Stop recursive symlink expansion and honor cancellation

internal/validation/destination_test.go

@@ -2,10 +2,12 @@ package validation
 
 import (
 	"context"
+	"errors"
 	"os"
 	"path/filepath"
 	"strings"
 	"testing"
+	"time"
 )
 
 func TestResolveDestinationClassifiesCommittedPaths(t *testing.T) {
@@ -81,6 +83,65 @@ func TestResolveDestinationClassifiesCommittedPaths(t *testing.T) {
 	}
 }
 
+func TestResolveDestinationHandlesExpandingCyclesAndRepeatedLinks(t *testing.T) {
+	repository := newTreeRepository(t)
+	writeTreeFile(t, repository, "README.md", []byte("source\n"))
+	for name, target := range map[string]string{
+		"direct":   "direct/child",
+		"mutual-a": "mutual-b/child",
+		"mutual-b": "mutual-a/child",
+		"alias":    ".",
+	} {
+		if err := os.Symlink(target, filepath.Join(repository, name)); err != nil {
+			t.Fatal(err)
+		}
+	}
+	gitTree(t, repository, "add", "--all")
+	gitTree(t, repository, "commit", "-m", "capture recursive links")
+	tree, err := ReadCandidateTree(t.Context(), repository, strings.TrimSpace(gitTree(t, repository, "rev-parse", "HEAD")))
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, test := range []struct {
+		destination string
+		kind        DestinationKind
+	}{
+		{"direct", DestinationSymlinkCycle},
+		{"mutual-a", DestinationSymlinkCycle},
+		{"alias/alias/README.md", DestinationEntry},
+	} {
+		t.Run(test.destination, func(t *testing.T) {
+			type result struct {
+				destination Destination
+				err         error
+			}
+			done := make(chan result, 1)
+			ctx, cancel := context.WithTimeout(t.Context(), time.Second)
+			defer cancel()
+			go func() {
+				destination, err := tree.ResolveDestination(ctx, "README.md", test.destination)
+				done <- result{destination, err}
+			}()
+			select {
+			case got := <-done:
+				if got.err != nil || got.destination.Kind != test.kind {
+					t.Fatalf("resolution = %#v, %v; want %s", got.destination, got.err, test.kind)
+				}
+				if test.kind == DestinationEntry && got.destination.Path != "README.md" {
+					t.Fatalf("finite repeated link resolved to %q", got.destination.Path)
+				}
+			case <-ctx.Done():
+				t.Fatal("symlink resolution did not finish")
+			}
+		})
+	}
+	ctx, cancel := context.WithCancel(t.Context())
+	cancel()
+	if _, err := tree.ResolveDestination(ctx, "README.md", "alias/README.md"); !errors.Is(err, context.Canceled) {
+		t.Fatalf("canceled resolution error = %v", err)
+	}
+}
+
 func TestResolveDestinationIgnoresNeighboringFiles(t *testing.T) {
 	repository := newTreeRepository(t)
 	writeTreeFile(t, repository, "docs/page.md", []byte("source\n"))