check for cancellation before apply confirmation

When executing an apply with no plan, it's possible for a cancellation
to arrive during the final batch of provider operations, resulting in no
errors in the plan. The run context was next checked during the
confirmation for apply, but in the case of -auto-approve that
confirmation is skipped, resulting in the canceled plan being applied.

Make sure we directly check for cancellation before confirming the plan.

Author: jbardin

internal/backend/local/backend_apply.go

@@ -2,6 +2,7 @@ package local
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"log"
 
@@ -16,6 +17,9 @@ import (
 	"github.com/hashicorp/terraform/internal/tfdiags"
 )
 
+// test hook called between plan+apply during opApply
+var testHookStopPlanApply func()
+
 func (b *Local) opApply(
 	stopCtx context.Context,
 	cancelCtx context.Context,
@@ -88,6 +92,22 @@ func (b *Local) opApply(
 		mustConfirm := hasUI && !op.AutoApprove && !trivialPlan
 		op.View.Plan(plan, schemas)
 
+		if testHookStopPlanApply != nil {
+			testHookStopPlanApply()
+		}
+
+		// Check if we've been stopped before going through confirmation, or
+		// skipping confirmation in the case of -auto-approve.
+		// This can currently happen if a single stop request was received
+		// during the final batch of resource plan calls, so no operations were
+		// forced to abort, and no errors were returned from Plan.
+		if stopCtx.Err() != nil {
+			diags = diags.Append(errors.New("execution halted"))
+			runningOp.Result = backend.OperationFailure
+			op.ReportResult(runningOp, diags)
+			return
+		}
+
 		if mustConfirm {
 			var desc, query string
 			switch op.PlanMode {

internal/backend/local/backend_apply_test.go

@@ -351,3 +351,36 @@ func applyFixtureSchema() *terraform.ProviderSchema {
 		},
 	}
 }
+
+func TestApply_applyCanceledAutoApprove(t *testing.T) {
+	b := TestLocal(t)
+
+	TestLocalProvider(t, b, "test", applyFixtureSchema())
+
+	op, configCleanup, done := testOperationApply(t, "./testdata/apply")
+	op.AutoApprove = true
+	defer configCleanup()
+	defer func() {
+		output := done(t)
+		if !strings.Contains(output.Stderr(), "execution halted") {
+			t.Fatal("expected 'execution halted', got:\n", output.All())
+		}
+	}()
+
+	ctx, cancel := context.WithCancel(context.Background())
+	testHookStopPlanApply = cancel
+	defer func() {
+		testHookStopPlanApply = nil
+	}()
+
+	run, err := b.Operation(ctx, op)
+	if err != nil {
+		t.Fatalf("error starting operation: %v", err)
+	}
+
+	<-run.Done()
+	if run.Result == backend.OperationSuccess {
+		t.Fatal("expected apply operation to fail")
+	}
+
+}