Avoid recursive FileSystemWatcher when not required in PhysicalFilesWatcher

[svick]

Note

This pull request was authored by GitHub Copilot.

PhysicalFilesWatcher previously set IncludeSubdirectories = true unconditionally on the wrapped FileSystemWatcher. On Linux this means inotify watches are created for every descendant directory under the root, which is expensive (and can hit the per-user inotify limit) when the root is something like / — as is the case when an application is launched by systemd without an explicit WorkingDirectory. The default Hosting setup only watches appsettings.json and appsettings.<env>.json, neither of which actually requires recursion.

Now IncludeSubdirectories is enabled only when at least one registered token's pattern actually references a subdirectory:

• a file path token whose path contains /
• a wildcard token whose pattern contains / or **
• or the wrapped FileSystemWatcher watches a strict ancestor of _root (where every event is in a subdirectory from the FSW's perspective)

The state is maintained via a small counter that is incremented when a qualifying token is actually added to the lookup and decremented when one is removed (via ReportChangeForMatchedEntries or OnError's CancelAll). The FSW's IncludeSubdirectories is re-evaluated every time TryEnableFileSystemWatcher runs (which is once per CreateFileChangeToken call), so the value can both go from false to true (when a subdirectory pattern is added) and back to false (when subdirectory tokens have all fired and a new root-only token is registered).

Fixes #126708.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-io
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull request overview

This PR changes Microsoft.Extensions.FileProviders.Physical’s PhysicalFilesWatcher to avoid enabling recursive FileSystemWatcher.IncludeSubdirectories unless it’s actually needed by the registered change-token patterns, reducing the risk of expensive recursive watch setup (notably on Linux/inotify) for common root-only patterns.

Changes:

• Track whether any currently-registered token patterns require subdirectory watching and toggle IncludeSubdirectories accordingly when enabling the watcher.
• Handle the “FSW path is an ancestor of _root” case by always requiring subdirectory watching.
• Add tests validating upgrade/downgrade behavior of IncludeSubdirectories as tokens are added and removed.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/libraries/Microsoft.Extensions.FileProviders.Physical/src/PhysicalFilesWatcher.cs	Introduces _fileWatcherIsAboveRoot and a token counter to conditionally set IncludeSubdirectories in TryEnableFileSystemWatcher, and keeps the counter in sync on add/remove/error paths.
src/libraries/Microsoft.Extensions.FileProviders.Physical/tests/PhysicalFilesWatcherTests.cs	Adds unit tests to verify IncludeSubdirectories is enabled only when patterns require it and that it can upgrade/downgrade as tokens come and go.