README.md

@@ -126,10 +126,12 @@ the list is named by its position, never printed. Keys are derived from the
 mnemonic's words joined by single spaces, whatever whitespace is around or
 between them, so one word per line, tabs or extra spaces give the same keys.
 
-The key index, `--words`, the `--to` recipients and the file `age encrypt` or
-`age decrypt` is given to read are checked before the mnemonic is looked for, so
-a run refused for one of them never prompts for the mnemonic or runs the
-mnemonic command.
+The key index, `--words`, the `--to` recipients, the file `age encrypt` or
+`age decrypt` is given to read and its `-o` path are checked before the mnemonic
+is looked for, so a run refused for one of them never prompts for the mnemonic
+or runs the mnemonic command. An `-o` path is refused at that point if it is a
+directory or a symlink to one, if its directory is missing or is not a
+directory, or if it is a symlink that points at nothing.
 
 `KEYFUNC_MNEMONIC` and `KEYFUNC_MNEMONIC_COMMAND` are removed from the
 environment before the mnemonic command, the system `ssh` (`keyfunc ssh to`) and

internal/cli/age/age.go

@@ -11,6 +11,7 @@ import (
 	"os"
 	"os/signal"
 	"path/filepath"
+	"syscall"
 
 	"github.com/spf13/cobra"
 	"sneak.berlin/go/keyfunc/internal/agekey"
@@ -108,9 +109,9 @@ func decrypt() *cobra.Command {
 }
 
 // runEncrypt encrypts to the derived recipient and any others given.
-// The recipients are read and the input opened before the key is
-// derived, so that a run refused for either never asks for the
-// mnemonic.
+// The recipients are read, the -o path looked at and the input opened
+// before the key is derived, so that a run refused for any of them
+// never asks for the mnemonic.
 func runEncrypt(cmd *cobra.Command, args []string) error {
 	names, err := cmd.Flags().GetStringArray("to")
 	if err != nil {
@@ -127,6 +128,11 @@ func runEncrypt(cmd *cobra.Command, args []string) error {
 		return fmt.Errorf("reading the armor flag: %w", err)
 	}
 
+	err = checkOutput(cmd)
+	if err != nil {
+		return err
+	}
+
 	src, closeSrc, err := input(cmd, args)
 	if err != nil {
 		return err
@@ -144,9 +150,14 @@ func runEncrypt(cmd *cobra.Command, args []string) error {
 	})
 }
 
-// runDecrypt decrypts with the derived identity. The input is opened
-// before the key is derived, as for runEncrypt.
+// runDecrypt decrypts with the derived identity. The -o path is looked
+// at and the input opened before the key is derived, as for runEncrypt.
 func runDecrypt(cmd *cobra.Command, args []string) error {
+	err := checkOutput(cmd)
+	if err != nil {
+		return err
+	}
+
 	src, closeSrc, err := input(cmd, args)
 	if err != nil {
 		return err
@@ -225,6 +236,52 @@ func input(cmd *cobra.Command, args []string) (io.Reader, func(), error) {
 	return file, func() { _ = file.Close() }, nil
 }
 
+// checkOutput refuses an -o path that output could not write to,
+// before anything is written: a directory or a symlink to one, one
+// whose directory is missing or is not a directory, or a symlink that
+// points at nothing. It creates nothing; output looks at the path again
+// when it writes.
+func checkOutput(cmd *cobra.Command) error {
+	name, err := cmd.Flags().GetString("output")
+	if err != nil {
+		return fmt.Errorf("reading the output file: %w", err)
+	}
+
+	if name == "" {
+		return nil
+	}
+
+	info, err := os.Lstat(name)
+
+	switch {
+	case errors.Is(err, fs.ErrNotExist):
+		// replace makes its new file in this directory. When the
+		// directory is a file instead, os.Lstat has already failed with
+		// ENOTDIR, the case below.
+		directory := filepath.Dir(name)
+
+		_, err = os.Stat(directory)
+		if err != nil {
+			return fmt.Errorf("looking at %s: %w", directory, err)
+		}
+
+		return nil
+	case err != nil:
+		return fmt.Errorf("looking at %s: %w", name, err)
+	case info.Mode().Type() == fs.ModeSymlink:
+		info, err = os.Stat(name)
+		if err != nil {
+			return fmt.Errorf("following %s: %w", name, err)
+		}
+	}
+
+	if info.IsDir() {
+		return fmt.Errorf("writing to %s: %w", name, syscall.EISDIR)
+	}
+
+	return nil
+}
+
 // output has the work write to the named path, going by what is there
 // without following a final symlink:
 //

internal/cli/age_test.go

@@ -108,6 +108,44 @@ func TestAnInputFileThatCannotBeOpenedIsRefused(t *testing.T) {
 	}
 }
 
+// The mnemonic command fails in these runs, so the refusal of the -o
+// path shows that it was looked at before the mnemonic was looked for.
+func TestAnOutputPathThatCannotBeWrittenIsRefused(t *testing.T) {
+	t.Setenv(mnemonic.CommandVariable, failingMnemonicCommand)
+
+	plain := written(t, "notes.txt", "the secret\n")
+	directory := t.TempDir()
+	missing := filepath.Join(directory, "missing")
+	dangling := filepath.Join(directory, "dangling")
+	require.NoError(t, os.Symlink(missing, dangling))
+
+	toDirectory := filepath.Join(directory, "to-directory")
+	require.NoError(t, os.Symlink(t.TempDir(), toDirectory))
+
+	refusals := map[string]error{
+		filepath.Join(missing, "notes.age"): fs.ErrNotExist,
+		filepath.Join(plain, "notes.age"):   syscall.ENOTDIR,
+		dangling:                            fs.ErrNotExist,
+		directory:                           syscall.EISDIR,
+		toDirectory:                         syscall.EISDIR,
+	}
+
+	for name, refusal := range refusals {
+		out, err := execute(t, "age", "encrypt", "-o", name, plain)
+		require.ErrorIs(t, err, refusal, name)
+		require.Empty(t, out, name)
+
+		out, err = execute(t, "age", "decrypt", "-o", name, plain)
+		require.ErrorIs(t, err, refusal, name)
+		require.Empty(t, out, name)
+	}
+
+	// Nothing was made beside the two symlinks.
+	left, err := os.ReadDir(directory)
+	require.NoError(t, err)
+	require.Len(t, left, 2)
+}
+
 func TestAFileForAnotherKeyIsRefused(t *testing.T) {
 	t.Setenv(mnemonic.Variable, example())