Projects: Switch to use a polling observer in its watcher

That's because it's more performant on Linux and behaves better on
Windows. It seems there are no issues with the current observer on Mac,
but I think it's better to have a single solution for all OSes.

spyder/plugins/projects/utils/watcher.py

@@ -7,19 +7,17 @@
 """Watcher to detect filesystem changes in the project's directory."""
 
 # Standard lib imports
+import os
 import logging
 
 # Third-party imports
 from qtpy.QtCore import QObject, Signal
-from qtpy.QtWidgets import QMessageBox
 from superqt.utils import qthrottled
 import watchdog
-from watchdog.observers import Observer
 from watchdog.events import FileSystemEventHandler, PatternMatchingEventHandler
+from watchdog.observers.polling import PollingObserverVFS
 
 # Local imports
-from spyder.config.base import _
-from spyder.py3compat import to_text_string
 from spyder.config.utils import get_edit_extensions
 
 
@@ -142,42 +140,26 @@ def connect_signals(self, project):
         self.sig_file_modified.connect(project.file_modified)
 
     def start(self, workspace_folder):
-        # Needed to handle an error caused by the inotify limit reached.
-        # See spyder-ide/spyder#10478
+        # We use a polling observer because:
+        # * It doesn't introduce long freezes on Linux when switching git
+        #   branches that have many changes between them. That's because the
+        #   OS-based observer (i.e. inotify) generates way too many events.
+        # * The OS-based observer on Windows has many shortcomings (see
+        #   openmsi/openmsistream#56).
+        # * There doesn't seem to be issues on Mac, but it's simpler to use a
+        #   single observer for all OSes.
+        self.observer = PollingObserverVFS(stat=os.stat, listdir=os.scandir)
+
+        self.observer.schedule(
+            self.event_handler, workspace_folder, recursive=True
+        )
+
         try:
-            self.observer = Observer()
-            self.observer.schedule(
-                self.event_handler, workspace_folder, recursive=True)
-            try:
-                self.observer.start()
-            except OSError:
-                # This error happens frequently on Linux
-                logger.debug("Watcher could not be started.")
-        except OSError as e:
-            self.observer = None
-            if u'inotify' in to_text_string(e):
-                QMessageBox.warning(
-                    self.parent(),
-                    "Spyder",
-                    _("File system changes for this project can't be tracked "
-                      "because it contains too many files. To fix this you "
-                      "need to increase the inotify limit in your system, "
-                      "with the following command:"
-                      "<br><br>"
-                      "<code>"
-                      "sudo sysctl -n -w fs.inotify.max_user_watches=524288"
-                      "</code>"
-                      "<br><br>For a permanent solution you need to add to"
-                      "<code>/etc/sysctl.conf</code>"
-                      "the following line:<br><br>"
-                      "<code>"
-                      "fs.inotify.max_user_watches=524288"
-                      "</code>"
-                      "<br><br>"
-                      "After doing that, you need to close and start Spyder "
-                      "again so those changes can take effect."))
-            else:
-                raise e
+            self.observer.start()
+        except Exception:
+            logger.debug(
+                f"Observer could not be started for: {workspace_folder}."
+            )
 
     def stop(self):
         if self.observer is not None: