cmd/tailscale: add file get options for dealing with existing files

A new flag --conflict=(skip|overwrite|rename) lets users specify
what to do when receiving files that match a same-named file in
the target directory.

Updates #3548

Signed-off-by: David Eger <david.eger@gmail.com>

cmd/tailscale/cli/file.go

@@ -15,6 +15,7 @@ import (
 	"mime"
 	"net/http"
 	"os"
+	"path"
 	"path/filepath"
 	"strings"
 	"time"
@@ -286,22 +287,116 @@ func runCpTargets(ctx context.Context, args []string) error {
 	return nil
 }
 
+// onConflict is a flag.Value for the --conflict flag's three string options.
+type onConflict string
+
+const (
+	skipOnExist         onConflict = "skip"
+	overwriteExisting   onConflict = "overwrite" //  Overwrite any existing file at the target location
+	createNumberedFiles onConflict = "rename"    //  Create an alternately named file in the style of Chrome Downloads
+)
+
+func (v *onConflict) String() string { return string(*v) }
+
+func (v *onConflict) Set(s string) error {
+	if s == "" {
+		*v = skipOnExist
+		return nil
+	}
+	*v = onConflict(strings.ToLower(s))
+	if *v != skipOnExist && *v != overwriteExisting && *v != createNumberedFiles {
+		return fmt.Errorf("%q is not one of (skip|overwrite|rename)", s)
+	}
+	return nil
+}
+
 var fileGetCmd = &ffcli.Command{
 	Name:       "get",
-	ShortUsage: "file get [--wait] [--verbose] <target-directory>",
+	ShortUsage: "file get [--wait] [--verbose] [--conflict=(skip|overwrite|rename)] <target-directory>",
 	ShortHelp:  "Move files out of the Tailscale file inbox",
 	Exec:       runFileGet,
 	FlagSet: (func() *flag.FlagSet {
 		fs := newFlagSet("get")
 		fs.BoolVar(&getArgs.wait, "wait", false, "wait for a file to arrive if inbox is empty")
 		fs.BoolVar(&getArgs.verbose, "verbose", false, "verbose output")
+		fs.Var(&getArgs.conflict, "conflict", `behavior when a conflicting (same-named) file already exists in the target directory.
+	skip:       skip conflicting files: leave them in the taildrop inbox and print an error. get any non-conflicting files
+	overwrite:  overwrite existing file
+	rename:     write to a new number-suffixed filename`)
 		return fs
 	})(),
 }
 
