Log fx's startup steps at debug; a refused setting is one line (closes #209)

fx's event lines, errors included, now log at debug level, so a normal
start shows none and DEBUG shows them. RunServer starts and stops the
fx app itself instead of calling Run: when the start fails, it logs the
error's root cause as one "cannot start" line, the backend's last, and
returns 1. Main returns the server's exit code.

Model: opus-5-5

cmd/netwatch-server/main_test.go

@@ -36,16 +36,94 @@ func TestMain(m *testing.M) {
 	os.Exit(m.Run())
 }
 
-// TestOutputIsJSON: off a terminal, every line the server writes from
-// start to stop is JSON, fx's own lines included.
-func TestOutputIsJSON(t *testing.T) {
+// TestOutputIsJSONWithoutFxLines: off a terminal, every line the server
+// writes from start to stop is JSON, and without DEBUG none of them is
+// one of fx's.
+func TestOutputIsJSONWithoutFxLines(t *testing.T) {
+	t.Parallel()
+
+	stdout, stderr := runServerUntilStopped(t)
+
+	requireJSONLines(t, stdout, stderr)
+
+	if !strings.Contains(stdout.String(), `"msg":"starting"`) {
+		t.Fatalf("no startup line in stdout:\n%s", stdout)
+	}
+
+	if strings.Contains(stdout.String(), `"msg":"started"`) {
+		t.Fatalf("fx line in stdout without DEBUG:\n%s", stdout)
+	}
+}
+
+// TestDebugLogsFxLines: with DEBUG set, fx's lines are logged, as
+// JSON.
+func TestDebugLogsFxLines(t *testing.T) {
+	t.Parallel()
+
+	stdout, stderr := runServerUntilStopped(t, "DEBUG=1")
+
+	requireJSONLines(t, stdout, stderr)
+
+	if !strings.Contains(stdout.String(), `"msg":"started"`) {
+		t.Fatalf("no fx line in stdout with DEBUG set:\n%s", stdout)
+	}
+}
+
+// TestRefusedSettingIsLastLine: a setting the server refuses stops the
+// start, and the last line logged is one error naming the setting and
+// why, without fx's text around it.
+func TestRefusedSettingIsLastLine(t *testing.T) {
 	t.Parallel()
 
 	ctx, cancel := context.WithTimeout(t.Context(), childTimeout)
 	defer cancel()
 
+	child, stdout, stderr := startServer(ctx, t, t.TempDir(),
+		freePort(ctx, t), "REPORTS_PER_MINUTE=0")
+
+	err := child.Wait()
+	if child.ProcessState.ExitCode() != 1 {
+		t.Fatalf("server exit = %v, want exit status 1", err)
+	}
+
+	if stderr.Len() != 0 {
+		t.Fatalf("stderr = %q, want nothing", stderr)
+	}
+
+	lines := strings.Split(strings.TrimSpace(stdout.String()), "\n")
+
+	var last struct {
+		Level string `json:"level"`
+		Error string `json:"error"`
+	}
+
+	err = json.Unmarshal([]byte(lines[len(lines)-1]), &last)
+	if err != nil {
+		t.Fatalf("last line is not JSON: %v\n%s", err, stdout)
+	}
+
+	want := `REPORTS_PER_MINUTE "0": must be a positive whole number`
+	if last.Level != "ERROR" || last.Error != want {
+		t.Fatalf("last line has level %q, error %q; want ERROR, %q\n%s",
+			last.Level, last.Error, want, stdout)
+	}
+}
+
+// runServerUntilStopped runs main() in a child process with env added
+// to its environment, waits until it answers its health check, stops it
+// with SIGTERM and returns its stdout and stderr. It fails the test if
+// the child does not exit cleanly.
+func runServerUntilStopped(
+	t *testing.T,
+	env ...string,
+) (*bytes.Buffer, *bytes.Buffer) {
+	t.Helper()
+
+	ctx, cancel := context.WithTimeout(t.Context(), childTimeout)
+	defer cancel()
+
 	port := freePort(ctx, t)
-	child, stdout, stderr := startServer(ctx, t, t.TempDir(), port)
+	child, stdout, stderr := startServer(ctx, t, t.TempDir(), port, env...)
 
 	waitForHealthcheck(ctx, t, port)
 
@@ -72,11 +150,7 @@ func TestOutputIsJSON(t *testing.T) {
 		t.Fatalf("server exit = %v, want success", err)
 	}
 
-	requireJSONLines(t, stdout, stderr)
-
-	if !strings.Contains(stdout.String(), `"msg":"starting"`) {
-		t.Fatalf("no startup line in stdout:\n%s", stdout)
-	}
+	return stdout, stderr
 }
 
 // TestMalformedConfigFileStopsTheStart: a config file the server finds
@@ -139,7 +213,8 @@ func TestRefusedSentryDSNStopsTheStart(t *testing.T) {
 
 // startServer runs main() in a child process listening on
 // 127.0.0.1:port, with home as its HOME and working directory and its
-// data directory in home, so it touches nothing outside home. Its
+// data directory in home, so it touches nothing outside home. env is
+// added to its environment, as NAME=value strings. Its
 // stdout and stderr go to the two buffers returned, which hold all of
 // it once child.Wait returns. The child is killed when ctx ends, and
 // killed and reaped when the test ends if nothing waited for it.
@@ -147,6 +222,7 @@ func startServer(
 	ctx context.Context,
 	t *testing.T,
 	home, port string,
+	env ...string,
 ) (*exec.Cmd, *bytes.Buffer, *bytes.Buffer) {
 	t.Helper()
 
@@ -166,6 +242,7 @@ func startServer(
 		"BIND_ADDRESS=127.0.0.1",
 		"PORT="+port,
 	)
+	child.Env = append(child.Env, env...)
 	child.Stdout = &stdout
 	child.Stderr = &stderr