localrpc: Add a package for local RPC over UNIX sockets

This patch adds a new package for doing local lightweight RPC calls over UNIX
sockets. This will be used in later patches for communication between chasquid
and chasquid-util.

internal/localrpc/client_test.go

@@ -0,0 +1,76 @@
+package localrpc
+
+import (
+	"bufio"
+	"errors"
+	"io/fs"
+	"net"
+	"net/textproto"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func NewFakeServer(t *testing.T, path, output string) {
+	t.Helper()
+	lis, err := net.Listen("unix", path)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	for {
+		conn, err := lis.Accept()
+		if err != nil {
+			t.Fatal(err)
+		}
+		t.Logf("FakeServer %v: accepted ", conn)
+
+		name, inS, err := readRequest(
+			textproto.NewReader(bufio.NewReader(conn)))
+		t.Logf("FakeServer %v: readRequest: %q %q / %v", conn, name, inS, err)
+
+		n, err := conn.Write([]byte(output))
+		t.Logf("FakeServer %v: writeMessage(%q): %d %v",
+			conn, output, n, err)
+
+		t.Logf("FakeServer %v: closing", conn)
+		conn.Close()
+	}
+}
+
+func TestBadServer(t *testing.T) {
+	tmpDir, err := os.MkdirTemp("", "rpc-test-*")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmpDir)
+	socketPath := filepath.Join(tmpDir, "rpc.sock")
+
+	// textproto client expects a numeric code, this should cause ReadCodeLine
+	// to fail with textproto.ProtocolError.
+	go NewFakeServer(t, socketPath, "xxx")
+	waitForServer(t, socketPath)
+
+	client := NewClient(socketPath)
+	_, err = client.Call("Echo")
+	if err == nil {
+		t.Fatal("expected error")
+	}
+	var protoErr textproto.ProtocolError
+	if !errors.As(err, &protoErr) {
+		t.Errorf("wanted textproto.ProtocolError, got: %v (%T)", err, err)
+	}
+}
+
+func TestBadSocket(t *testing.T) {
+	c := NewClient("/does/not/exist")
+	_, err := c.Call("Echo")
+
+	opErr, ok := err.(*net.OpError)
+	if !ok {
+		t.Fatalf("expected net.OpError, got %q (%T)", err, err)
+	}
+	if !errors.Is(err, fs.ErrNotExist) {
+		t.Errorf("wanted ErrNotExist, got: %q (%T)", opErr.Err, opErr.Err)
+	}
+}