Recognize circular links

internal/smartlink/smartlink.go

@@ -40,7 +40,15 @@ func LinkContents(src, dest string) error {
 // Link creates a link from src to target. MS decided to support Symlinks but only if you opt into developer mode (go figure),
 // which we cannot reasonably force on our users. So on Windows we will instead create dirs and hardlinks.
 func Link(src, dest string) error {
-	var err error
+	isCircular, err := isCircularLink(src)
+	if err != nil {
+		return errs.Wrap(err, "could not check for circular link")
+	}
+	if isCircular {
+		logging.Warning("Skipping linking '%s' to '%s' as it is a circular link", src, dest)
+		return nil
+	}
+
 	src, dest, err = resolvePaths(src, dest)
 	if err != nil {
 		return errs.Wrap(err, "Could not resolve src and dest paths")
@@ -80,6 +88,32 @@ func Link(src, dest string) error {
 	return nil
 }
 
+func isCircularLink(src string) (bool, error) {
+	if !fileutils.IsSymlink(src) || !fileutils.IsDir(src) {
+		return false, nil
+	}
+
+	target, err := os.Readlink(src)
+	if err != nil {
+		return false, errs.Wrap(err, "could not read symlink %s", src)
+	}
+
+	if !filepath.IsAbs(target) {
+		resolved, err := fileutils.ResolveUniquePath(src)
+		if err != nil {
+			return false, errs.Wrap(err, "Could not resolve src path")
+		}
+		target = resolved
+	}
+
+	// If the target points at the parent directory, we have a circular link
+	if filepath.Dir(src) == target {
+		return true, nil
+	}
+
+	return false, nil
+}
+
 // UnlinkContents will unlink the contents of src to dest if the links exist
 // WARNING: on windows smartlinks are hard links, and relating hard links back to their source is non-trivial, so instead
 // we just delete the target path. If the user modified the target in any way their changes will be lost.

internal/smartlink/smartlink_test.go

@@ -0,0 +1,67 @@
+package smartlink
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/ActiveState/cli/internal/fileutils"
+	"github.com/stretchr/testify/require"
+)
+
+func TestLinkContentsWithCircularLink(t *testing.T) {
+	srcDir, err := os.MkdirTemp("", "src")
+	require.NoError(t, err)
+	defer os.RemoveAll(srcDir)
+
+	destDir, err := os.MkdirTemp("", "dest")
+	require.NoError(t, err)
+	defer os.RemoveAll(destDir)
+
+	// Create test file structure:
+	// src/
+	//   ├── regular.txt
+	//   └── subdir/
+	//        ├── circle -> subdir (circular link)
+	//        └── subfile.txt
+
+	testFile := filepath.Join(srcDir, "regular.txt")
+	err = os.WriteFile(testFile, []byte("test content"), 0644)
+	require.NoError(t, err)
+
+	subDir := filepath.Join(srcDir, "subdir")
+	err = os.Mkdir(subDir, 0755)
+	require.NoError(t, err)
+
+	subFile := filepath.Join(subDir, "subfile.txt")
+	err = os.WriteFile(subFile, []byte("sub content"), 0644)
+	require.NoError(t, err)
+
+	circularLink := filepath.Join(subDir, "circle")
+	err = os.Symlink(subDir, circularLink)
+	require.NoError(t, err)
+
+	err = LinkContents(srcDir, destDir)
+	require.NoError(t, err)
+
+	// Verify file structure:
+	// dest/
+	//   ├── regular.txt
+	//   └── subdir/
+	//        └── subfile.txt
+
+	destFile := filepath.Join(destDir, "regular.txt")
+	require.FileExists(t, destFile)
+	content, err := os.ReadFile(destFile)
+	require.NoError(t, err)
+	require.Equal(t, "test content", string(content))
+
+	destSubFile := filepath.Join(destDir, "subdir", "subfile.txt")
+	require.FileExists(t, destSubFile)
+	subContent, err := os.ReadFile(destSubFile)
+	require.NoError(t, err)
+	require.Equal(t, "sub content", string(subContent))
+
+	destCircular := filepath.Join(destDir, "subdir", "circle")
+	require.False(t, fileutils.TargetExists(destCircular), "Circular link should not have been created")
+}