Share link dependency evaluation with host access

docs/roadmap/implementation.md

@@ -294,6 +294,12 @@ Focused check: `go test ./internal/engine -run 'Link|Directories|Glob'`.
 Finding: remaining host part of **C02**.
 Owners: `internal/engine/filesystem_links.go` and the same link/discovery tests.
 
+Status: **complete** (C02). Prior completed commits: `2736ddf`, `deb9aa5`,
+`ae3408a`, `ea27f17`, `08a8c25`, `877c497`, `8aba43d`, `7bf2f0c`.
+Validation passed: `env GOTOOLCHAIN=local GOPROXY=off GOTELEMETRY=off go test ./internal/engine -run 'Link|Directory|Glob|Filesystem|Discovery' -count=1`,
+`env GOTOOLCHAIN=local GOPROXY=off GOTELEMETRY=off go test ./internal/engine -count=1`,
+and `git diff --check`.
+
 Bind rooted host `Lstat`/`Readlink` operations to Stage 8's evaluator and retire the
 duplicate host frame loop. Retain portable component evaluation, rooted access,
 host-relative targets, and contained absolute targets translated with `filepath`.

internal/engine/filesystem_links.go

@@ -119,8 +119,8 @@ 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.
+// A link remains active until its own target resolves. Completed targets can be
+// reused while the caller continues resolving its remaining components.
 type importLinkFrame struct {
 	pending []string
 	link    string
@@ -190,38 +190,18 @@ func resolveImportLinkPath(name string, operations importLinkOperations) (string
 }
 
 func resolveHostImportPath(access *filesystemAccess, name string) (string, error) {
-	frames := []importLinkFrame{{pending: strings.Split(filepath.ToSlash(name), "/")}}
-	resolved := ""
-	activeLinks := make(map[string]struct{})
-	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 = "."
-		}
-		info, err := access.root.Lstat(filepath.FromSlash(candidate))
-		if err != nil {
-			return "", err
-		}
-		if info.Mode()&fs.ModeSymlink == 0 {
-			resolved = candidate
-			continue
-		}
-		if _, exists := activeLinks[candidate]; exists {
-			return "", errImportDirectoryLinkCycle
-		}
-		activeLinks[candidate] = struct{}{}
-		target, err := access.root.Readlink(filepath.FromSlash(candidate))
-		if err != nil {
-			return "", err
+	return resolveImportLinkPath(filepath.ToSlash(name), hostImportLinkOperations(access))
 }
+
+func hostImportLinkOperations(access *filesystemAccess) importLinkOperations {
+	return importLinkOperations{
+		lstat: func(candidate string) (fs.FileInfo, error) {
+			return access.root.Lstat(filepath.FromSlash(candidate))
+		},
+		readlink: func(candidate string) (string, error) {
+			return access.root.Readlink(filepath.FromSlash(candidate))
+		},
+		normalizeTarget: func(candidate, target string) (string, error) {
 			var targetPath string
 			if filepath.IsAbs(target) {
 				relative, err := filepath.Rel(access.rootPath, filepath.Clean(target))
@@ -235,13 +215,9 @@ func resolveHostImportPath(access *filesystemAccess, name string) (string, error
 			if !withinPortableRoot(targetPath) {
 				return "", errImportPathOutsideRoot
 			}
-		frames = append(frames, importLinkFrame{pending: strings.Split(targetPath, "/"), link: candidate})
-		resolved = ""
+			return targetPath, nil
+		},
 	}
-	if resolved == "" {
-		resolved = "."
-	}
-	return resolved, nil
 }
 
 func withinPortableRoot(name string) bool {

internal/engine/filesystem_links_test.go

@@ -136,6 +136,129 @@ func TestNativeLinkResolutionRejectsEscapesAndSkipsWhenFollowingIsDisabled(t *te
 	}
 }
 
