Core, WebHost: lazy-load worlds in unpickler, WebHost and WebHostLib (#2156)

* Core: lazy-load worlds in unpickler

this should hopefully fix customserver's memory consumption

* WebHost: move imports around to save memory in MP

* MultiServer: prefer loading _speedups without pyximport

This saves ~15MB per MP and speeds up module import if it was built in-place.

* Tests: fix tests for changed WebHost imports

* CustomServer: run GC after setup

* CustomServer: cleanup exception handling

WebHostLib/locker.py

@@ -0,0 +1,51 @@
+import os
+import sys
+
+
+class CommonLocker:
+    """Uses a file lock to signal that something is already running"""
+    lock_folder = "file_locks"
+
+    def __init__(self, lockname: str, folder=None):
+        if folder:
+            self.lock_folder = folder
+        os.makedirs(self.lock_folder, exist_ok=True)
+        self.lockname = lockname
+        self.lockfile = os.path.join(self.lock_folder, f"{self.lockname}.lck")
+
+
+class AlreadyRunningException(Exception):
+    pass
+
+
+if sys.platform == 'win32':
+    class Locker(CommonLocker):
+        def __enter__(self):
+            try:
+                if os.path.exists(self.lockfile):
+                    os.unlink(self.lockfile)
+                self.fp = os.open(
+                    self.lockfile, os.O_CREAT | os.O_EXCL | os.O_RDWR)
+            except OSError as e:
+                raise AlreadyRunningException() from e
+
+        def __exit__(self, _type, value, tb):
+            fp = getattr(self, "fp", None)
+            if fp:
+                os.close(self.fp)
+                os.unlink(self.lockfile)
+else:  # unix
+    import fcntl
+
+
+    class Locker(CommonLocker):
+        def __enter__(self):
+            try:
+                self.fp = open(self.lockfile, "wb")
+                fcntl.flock(self.fp.fileno(), fcntl.LOCK_EX | fcntl.LOCK_NB)
+            except OSError as e:
+                raise AlreadyRunningException() from e
+
+        def __exit__(self, _type, value, tb):
+            fcntl.flock(self.fp.fileno(), fcntl.LOCK_UN)
+            self.fp.close()