdata/plan")
 	defer configCleanup()
 
-	op.Parallelism = 3
+	if b.ContextOpts == nil {
+		b.ContextOpts = &terraform.ContextOpts{}
+	}
+	b.ContextOpts.Parallelism = 3
 	op.Workspace = backend.DefaultStateName
 
 	run, err := b.Operation(context.Background(), op)

internal/command/meta_backend.go

@@ -349,7 +349,6 @@ func (m *Meta) Operation(b backend.Backend) *backend.Operation {
 
 	return &backend.Operation{
 		PlanOutBackend: planOutBackend,
-		Parallelism:    m.parallelism,
 		Targets:        m.targets,
 		UIIn:           m.UIInput(),
 		UIOut:          m.Ui,