taildrop: document and cleanup the package (#9699)

Changes made:
* Unexport declarations specific to internal taildrop functionality.
* Document all exported functionality.
* Move TestRedactErr to the taildrop package.
* Rename and invert Handler.DirectFileDoFinalRename as AvoidFinalRename.

Updates tailscale/corp#14772

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

taildrop/taildrop_test.go

@@ -4,6 +4,9 @@
 package taildrop
 
 import (
+	"errors"
+	"fmt"
+	"io/fs"
 	"os"
 	"path/filepath"
 	"runtime"
@@ -13,7 +16,7 @@ import (
 // Tests "foo.jpg.deleted" marks (for Windows).
 func TestDeletedMarkers(t *testing.T) {
 	dir := t.TempDir()
-	h := &Handler{RootDir: dir}
+	h := &Handler{Dir: dir}
 
 	nothingWaiting := func() {
 		t.Helper()
@@ -24,7 +27,7 @@ func TestDeletedMarkers(t *testing.T) {
 	}
 	touch := func(base string) {
 		t.Helper()
-		if err := TouchFile(filepath.Join(dir, base)); err != nil {
+		if err := touchFile(filepath.Join(dir, base)); err != nil {
 			t.Fatal(err)
 		}
 	}
@@ -86,3 +89,67 @@ func TestDeletedMarkers(t *testing.T) {
 		rc.Close()
 	}
 }
+
+func TestRedactErr(t *testing.T) {
+	testCases := []struct {
+		name string
+		err  func() error
+		want string
+	}{
+		{
+			name: "PathError",
+			err: func() error {
+				return &os.PathError{
+					Op:   "open",
+					Path: "/tmp/sensitive.txt",
+					Err:  fs.ErrNotExist,
+				}
+			},
+			want: `open redacted.41360718: file does not exist`,
+		},
+		{
+			name: "LinkError",
+			err: func() error {
+				return &os.LinkError{
+					Op:  "symlink",
+					Old: "/tmp/sensitive.txt",
+					New: "/tmp/othersensitive.txt",
+					Err: fs.ErrNotExist,
+				}
+			},
+			want: `symlink redacted.41360718 redacted.6bcf093a: file does not exist`,
+		},
+		{
+			name: "something else",
+			err:  func() error { return errors.New("i am another error type") },
+			want: `i am another error type`,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			// For debugging
+			var i int
+			for err := tc.err(); err != nil; err = errors.Unwrap(err) {
+				t.Logf("%d: %T @ %p", i, err, err)
+				i++
+			}
+
+			t.Run("Root", func(t *testing.T) {
+				got := redactErr(tc.err()).Error()
+				if got != tc.want {
+					t.Errorf("err = %q; want %q", got, tc.want)
+				}
+			})
+			t.Run("Wrapped", func(t *testing.T) {
+				wrapped := fmt.Errorf("wrapped error: %w", tc.err())
+				want := "wrapped error: " + tc.want
+
+				got := redactErr(wrapped).Error()
+				if got != want {
+					t.Errorf("err = %q; want %q", got, want)
+				}
+			})
+		})
+	}
+}