bpo-30014: make poll-like selector's modify() method faster (#1030)

* #30014: make selectors.DefaultSelector.modify() faster by relying on selector's modify() method instead of un/register()ing the fd

* #30014: add unit test

* speedup poll/epoll/devpoll modify() method by using internal modify() call

* update doc

* address PR comments

* update NEWS entries

* use != instead of 'is not'

1 files changed, 29 insertions, 1 deletions

diff --git a/Lib/selectors.py b/Lib/selectors.py
index f29d11fc6a..edde22c634 100644
--- a/Lib/selectors.py
+++ b/Lib/selectors.py
@@ -252,7 +252,6 @@ class _BaseSelectorImpl(BaseSelector):
         return key
 
     def modify(self, fileobj, events, data=None):
-        # TODO: Subclasses can probably optimize this even further.
         try:
             key = self._fd_to_key[self._fileobj_lookup(fileobj)]
         except KeyError:
@@ -342,6 +341,8 @@ class SelectSelector(_BaseSelectorImpl):
 class _PollLikeSelector(_BaseSelectorImpl):
     """Base class shared between poll, epoll and devpoll selectors."""
     _selector_cls = None
+    _EVENT_READ = None
+    _EVENT_WRITE = None
 
     def __init__(self):
         super().__init__()
@@ -371,6 +372,33 @@ class _PollLikeSelector(_BaseSelectorImpl):
             pass
         return key
 
+    def modify(self, fileobj, events, data=None):
+        try:
+            key = self._fd_to_key[self._fileobj_lookup(fileobj)]
+        except KeyError:
+            raise KeyError(f"{fileobj!r} is not registered") from None
+
+        changed = False
+        if events != key.events:
+            selector_events = 0
+            if events & EVENT_READ:
+                selector_events |= self._EVENT_READ
+            if events & EVENT_WRITE:
+                selector_events |= self._EVENT_WRITE
+            try:
+                self._selector.modify(key.fd, selector_events)
+            except Exception:
+                super().unregister(fileobj)
+                raise
+            changed = True
+        if data != key.data:
+            changed = True
+
+        if changed:
+            key = key._replace(events=events, data=data)
+            self._fd_to_key[key.fd] = key
+        return key
+
     def select(self, timeout=None):
         # This is shared between poll() and epoll().
         # epoll() has a different signature and handling of timeout parameter.