serve s3: skip TestS3Minio when the docker test framework is unavailable

TestS3Minio brings up a minio container via the fstest/testserver
framework, which exec's bash init.d scripts that shell out to docker.
This is not available on all platforms - Windows has no POSIX shell to
run the scripts, and macOS CI runners have no docker daemon - which
caused the build to fail there.

Add testy.SkipUnlessDocker to detect whether the framework can run and
skip the test when it cannot.

cmd/serve/s3/s3_test.go

@@ -27,6 +27,7 @@ import (
 	"github.com/rclone/rclone/fs/object"
 	"github.com/rclone/rclone/fs/rc"
 	"github.com/rclone/rclone/fstest"
+	"github.com/rclone/rclone/fstest/testy"
 	"github.com/rclone/rclone/lib/random"
 	"github.com/rclone/rclone/vfs/vfscommon"
 	"github.com/stretchr/testify/assert"
@@ -84,6 +85,7 @@ func TestS3(t *testing.T) {
 // this exercises that streaming path in serve s3 - the path the local
 // backing in TestS3 cannot cover.
 func TestS3Minio(t *testing.T) {
+	testy.SkipUnlessDocker(t)
 	servetest.RunWithBackend(t, "s3", startS3(t), "TestS3Minio:")
 }
 

fstest/testy/testy.go

@@ -3,6 +3,9 @@ package testy
 
 import (
 	"os"
+	"os/exec"
+	"runtime"
+	"sync"
 	"testing"
 )
 
@@ -18,3 +21,37 @@ func SkipUnreliable(t *testing.T) {
 	}
 	t.Skip("Skipping Unreliable Test on CI")
 }
+
+var dockerOnce struct {
+	sync.Once
+	ok bool
+}
+
+// HaveDocker returns true if the fstest/testserver docker framework can
+// be used on this host. The framework brings up containers (e.g. minio)
+// by exec'ing the bash init.d scripts which in turn shell out to docker,
+// so it needs both a POSIX shell (not available on Windows) and a
+// reachable docker daemon. The result is cached after the first call.
+func HaveDocker() bool {
+	dockerOnce.Do(func() {
+		// The init.d scripts are bash and cannot be exec'd on Windows.
+		if runtime.GOOS == "windows" {
+			return
+		}
+		// docker version exits non-zero if the client is installed but
+		// the daemon is unreachable, which is exactly what we want to
+		// detect.
+		dockerOnce.ok = exec.Command("docker", "version").Run() == nil
+	})
+	return dockerOnce.ok
+}
+
+// SkipUnlessDocker skips this test unless a working docker daemon is
+// available. Use it for tests that rely on the fstest/testserver docker
+// framework, which is not available on all platforms (e.g. Windows and
+// macOS CI runners).
+func SkipUnlessDocker(t *testing.T) {
+	if !HaveDocker() {
+		t.Skip("Skipping test as docker is not available")
+	}
+}