Turn IdentitiesOnly off for ssh to (closes #74)

ssh to now gives ssh -o IdentitiesOnly=no before the host, next to
-o IdentityAgent. With IdentitiesOnly yes in the user's ssh
configuration, ssh listed the agent's key but never offered it, and
the login failed with Permission denied. An option on the command
line wins over the configuration, and the tool's agent holds only the
derived key, so turning the option off adds no other key. A test
against the stand-in ssh requires the option before the host, and the
README's ssh to section names both options.

Model: opus-5-5

README.md

@@ -219,11 +219,15 @@ first, or pass `-o StrictHostKeyChecking=accept-new` after `--`.
 
 Derives the key, serves it from an SSH agent that runs inside the tool on a unix
 socket in a new private `0700` temporary directory, then runs the system `ssh`
-with `-o IdentityAgent=<that socket>` followed by the host and all remaining
+with `-o IdentityAgent=<that socket> -o IdentitiesOnly=no` followed by the host
-arguments unchanged. The tool exits with `ssh`'s exit status and removes the
+and all remaining arguments unchanged. Without `IdentitiesOnly=no`, an
-socket and directory on the way out. The private key is never written to disk. A
+`IdentitiesOnly yes` in the user's `ssh` configuration keeps the derived key
-SIGINT, SIGTERM or SIGHUP ends `ssh` and still removes the socket and directory,
+from being offered. The agent holds no other key, so turning it off adds nothing
-and the tool then exits with status 1 unless `ssh` reported one of its own.
+else to what `ssh` offers. The tool exits with `ssh`'s exit status and removes
+the socket and directory on the way out. The private key is never written to
+disk. A SIGINT, SIGTERM or SIGHUP ends `ssh` and still removes the socket and
+directory, and the tool then exits with status 1 unless `ssh` reported one of
+its own.
 
 ## age identities: `keyfunc age`
 

internal/cli/ssh/to.go

@@ -57,8 +57,13 @@ func to() *cobra.Command {
 
 			defer served.Stop()
 
+			// IdentitiesOnly yes in the user's ssh configuration
+			// would keep the agent's key from being offered. The
+			// agent holds only the derived key, so turning it off
+			// adds no other key.
 			argv := slices.Concat([]string{
 				"-o", "IdentityAgent=" + served.Socket(),
+				"-o", "IdentitiesOnly=no",
 			}, args)
 
 			return connect(ctx, argv)

internal/cli/ssh_test.go

@@ -587,7 +587,7 @@ func TestSSHIsPointedAtTheAgentAndItsStatusIsHandedOn(t *testing.T) {
 
 	given := recorded(t, arguments)
 	require.Equal(t, "-o", given[0])
-	require.Equal(t, []string{host, remoteCommand}, given[2:])
+	require.Equal(t, []string{host, remoteCommand}, given[4:])
 
 	// The stand-in wrote the path down only because there really was
 	// a socket there while it ran.
@@ -596,6 +596,26 @@ func TestSSHIsPointedAtTheAgentAndItsStatusIsHandedOn(t *testing.T) {
 	require.NoDirExists(t, filepath.Dir(socket))
 }
 
+func TestIdentitiesOnlyIsTurnedOffBeforeTheHost(t *testing.T) {
+	t.Setenv(mnemonic.Variable, example())
+
+	arguments, _ := pretendCall(t)
+
+	_, err := execute(t, subcommand, "to", host, remoteCommand)
+
+	var passed ssh.StatusError
+
+	require.ErrorAs(t, err, &passed)
+
+	// IdentitiesOnly yes in the user's ssh configuration keeps ssh
+	// from offering the agent's key. An option given before the host
+	// wins over the configuration.
+	given := recorded(t, arguments)
+	require.Equal(t,
+		[]string{"-o", "IdentitiesOnly=no", host, remoteCommand}, given[2:],
+	)
+}
+
 func TestTheToolEndsWithTheStatusSSHEndedWith(t *testing.T) {
 	t.Setenv(mnemonic.Variable, example())