root: handle SIGHUP and SIGUSR2, healthcheck gunicorn (#6630)

Co-authored-by: Jens Langhammer <jens@goauthentik.io>

internal/utils/process.go

@@ -0,0 +1,39 @@
+package utils
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"syscall"
+)
+
+func FindProcess(pid int) (*os.Process, error) {
+	if pid <= 0 {
+		return nil, fmt.Errorf("invalid pid %v", pid)
+	}
+	// The error doesn't mean anything on Unix systems, let's just check manually
+	// that the new gunicorn master has properly started
+	// https://github.com/golang/go/issues/34396
+	proc, err := os.FindProcess(pid)
+	if err != nil {
+		return nil, err
+	}
+	err = proc.Signal(syscall.Signal(0))
+	if err == nil {
+		return proc, nil
+	}
+	if errors.Is(err, os.ErrProcessDone) {
+		return nil, nil
+	}
+	errno, ok := err.(syscall.Errno)
+	if !ok {
+		return nil, err
+	}
+	switch errno {
+	case syscall.ESRCH:
+		return nil, nil
+	case syscall.EPERM:
+		return proc, nil
+	}
+	return nil, err
+}