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>

ipn/ipnlocal/local.go

@@ -3545,11 +3545,11 @@ func (b *LocalBackend) initPeerAPIListener() {
 	ps := &peerAPIServer{
 		b: b,
 		taildrop: &taildrop.Handler{
-			Logf:                    b.logf,
-			Clock:                   b.clock,
-			RootDir:                 fileRoot,
-			DirectFileMode:          b.directFileRoot != "",
-			DirectFileDoFinalRename: b.directFileDoFinalRename,
+			Logf:             b.logf,
+			Clock:            b.clock,
+			Dir:              fileRoot,
+			DirectFileMode:   b.directFileRoot != "",
+			AvoidFinalRename: !b.directFileDoFinalRename,
 		},
 	}
 	if dm, ok := b.sys.DNSManager.GetOK(); ok {

ipn/ipnlocal/peerapi_test.go

@@ -5,7 +5,6 @@ package ipnlocal
 
 import (
 	"bytes"
-	"errors"
 	"fmt"
 	"io"
 	"io/fs"
@@ -67,7 +66,7 @@ func bodyNotContains(sub string) check {
 
 func fileHasSize(name string, size int) check {
 	return func(t *testing.T, e *peerAPITestEnv) {
-		root := e.ph.ps.taildrop.RootDir
+		root := e.ph.ps.taildrop.Dir
 		if root == "" {
 			t.Errorf("no rootdir; can't check whether %q has size %v", name, size)
 			return
@@ -83,7 +82,7 @@ func fileHasSize(name string, size int) check {
 
 func fileHasContents(name string, want string) check {
 	return func(t *testing.T, e *peerAPITestEnv) {
-		root := e.ph.ps.taildrop.RootDir
+		root := e.ph.ps.taildrop.Dir
 		if root == "" {
 			t.Errorf("no rootdir; can't check contents of %q", name)
 			return
@@ -498,7 +497,7 @@ func TestHandlePeerAPI(t *testing.T) {
 						Clock: &tstest.Clock{},
 					}
 				}
-				e.ph.ps.taildrop.RootDir = rootDir
+				e.ph.ps.taildrop.Dir = rootDir
 			}
 			for _, req := range tt.reqs {
 				e.rr = httptest.NewRecorder()
@@ -538,9 +537,9 @@ func TestFileDeleteRace(t *testing.T) {
 			clock:          &tstest.Clock{},
 		},
 		taildrop: &taildrop.Handler{
-			Logf:    t.Logf,
-			Clock:   &tstest.Clock{},
-			RootDir: dir,
+			Logf:  t.Logf,
+			Clock: &tstest.Clock{},
+			Dir:   dir,
 		},
 	}
 	ph := &peerAPIHandler{
@@ -635,67 +634,3 @@ func TestPeerAPIReplyToDNSQueries(t *testing.T) {
 		t.Errorf("unexpectedly IPv6 deny; wanted to be a DNS server")
 	}
 }
-
-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 := taildrop.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 := taildrop.RedactErr(wrapped).Error()
-				if got != want {
-					t.Errorf("err = %q; want %q", got, want)
-				}
-			})
-		})
-	}
-}