clientupdate: do not recursively delete dirs in cleanupOldDownloads (#10093)

In case there's a wild symlink in one of the target paths, we don't want
to accidentally delete too much. Limit `cleanupOldDownloads` to deleting
individual files only.

Updates https://github.com/tailscale/tailscale/issues/10082

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>
(cherry picked from commit 7145016414)

clientupdate/clientupdate.go

@@ -279,7 +279,7 @@ func (up *Updater) updateSynology() error {
 		return nil
 	}
 
-	up.cleanupOldDownloads(filepath.Join(os.TempDir(), "tailscale-update*"))
+	up.cleanupOldDownloads(filepath.Join(os.TempDir(), "tailscale-update*", "*.spk"))
 	// Download the SPK into a temporary directory.
 	spkDir, err := os.MkdirTemp("", "tailscale-update")
 	if err != nil {
@@ -823,6 +823,9 @@ func (up *Updater) installMSI(msi string) error {
 	return err
 }
 
+// cleanupOldDownloads removes all files matching glob (see filepath.Glob).
+// Only regular files are removed, so the glob must match specific files and
+// not directories.
 func (up *Updater) cleanupOldDownloads(glob string) {
 	matches, err := filepath.Glob(glob)
 	if err != nil {
@@ -830,7 +833,15 @@ func (up *Updater) cleanupOldDownloads(glob string) {
 		return
 	}
 	for _, m := range matches {
-		if err := os.RemoveAll(m); err != nil {
+		s, err := os.Lstat(m)
+		if err != nil {
+			up.Logf("cleaning up old downloads: %v", err)
+			continue
+		}
+		if !s.Mode().IsRegular() {
+			continue
+		}
+		if err := os.Remove(m); err != nil {
 			up.Logf("cleaning up old downloads: %v", err)
 		}
 	}

clientupdate/clientupdate_test.go

@@ -11,6 +11,8 @@ import (
 	"maps"
 	"os"
 	"path/filepath"
+	"slices"
+	"sort"
 	"strings"
 	"testing"
 )
@@ -683,3 +685,113 @@ func TestWriteFileSymlink(t *testing.T) {
 		}
 	}
 }
+
+func TestCleanupOldDownloads(t *testing.T) {
+	tests := []struct {
+		desc     string
+		before   []string
+		symlinks map[string]string
+		glob     string
+		after    []string
+	}{
+		{
+			desc: "MSIs",
+			before: []string{
+				"MSICache/tailscale-1.0.0.msi",
+				"MSICache/tailscale-1.1.0.msi",
+				"MSICache/readme.txt",
+			},
+			glob: "MSICache/*.msi",
+			after: []string{
+				"MSICache/readme.txt",
+			},
+		},
+		{
+			desc: "SPKs",
+			before: []string{
+				"tmp/tailscale-update-1/tailscale-1.0.0.spk",
+				"tmp/tailscale-update-2/tailscale-1.1.0.spk",
+				"tmp/readme.txt",
+				"tmp/tailscale-update-3",
+				"tmp/tailscale-update-4/tailscale-1.3.0",
+			},
+			glob: "tmp/tailscale-update*/*.spk",
+			after: []string{
+				"tmp/readme.txt",
+				"tmp/tailscale-update-3",
+				"tmp/tailscale-update-4/tailscale-1.3.0",
+			},
+		},
+		{
+			desc:   "empty-target",
+			before: []string{},
+			glob:   "tmp/tailscale-update*/*.spk",
+			after:  []string{},
+		},
+		{
+			desc: "keep-dirs",
+			before: []string{
+				"tmp/tailscale-update-1/tailscale-1.0.0.spk",
+			},
+			glob: "tmp/tailscale-update*",
+			after: []string{
+				"tmp/tailscale-update-1/tailscale-1.0.0.spk",
+			},
+		},
+		{
+			desc: "no-follow-symlinks",
+			before: []string{
+				"MSICache/tailscale-1.0.0.msi",
+				"MSICache/tailscale-1.1.0.msi",
+				"MSICache/readme.txt",
+			},
+			symlinks: map[string]string{
+				"MSICache/tailscale-1.3.0.msi": "MSICache/tailscale-1.0.0.msi",
+				"MSICache/tailscale-1.4.0.msi": "MSICache/readme.txt",
+			},
+			glob: "MSICache/*.msi",
+			after: []string{
+				"MSICache/tailscale-1.3.0.msi",
+				"MSICache/tailscale-1.4.0.msi",
+				"MSICache/readme.txt",
+			},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.desc, func(t *testing.T) {
+			dir := t.TempDir()
+			for _, p := range tt.before {
+				if err := os.MkdirAll(filepath.Join(dir, filepath.Dir(p)), 0700); err != nil {
+					t.Fatal(err)
+				}
+				if err := os.WriteFile(filepath.Join(dir, p), []byte(tt.desc), 0600); err != nil {
+					t.Fatal(err)
+				}
+			}
+			for from, to := range tt.symlinks {
+				if err := os.Symlink(filepath.Join(dir, to), filepath.Join(dir, from)); err != nil {
+					t.Fatal(err)
+				}
+			}
+
+			up := &Updater{Arguments: Arguments{Logf: t.Logf}}
+			up.cleanupOldDownloads(filepath.Join(dir, tt.glob))
+
+			var after []string
+			if err := filepath.WalkDir(dir, func(path string, d fs.DirEntry, err error) error {
+				if !d.IsDir() {
+					after = append(after, strings.TrimPrefix(filepath.ToSlash(path), filepath.ToSlash(dir)+"/"))
+				}
+				return nil
+			}); err != nil {
+				t.Fatal(err)
+			}
+
+			sort.Strings(after)
+			sort.Strings(tt.after)
+			if !slices.Equal(after, tt.after) {
+				t.Errorf("got files after cleanup: %q, want: %q", after, tt.after)
+			}
+		})
+	}
+}