README says secret add drops one trailing newline and import keeps it (closes #158)

`secret add` drops one trailing newline from stdin, so
`echo value | secret add name` stores `value`. `secret import` stores a
file's bytes unchanged, and `secret get` writes the value with no
newline added. The README said none of this, so a PEM key piped into
`add` came back one byte short with no hint why. Its entries for `add`,
`get` and `import` now state each rule. A new test stores `a\n\n`
through `add` and through `import` and checks that `add` kept two bytes
and `import` three. No program code changes.

The new test passes on the code before this change; it fails when `add` keeps the newline.

Model: opus-5-5

README.md

@@ -142,7 +142,11 @@ value may be over 100 MiB, whatever the limit.
 
 #### `secret add <secret-name> [--force]`
 
-Adds a secret to the current vault. Reads the secret value from stdin.
+Adds a secret to the current vault. Reads the secret value from stdin. When the
+value ends in a newline, that one newline is dropped, so
+`echo value | secret add name` stores `value`; from `printf 'a\n\n'` it stores
+`a` and one newline. To keep a file's final newline, as in a PEM key, use
+`secret import`, which stores the file's bytes unchanged.
 
 - `--force, -f`: Overwrite existing secret
 
@@ -155,13 +159,13 @@ contain `//`, or have `..` as a path segment.
 
 #### `secret get <secret-name> [--version <version>]`
 
-Retrieves and outputs a secret value to stdout. A failed write of the value,
-such as to a file on a full disk, prints the error and exits 1, with two
-exceptions. If the program reading from a pipe has already exited, as in
-`secret get s | true`, the broken-pipe signal (SIGPIPE) ends `secret` with no
-message and the shell reports exit status 141. If the shell closed stdout before
-`secret` started (`secret get s >&-`), the value is written to `/dev/null` and
-`secret get` exits 0.
+Writes the secret's value to stdout exactly as stored, with no newline added
+after it. A failed write of the value, such as to a file on a full disk, prints
+the error and exits 1, with two exceptions. If the program reading from a pipe
+has already exited, as in `secret get s | true`, the broken-pipe signal
+(SIGPIPE) ends `secret` with no message and the shell reports exit status 141.
+If the shell closed stdout before `secret` started (`secret get s >&-`), the
+value is written to `/dev/null` and `secret get` exits 0.
 
 - `--version, -v`: Get a specific version (default: current)
 
@@ -300,7 +304,7 @@ Selects an unlocker as the current default for operations.
 #### `secret import <secret-name> --source <filename>`
 
 Imports a secret from a file and stores it in the current vault under the given
-name.
+name. The file's bytes are stored unchanged, a final newline included.
 
 ### Encryption Operations
 

internal/cli/trailing_newline_test.go

@@ -0,0 +1,47 @@
+//nolint:testpackage // white-box test of unexported internals
+package cli
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/spf13/afero"
+	"github.com/spf13/cobra"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// TestAddDropsOneTrailingNewlineImportKeepsIt pins the rule the README
+// gives: `secret add` drops one trailing newline from stdin, and
+// `secret import` stores the file's bytes unchanged.
+func TestAddDropsOneTrailingNewlineImportKeepsIt(t *testing.T) {
+	t.Parallel()
+
+	fs, vlt := newSizeTestVault(t)
+	value := []byte("a\n\n")
+
+	cmd := &cobra.Command{}
+	cmd.SetIn(bytes.NewReader(value))
+
+	cli := NewCLIInstanceWithStateDir(fs, testStateDir)
+	cli.cmd = cmd
+
+	require.NoError(t, cli.AddSecret("added", false))
+
+	sourceFile := "/test/value"
+	require.NoError(t, afero.WriteFile(fs, sourceFile, value, 0o600))
+	require.NoError(t, cli.ImportSecret(cmd, "imported", sourceFile, false))
+
+	added, err := vlt.GetSecret("added")
+	require.NoError(t, err)
+
+	defer added.Destroy()
+
+	imported, err := vlt.GetSecret("imported")
+	require.NoError(t, err)
+
+	defer imported.Destroy()
+
+	assert.Equal(t, []byte("a\n"), added.Bytes())
+	assert.Equal(t, []byte("a\n\n"), imported.Bytes())
+}