+func TestHostLinkResolutionMemoizesCompletedDependencies(t *testing.T) {
+	// The small graph stays within host symlink-depth limits while exceeding the
+	// bounded linear operation allowance without completed-target reuse.
+	const linkCount = 5
+	root := t.TempDir()
+	configurationRoot := filepath.Join(root, "workspace", "config")
+	graph := filepath.Join(configurationRoot, "graph")
+	selected := filepath.Join(configurationRoot, "selected")
+	for _, directory := range []string{graph, selected} {
+		if err := os.MkdirAll(directory, 0o700); err != nil {
+			t.Fatal(err)
+		}
+	}
+	for index := 0; index < linkCount; index++ {
+		name := fmt.Sprintf("s%02d", index)
+		target := "."
+		if index > 0 {
+			previous := fmt.Sprintf("s%02d", index-1)
+			target = previous + "/" + previous
+		}
+		if err := os.Symlink(target, filepath.Join(graph, name)); err != nil {
+			t.Skipf("symlinks unavailable: %v", err)
+		}
+	}
+	if err := os.WriteFile(filepath.Join(graph, "item.yml"), []byte("value: linked\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	aliasTarget := fmt.Sprintf("../graph/s%02d", linkCount-1)
+	if err := os.Symlink(aliasTarget, filepath.Join(selected, "entry")); err != nil {
+		t.Skipf("symlinks unavailable: %v", err)
+	}
+	containedAbsolute := filepath.Join(graph, "s00")
+	if err := os.Symlink(containedAbsolute, filepath.Join(selected, "absolute")); err != nil {
+		t.Skipf("absolute symlinks unavailable: %v", err)
+	}
+	outside := t.TempDir()
+	if err := os.Symlink(outside, filepath.Join(selected, "escape")); err != nil {
+		t.Skipf("symlinks unavailable: %v", err)
+	}
+
+	access, err := openHostAccess(root)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer access.close()
+	name := filepath.ToSlash(filepath.Join("workspace", "config", "selected", "entry"))
+	operations := hostImportLinkOperations(access)
+	var operationCount int
+	lstat := operations.lstat
+	readlink := operations.readlink
+	operations.lstat = func(candidate string) (fs.FileInfo, error) {
+		operationCount++
+		if operationCount > linkCount*20+40 {
+			return nil, errors.New("host link operation limit exceeded")
+		}
+		return lstat(candidate)
+	}
+	operations.readlink = func(candidate string) (string, error) {
+		operationCount++
+		if operationCount > linkCount*20+40 {
+			return "", errors.New("host link operation limit exceeded")
+		}
+		return readlink(candidate)
+	}
+	resolved, err := resolveImportLinkPath(name, operations)
+	if err != nil || resolved != filepath.ToSlash(filepath.Join("workspace", "config", "graph")) {
+		t.Fatalf("host repeated-link resolution = %q, %v", resolved, err)
+	}
+	if operationCount == 0 || operationCount > linkCount*20+40 {
+		t.Fatalf("host link operations = %d, want 1..%d", operationCount, linkCount*20+40)
+	}
+
+	contained, err := resolveHostImportPath(access, filepath.Join("workspace", "config", "selected", "absolute"))
+	if err != nil || contained != filepath.ToSlash(filepath.Join("workspace", "config", "graph")) {
+		t.Fatalf("contained absolute link = %q, %v", contained, err)
+	}
+	if _, err := resolveHostImportPath(access, filepath.Join("workspace", "config", "selected", "escape")); !errors.Is(err, errImportPathOutsideRoot) {
+		t.Fatalf("escaping absolute link error = %v, want outside-root rejection", err)
+	}
+
+	for _, selector := range []string{"directory", "glob"} {
+		t.Run(selector, func(t *testing.T) {
+			var diagnostics []Diagnostic
+			if selector == "directory" {
+				_, diagnostics = selectDirectoryWithDiagnostics(t, access, root, "    - dir: workspace/config/selected/entry\n      follow: true\n")
+			} else {
+				_, diagnostics = selectGlobForTestWithDiagnostics(t, access, root, "    - glob: 'workspace/config/selected/entry/*.yml'\n      follow: true\n")
+			}
+			if len(diagnostics) != 0 {
+				t.Fatalf("host selector diagnostics = %#v", diagnostics)
+			}
+		})
+	}
+}
+
+func TestHostLinkResolutionKeepsNamespacesIndependent(t *testing.T) {
+	for _, test := range []struct {
+		name   string
+		target string
+	}{
+		{name: "left", target: "left-target"},
+		{name: "right", target: "right-target"},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			root := t.TempDir()
+			if err := os.Mkdir(filepath.Join(root, test.target), 0o700); err != nil {
+				t.Fatal(err)
+			}
+			if err := os.Symlink(test.target, filepath.Join(root, "alias")); err != nil {
+				t.Skipf("symlinks unavailable: %v", err)
+			}
+			access, err := openHostAccess(root)
+			if err != nil {
+				t.Fatal(err)
+			}
+			defer access.close()
+			if got, err := resolveHostImportPath(access, "alias"); err != nil || got != test.target {
+				t.Fatalf("namespace link = %q, %v; want %q", got, err, test.target)
+			}
+		})
+	}
+}
+
 type countingReadLinkFS struct {
 	filesystem fs.FS
 	links      fs.ReadLinkFS