test: Use modern testing features instead of custom helpers

Many of our tests rely on helpers that predate features which are now
available in the standard library.

This patch replaces them with the standard equivalents:

- MustTempDir/RemoveIfOk -> t.ArtifactDir() or t.TempDir() (depending on
  whether it is useful to keep the data on request).
- GetFreePort + waitForServer -> opening the listeners before starting
  the servers. To support this, localrpc.Server gains a Serve method. We
  may re-introduce these in the future if more use cases appear.
- WaitFor and real-time timing checks -> testing/synctest.
- context.Background() -> t.Context().
- Benchmark b.N loops -> b.Loop().
- reflect.DeepEqual -> cmp.Diff, and proto.Equal for protobufs.

internal/testlib/testlib.go

@@ -11,78 +11,18 @@ import (
 	"math/big"
 	"net"
 	"os"
-	"strings"
 	"sync"
 	"testing"
 	"time"
 )
 
-// MustTempDir creates a temporary directory, or dies trying.
-func MustTempDir(t *testing.T) string {
-	dir, err := os.MkdirTemp("", "testlib_")
-	if err != nil {
-		t.Fatal(err)
-	}
-
-	err = os.Chdir(dir)
-	if err != nil {
-		t.Fatal(err)
-	}
-
-	t.Logf("test directory: %q", dir)
-	return dir
-}
-
-// RemoveIfOk removes the given directory, but only if we have not failed. We
-// want to keep the failed directories for debugging.
-func RemoveIfOk(t *testing.T, dir string) {
-	// Safeguard, to make sure we only remove test directories.
-	// This should help prevent accidental deletions.
-	if !strings.Contains(dir, "testlib_") {
-		panic("invalid/dangerous directory")
-	}
-
-	if !t.Failed() {
-		os.RemoveAll(dir)
-	}
-}
-
-// Rewrite a file with the given contents.
-func Rewrite(t *testing.T, path, contents string) error {
-	// Safeguard, to make sure we only mess with test files.
-	if !strings.Contains(path, "testlib_") {
-		panic("invalid/dangerous path")
-	}
-
+// Rewrite a file with the given contents, or fail the test.
+func Rewrite(t *testing.T, path, contents string) {
+	t.Helper()
 	err := os.WriteFile(path, []byte(contents), 0600)
 	if err != nil {
-		t.Errorf("failed to rewrite file: %v", err)
+		t.Fatalf("failed to rewrite file: %v", err)
 	}
-
-	return err
-}
-
-// GetFreePort returns a free TCP port. This is hacky and not race-free, but
-// it works well enough for testing purposes.
-func GetFreePort() string {
-	l, err := net.Listen("tcp", "localhost:0")
-	if err != nil {
-		panic(err)
-	}
-	defer l.Close()
-	return l.Addr().String()
-}
-
-// WaitFor f to return true (returns true), or d to pass (returns false).
-func WaitFor(f func() bool, d time.Duration) bool {
-	start := time.Now()
-	for time.Since(start) < d {
-		if f() {
-			return true
-		}
-		time.Sleep(20 * time.Millisecond)
-	}
-	return false
 }
 
 type deliverRequest struct {
@@ -97,27 +37,27 @@ type TestCourier struct {
 	wg       sync.WaitGroup
 	Requests []*deliverRequest
 	ReqFor   map[string]*deliverRequest
-	sync.Mutex
+	mu       sync.Mutex
 }
 
 // Deliver the given mail (saving it in tc.Requests).
 func (tc *TestCourier) Deliver(from string, to string, data []byte) (error, bool) {
 	defer tc.wg.Done()
 	dr := &deliverRequest{from, to, data, nil}
-	tc.Lock()
+	tc.mu.Lock()
 	tc.Requests = append(tc.Requests, dr)
 	tc.ReqFor[to] = dr
-	tc.Unlock()
+	tc.mu.Unlock()
 	return nil, false
 }
 
 func (tc *TestCourier) Forward(from string, to string, data []byte, servers []string) (error, bool) {
 	defer tc.wg.Done()
 	dr := &deliverRequest{from, to, data, servers}
-	tc.Lock()
+	tc.mu.Lock()
 	tc.Requests = append(tc.Requests, dr)
 	tc.ReqFor[to] = dr
-	tc.Unlock()
+	tc.mu.Unlock()
 	return nil, false
 }