Disable multiprocessing if number of workers is 0

This should allow support of Digital Ocean and other hosting
environments that don't support multiprocessing. Note thought there is
no intention of supporting the reloader in this state (as pre 14.3).

src/hypercorn/run.py

@@ -37,50 +37,56 @@ def run(config: Config) -> None:
 
     sockets = config.create_sockets()
 
-    if config.use_reloader:
+    if config.use_reloader and config.workers == 0:
+        raise RuntimeError("Cannot reload without workers")
+
+    if config.use_reloader or config.workers == 0:
         # Load the application so that the correct paths are checked for
         # changes, but only when the reloader is being used.
         load_application(config.application_path, config.wsgi_max_body_size)
 
-    ctx = get_context("spawn")
-
-    active = True
-    while active:
-        # Ignore SIGINT before creating the processes, so that they
-        # inherit the signal handling. This means that the shutdown
-        # function controls the shutdown.
-        signal.signal(signal.SIGINT, signal.SIG_IGN)
-
-        shutdown_event = ctx.Event()
-        processes = start_processes(config, worker_func, sockets, shutdown_event, ctx)
-
-        def shutdown(*args: Any) -> None:
-            nonlocal active, shutdown_event
-            shutdown_event.set()
-            active = False
-
-        for signal_name in {"SIGINT", "SIGTERM", "SIGBREAK"}:
-            if hasattr(signal, signal_name):
-                signal.signal(getattr(signal, signal_name), shutdown)
-
-        if config.use_reloader:
-            wait_for_changes(shutdown_event)
-            shutdown_event.set()
-            # Recreate the sockets to be used again in the next
-            # iteration of the loop.
-            sockets = config.create_sockets()
-        else:
-            active = False
-
-    for process in processes:
-        process.join()
-    for process in processes:
-        process.terminate()
-
-    for sock in sockets.secure_sockets:
-        sock.close()
-    for sock in sockets.insecure_sockets:
-        sock.close()
+    if config.workers == 0:
+        worker_func(config, sockets)
+    else:
+        ctx = get_context("spawn")
+
+        active = True
+        while active:
+            # Ignore SIGINT before creating the processes, so that they
+            # inherit the signal handling. This means that the shutdown
+            # function controls the shutdown.
+            signal.signal(signal.SIGINT, signal.SIG_IGN)
+
+            shutdown_event = ctx.Event()
+            processes = start_processes(config, worker_func, sockets, shutdown_event, ctx)
+
+            def shutdown(*args: Any) -> None:
+                nonlocal active, shutdown_event
+                shutdown_event.set()
+                active = False
+
+            for signal_name in {"SIGINT", "SIGTERM", "SIGBREAK"}:
+                if hasattr(signal, signal_name):
+                    signal.signal(getattr(signal, signal_name), shutdown)
+
+            if config.use_reloader:
+                wait_for_changes(shutdown_event)
+                shutdown_event.set()
+                # Recreate the sockets to be used again in the next
+                # iteration of the loop.
+                sockets = config.create_sockets()
+            else:
+                active = False
+
+        for process in processes:
+            process.join()
+        for process in processes:
+            process.terminate()
+
+        for sock in sockets.secure_sockets:
+            sock.close()
+        for sock in sockets.insecure_sockets:
+            sock.close()
 
 
 def start_processes(