fix(testrun): pass the jvm the flag that silences the jdk 24 unsafe warning

internal/testrun/driver.go

@@ -154,13 +154,7 @@ func buildDriver(ctx context.Context, options Options, stdout io.Writer) (driver
 	if err != nil {
 		return nil, nil, err
 	}
-	sidecarArgs := []string{"-jar", jarPath,
+	sidecarArgs := sidecarArguments(ctx, jarPath, sidecarPort, options.Platform, options.Device)
-		"--port", strconv.Itoa(sidecarPort),
-		"--platform", options.Platform,
-	}
-	if options.Device != "" {
-		sidecarArgs = append(sidecarArgs, "--serial", options.Device)
-	}
 	adbPath, err := android.AdbBinary()
 	if err != nil {
 		return nil, nil, preflightFailure("android", err)
@@ -212,6 +206,25 @@ func buildDriver(ctx context.Context, options Options, stdout io.Writer) (driver
 	return driverClient, cleanup, nil
 }
 
+// From JDK 24 the JVM prints a four-line sun.misc.Unsafe deprecation warning
+// on every sidecar start, because the netty that grpc pulls in still reads
+// field offsets through it. The flag that silences it did not exist before JDK
+// 23 and an older JVM refuses to start when handed it, so ask this JVM whether
+// it takes the flag rather than reading its version string.
+const unsafeMemoryAccessAllow = "--sun-misc-unsafe-memory-access=allow"
+
+func sidecarArguments(ctx context.Context, jarPath string, port int, platform, serial string) []string {
+	var args []string
+	if exec.CommandContext(ctx, "java", unsafeMemoryAccessAllow, "-version").Run() == nil {
+		args = append(args, unsafeMemoryAccessAllow)
+	}
+	args = append(args, "-jar", jarPath, "--port", strconv.Itoa(port), "--platform", platform)
+	if serial != "" {
+		args = append(args, "--serial", serial)
+	}
+	return args
+}
+
 // watchSidecar reaps the sidecar and publishes its exit status. The channel is
 // closed after the send so the shutdown path can still receive once the startup
 // path has taken the status.

internal/testrun/driver_test.go

@@ -4,7 +4,10 @@ import (
 	"context"
 	"errors"
 	"io"
+	"os"
 	"os/exec"
+	"path/filepath"
+	"slices"
 	"strings"
 	"testing"
 	"time"
@@ -305,3 +308,41 @@ func TestResolveIOSTargetDeviceResolutionErrorSurfaces(t *testing.T) {
 		t.Fatal("expected the device-resolution error to surface")
 	}
 }
+
+func fakeJava(t *testing.T, script string) {
+	t.Helper()
+	directory := t.TempDir()
+	path := filepath.Join(directory, "java")
+	if err := os.WriteFile(path, []byte("#!/bin/sh\n"+script+"\n"), 0o755); err != nil {
+		t.Fatalf("write fake java: %v", err)
+	}
+	t.Setenv("PATH", directory)
+}
+
+func TestSidecarArgumentsSilencesTheUnsafeWarningOnAJvmThatTakesTheFlag(t *testing.T) {
+	fakeJava(t, "exit 0")
+
+	args := sidecarArguments(context.Background(), "/tmp/sidecar.jar", 51129, "android", "663c91b1")
+
+	want := []string{
+		"--sun-misc-unsafe-memory-access=allow",
+		"-jar", "/tmp/sidecar.jar",
+		"--port", "51129",
+		"--platform", "android",
+		"--serial", "663c91b1",
+	}
+	if !slices.Equal(args, want) {
+		t.Fatalf("sidecar argv = %q, want %q", args, want)
+	}
+}
+
+func TestSidecarArgumentsOmitsTheFlagAJvmBefore23WouldRefuseToStartWith(t *testing.T) {
+	fakeJava(t, `case "$1" in --sun-misc-unsafe-memory-access=*) echo "Unrecognized option: $1" >&2; exit 1;; esac; exit 0`)
+
+	args := sidecarArguments(context.Background(), "/tmp/sidecar.jar", 51129, "android", "")
+
+	want := []string{"-jar", "/tmp/sidecar.jar", "--port", "51129", "--platform", "android"}
+	if !slices.Equal(args, want) {
+		t.Fatalf("sidecar argv = %q, want %q", args, want)
+	}
+}