-var getArgs struct {
-	wait    bool
-	verbose bool
+var getArgs = struct {
+	wait     bool
+	verbose  bool
+	conflict onConflict
+}{conflict: skipOnExist}
+
+func numberedFileName(dir, name string, i int) string {
+	ext := path.Ext(name)
+	return filepath.Join(dir, fmt.Sprintf("%s (%d)%s",
+		strings.TrimSuffix(name, ext),
+		i, ext))
+}
+
+func openFileOrSubstitute(dir, base string, action onConflict) (*os.File, error) {
+	targetFile := filepath.Join(dir, base)
+	f, err := os.OpenFile(targetFile, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0644)
+	if err == nil {
+		return f, nil
+	}
+	// Something went wrong trying to open targetFile as a new file for writing.
+	switch action {
+	default:
+		// This should not happen.
+		return nil, fmt.Errorf("file issue. how to resolve this conflict? no one knows.")
+	case skipOnExist:
+		if _, statErr := os.Stat(targetFile); statErr == nil {
+			// we can stat a file at that path: so it already exists.
+			return nil, fmt.Errorf("refusing to overwrite file: %w", err)
+		}
+		return nil, fmt.Errorf("failed to write; %w", err)
+	case overwriteExisting:
+		// remove the target file and create it anew so we don't fall for an 
+		// attacker who symlinks a known target name to a file he wants changed.
+		if err = os.Remove(targetFile); err != nil {
+			return nil, fmt.Errorf("unable to remove target file: %w", err)
+		}
+		if f, err = os.OpenFile(targetFile, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0644); err != nil {
+			return nil, fmt.Errorf("unable to overwrite: %w", err)
+		}
+		return f, nil
+	case createNumberedFiles:
+		// It's possible the target directory or filesystem isn't writable by us,
+		// not just that the target file(s) already exists.  For now, give up after
+		// a limited number of attempts.  In future, maybe distinguish this case
+		// and follow in the style of https://tinyurl.com/chromium100
+		maxAttempts := 100
+		for i := 1; i < maxAttempts; i++ {
+			if f, err = os.OpenFile(numberedFileName(dir, base, i), os.O_RDWR|os.O_CREATE|os.O_EXCL, 0644); err == nil {
+				return f, nil
+			}
+		}
+		return nil, fmt.Errorf("unable to find a name for writing %v, final attempt: %w", targetFile, err)
+	}
+}
+
+func receiveFile(ctx context.Context, wf apitype.WaitingFile, dir string) (targetFile string, size int64, err error) {
+	rc, size, err := tailscale.GetWaitingFile(ctx, wf.Name)
+	if err != nil {
+		return "", 0, fmt.Errorf("opening inbox file %q: %w", wf.Name, err)
+	}
+	f, err := openFileOrSubstitute(dir, wf.Name, getArgs.conflict)
+	if err != nil {
+		return "", 0, err
+	}
+	_, err = io.Copy(f, rc)
+	rc.Close()
+	if err != nil {
+		return "", 0, fmt.Errorf("failed to write %v: %v", f.Name(), err)
+	}
+	return f.Name(), size, f.Close()
 }
 
 func runFileGet(ctx context.Context, args []string) error {
@@ -330,47 +425,40 @@ func runFileGet(ctx context.Context, args []string) error {
 			break
 		}
 		if getArgs.verbose {
-			log.Printf("waiting for file...")
+			printf("waiting for file...")
 		}
 		if err := waitForFile(ctx); err != nil {
 			return err
 		}
 	}
 
+	var errs []error
 	deleted := 0
 	for _, wf := range wfs {
-		rc, size, err := tailscale.GetWaitingFile(ctx, wf.Name)
+		writtenFile, size, err := receiveFile(ctx, wf, dir)
 		if err != nil {
-			return fmt.Errorf("opening inbox file %q: %v", wf.Name, err)
-		}
-		targetFile := filepath.Join(dir, wf.Name)
-		of, err := os.OpenFile(targetFile, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0644)
-		if err != nil {
-			if _, err := os.Stat(targetFile); err == nil {
-				return fmt.Errorf("refusing to overwrite %v", targetFile)
-			}
-			return err
-		}
-		_, err = io.Copy(of, rc)
-		rc.Close()
-		if err != nil {
-			return fmt.Errorf("failed to write %v: %v", targetFile, err)
-		}
-		if err := of.Close(); err != nil {
-			return err
+			errs = append(errs, err)
+			continue
 		}
 		if getArgs.verbose {
-			log.Printf("wrote %v (%d bytes)", wf.Name, size)
+			printf("wrote %v as %v (%d bytes)\n", wf.Name, writtenFile, size)
 		}
-		if err := tailscale.DeleteWaitingFile(ctx, wf.Name); err != nil {
-			return fmt.Errorf("deleting %q from inbox: %v", wf.Name, err)
+		if err = tailscale.DeleteWaitingFile(ctx, wf.Name); err != nil {
+			errs = append(errs, fmt.Errorf("deleting %q from inbox: %v", wf.Name, err))
+			continue
 		}
 		deleted++
 	}
 	if getArgs.verbose {
-		log.Printf("moved %d files", deleted)
+		printf("moved %d/%d files\n", deleted, len(wfs))
 	}
-	return nil
+	if len(errs) == 0 {
+		return nil
+	}
+	for _, err := range errs[:len(errs)-1] {
+		outln(err)
+	}
+	return errs[len(errs)-1]
 }
 
 func wipeInbox(ctx context.Context) error {