The ssh install and ssh to commands (closes #2)

install runs the system ssh and hands the host a short shell script.
The public key line reaches it on standard input, never on a command
line others on the host could read. The script makes ~/.ssh and
authorized_keys if missing, adds the line unless it is already there,
and says which of the two it did.

to serves the key from an agent inside the tool, on a unix socket in a
temporary directory only its owner can enter, and points ssh at it with
-o IdentityAgent. Socket and directory go when the command ends and the
private key is never written to disk. Only this command ends with the
status ssh ended with rather than status 1.

Model: opus-5

internal/sshkey/sshkey_test.go

@@ -1,6 +1,9 @@
 package sshkey_test
 
 import (
+	"net"
+	"os"
+	"path/filepath"
 	"strings"
 	"testing"
 
@@ -8,8 +11,16 @@ import (
 	"git.eeqj.de/sneak/keyfunc/internal/sshkey"
 	"github.com/stretchr/testify/require"
 	"golang.org/x/crypto/ssh"
+	"golang.org/x/crypto/ssh/agent"
 )
 
+// agentDirectoryMode is what the directory holding the agent socket
+// has to be: nobody but its owner may enter it.
+const agentDirectoryMode = 0o700
+
+// exampleIndex is the key index every test here derives at.
+const exampleIndex = 0
+
 // example returns the mnemonic every BIP-39 document uses to show its
 // test vectors: eleven abandons and about.
 func example() string {
@@ -26,7 +37,7 @@ func TestTooFewBytesAreRefused(t *testing.T) {
 func TestTheCommentIsPutAtTheEndOfTheLine(t *testing.T) {
 	t.Parallel()
 
-	key := forIndex(t, 0)
+	key := exampleKey(t)
 
 	line, err := key.Line("hello")
 	require.NoError(t, err)
@@ -37,7 +48,7 @@ func TestTheCommentIsPutAtTheEndOfTheLine(t *testing.T) {
 func TestThePrivateKeyCarriesTheSamePublicKey(t *testing.T) {
 	t.Parallel()
 
-	key := forIndex(t, 0)
+	key := exampleKey(t)
 
 	line, err := key.Line("")
 	require.NoError(t, err)
@@ -54,11 +65,60 @@ func TestThePrivateKeyCarriesTheSamePublicKey(t *testing.T) {
 	require.Equal(t, line, back)
 }
 
-// forIndex derives the key for one index.
-func forIndex(t *testing.T, index uint32) *sshkey.Key {
+func TestTheAgentServesTheOneKeyAndNothingElse(t *testing.T) {
+	t.Parallel()
+
+	key := exampleKey(t)
+
+	served, err := key.Serve(t.Context(), "a comment")
+	require.NoError(t, err)
+	t.Cleanup(served.Stop)
+
+	directory, err := os.Stat(filepath.Dir(served.Socket()))
+	require.NoError(t, err)
+	require.Equal(t,
+		os.FileMode(agentDirectoryMode), directory.Mode().Perm(),
+	)
+
+	var dialer net.Dialer
+
+	connection, err := dialer.DialContext(t.Context(), "unix", served.Socket())
+	require.NoError(t, err)
+
+	defer func() { _ = connection.Close() }()
+
+	held, err := agent.NewClient(connection).List()
+	require.NoError(t, err)
+	require.Len(t, held, 1)
+
+	line, err := key.Line("a comment")
+	require.NoError(t, err)
+	require.Equal(t, line, held[0].String())
+}
+
+func TestStoppingTheAgentLeavesNothingBehind(t *testing.T) {
+	t.Parallel()
+
+	served, err := exampleKey(t).Serve(t.Context(), "a comment")
+	require.NoError(t, err)
+
+	directory := filepath.Dir(served.Socket())
+	require.DirExists(t, directory)
+
+	served.Stop()
+	require.NoDirExists(t, directory)
+
+	var dialer net.Dialer
+
+	_, err = dialer.DialContext(t.Context(), "unix", served.Socket())
+	require.Error(t, err)
+}
+
+// exampleKey derives the key the example mnemonic gives.
+func exampleKey(t *testing.T) *sshkey.Key {
 	t.Helper()
 
-	material, err := derive.Bytes(example(), sshkey.Application, index)
+	material, err := derive.Bytes(example(), sshkey.Application, exampleIndex)
 	require.NoError(t, err)
 
 	key, err := sshkey.New(material)