Track active symlink expansions precisely

internal/engine/filesystem_links.go

@@ -109,13 +109,26 @@ func (a *filesystemAccess) importEntryTarget(name string, entry fs.DirEntry, fol
 	return importEntryTarget{symlink: symlink, dir: info.IsDir(), identity: name}, nil
 }
 
+// A link stays active until its target has resolved, not until the entire
+// original pathname has resolved. Remaining segments may reuse a finished link.
+type importLinkFrame struct {
+	pending []string
+	link    string
+}
+
 func resolveHostImportPath(access *filesystemAccess, name string) (string, error) {
-	pending := strings.Split(filepath.ToSlash(name), "/")
+	frames := []importLinkFrame{{pending: strings.Split(filepath.ToSlash(name), "/")}}
 	resolved := ""
 	activeLinks := make(map[string]struct{})
-	for len(pending) > 0 {
-		segment := pending[0]
-		pending = pending[1:]
+	for len(frames) > 0 {
+		frame := &frames[len(frames)-1]
+		if len(frame.pending) == 0 {
+			delete(activeLinks, frame.link)
+			frames = frames[:len(frames)-1]
+			continue
+		}
+		segment := frame.pending[0]
+		frame.pending = frame.pending[1:]
 		candidate := path.Join(resolved, segment)
 		if candidate == "" {
 			candidate = "."
@@ -149,8 +162,7 @@ func resolveHostImportPath(access *filesystemAccess, name string) (string, error
 		if !withinPortableRoot(targetPath) {
 			return "", errImportPathOutsideRoot
 		}
-		remaining := append([]string(nil), pending...)
-		pending = append(strings.Split(targetPath, "/"), remaining...)
+		frames = append(frames, importLinkFrame{pending: strings.Split(targetPath, "/"), link: candidate})
 		resolved = ""
 	}
 	if resolved == "" {
@@ -167,12 +179,18 @@ func resolveNativeImportPath(access *filesystemAccess, filesystem fs.ReadLinkFS,
 	if !fs.ValidPath(name) || !access.contains(name) {
 		return "", fs.ErrInvalid
 	}
-	pending := strings.Split(name, "/")
+	frames := []importLinkFrame{{pending: strings.Split(name, "/")}}
 	resolved := ""
 	activeLinks := make(map[string]struct{})
-	for len(pending) > 0 {
-		segment := pending[0]
-		pending = pending[1:]
+	for len(frames) > 0 {
+		frame := &frames[len(frames)-1]
+		if len(frame.pending) == 0 {
+			delete(activeLinks, frame.link)
+			frames = frames[:len(frames)-1]
+			continue
+		}
+		segment := frame.pending[0]
+		frame.pending = frame.pending[1:]
 		candidate := path.Join(resolved, segment)
 		if candidate == "" {
 			candidate = "."
@@ -205,8 +223,7 @@ func resolveNativeImportPath(access *filesystemAccess, filesystem fs.ReadLinkFS,
 		if !fs.ValidPath(targetPath) || !access.contains(targetPath) {
 			return "", errImportPathOutsideRoot
 		}
-		remaining := append([]string(nil), pending...)
-		pending = append(strings.Split(targetPath, "/"), remaining...)
+		frames = append(frames, importLinkFrame{pending: strings.Split(targetPath, "/"), link: candidate})
 		resolved = ""
 	}
 	if resolved == "" {

internal/engine/filesystem_links_test.go

@@ -0,0 +1,47 @@
+package engine
+
+import (
+	"errors"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestImportLinkResolutionTracksActiveExpansions(t *testing.T) {
+	root := t.TempDir()
+	if err := os.Mkdir(filepath.Join(root, "parts"), 0o700); err != nil {
+		t.Fatal(err)
+	}
+	for name, target := range map[string]string{"parts/up": "..", "self": "self", "first": "second", "second": "first"} {
+		if err := os.Symlink(target, filepath.Join(root, filepath.FromSlash(name))); err != nil {
+			t.Skipf("symlinks unavailable: %v", err)
+		}
+	}
+	host, err := openHostAccess(root)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer host.close()
+	filesystem := os.DirFS(root)
+	links, ok := filesystem.(fs.ReadLinkFS)
+	if !ok {
+		t.Fatal("native directory filesystem does not expose link inspection")
+	}
+	native := newNativeAccess(filesystem, ".", &namespaceToken{})
+	for name, resolve := range map[string]func(string) (string, error){
+		"host":   func(name string) (string, error) { return resolveHostImportPath(host, name) },
+		"native": func(name string) (string, error) { return resolveNativeImportPath(native, links, name) },
+	} {
+		t.Run(name, func(t *testing.T) {
+			if got, err := resolve("parts/up/parts/up/parts"); err != nil || got != "parts" {
+				t.Errorf("finite link reuse = %q, %v", got, err)
+			}
+			for _, cycle := range []string{"self", "first"} {
+				if _, err := resolve(cycle); !errors.Is(err, errImportDirectoryLinkCycle) {
+					t.Errorf("%s cycle = %v", cycle, err)
+				}
+			}
+		})
+	}
+}