journal: add proper StderrIsJournalStream test

To ensure that StderrIsJournalStream properly works in real conditions,
this test re-executes itself with systemd-run, and observes exit code
and logged entries.

journal/journal_unix_test.go

@@ -18,15 +18,19 @@
 package journal_test
 
 import (
+	"crypto/rand"
+	"encoding/json"
 	"fmt"
 	"os"
+	"os/exec"
 	"syscall"
 	"testing"
+	"time"
 
 	"github.com/coreos/go-systemd/v22/journal"
 )
 
-func TestStderrIsJournalStream(t *testing.T) {
+func TestJournalStreamParsing(t *testing.T) {
 	if _, ok := os.LookupEnv("JOURNAL_STREAM"); ok {
 		t.Fatal("unset JOURNAL_STREAM before running this test")
 	}
@@ -84,6 +88,98 @@ func TestStderrIsJournalStream(t *testing.T) {
 	})
 }
 
+func TestStderrIsJournalStream(t *testing.T) {
+	const (
+		message = "TEST_MESSAGE"
+	)
+
+	userOrSystem := "--user"
+	if os.Getuid() == 0 {
+		userOrSystem = "--system"
+	}
+
+	if _, ok := os.LookupEnv("JOURNAL_STREAM"); !ok {
+		// Re-execute this test under systemd (see the else branch),
+		// and observe its exit code and logged entries.
+
+		var randBytes [16]byte
+		_, err := rand.Read(randBytes[:])
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		unitName := fmt.Sprintf("run-r%x.service", randBytes[:])
+
+		args := []string{
+			"systemd-run",
+			"--unit", unitName,
+			userOrSystem,
+			"--wait",
+			"--quiet",
+			"--",
+			os.Args[0],
+			"-test.run=TestStderrIsJournalStream",
+			"-test.count=1", // inhibit caching
+		}
+
+		cmd := exec.Command(args[0], args[1:]...)
+		cmd.Stderr = os.Stderr
+		err = cmd.Run()
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		cmd = exec.Command(
+			"journalctl",
+			userOrSystem,
+			"-u", unitName,
+			"-o", "json",
+			"MESSAGE="+message,
+		)
+		output, err := cmd.Output()
+		if err != nil {
+			t.Fatal(err)
+		}
+		t.Logf("Journalled output: %s", output)
+
+		var entry map[string]interface{}
+
+		err = json.Unmarshal(output, &entry)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		if expected, got := "journal", entry["_TRANSPORT"]; expected != got {
+			t.Errorf("wrong _TRANSPORT field value: expected %q got %q", expected, got)
+		}
+
+	} else {
+		ok, err := journal.StderrIsJournalStream()
+		if err != nil {
+			t.Fatal(err)
+		}
+		if !ok {
+			t.Fatal("StderrIsJournalStream should've returned true")
+		}
+
+		err = journal.Send(message, journal.PriInfo, nil)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		if userOrSystem == "--system" {
+			err = exec.Command("journalctl", "--sync").Run()
+			if err != nil {
+				t.Fatal(err)
+			}
+		} else {
+			// XXX there's no good way to ensure that journald has received
+			// and stored our message.
+			time.Sleep(time.Second)
+		}
+	}
+}
+
 func ExampleStderrIsJournalStream() {
 	// NOTE: this is just an example. Production code
 	// will likely use this to setup a logging library