chore(fs): build kqueue instead of fsevents watcher on iOS (#9950)

### Purpose

On iOS, the FSEvents API for watching files (also used on macOS) is not
available, but `kqueue` is. This PR ensures `kqueue` support is built on
iOS instead of the FSEvents based watcher implementation.

Before this PR, you could already use the `kqueue` build option to force
its usage. Unfortunately `gomobile`, the tool that I use to build
Syncthing for iOS and macOS for Synctrain, does not support setting
different build flags for iOS and macOS (unless I build separately for
each, which is a bit of a hassle because XCode nonsense). I am assuming
there are good reasons to support FSEvents even though `kqueue` is also
available on macOS (but I'm not sure why?). I do know FSEvents has been
working fine for me on macOS so it seems best to use FSEvents on macOS
and kqueue on iOS.

Note that this also requires https://github.com/syncthing/notify/pull/4
to be merged in `synchting/notify` (until that is done, this PR will
fail to build on iOS due to `notify` still trying to link to `fsevents`
stuff when the `kqueue` build flag is not set).

### Testing

I compiled both `syncthing/notify` and syncthing with this PR applied,
and used that to successfully build the Synctrain iOS app, which after
this PR works fine and should follow up file changes a bit quicker.

### Screenshots

n/a

### Documentation

n/a

## Authorship

Your name and email will be added automatically to the AUTHORS file
based on the commit metadata.

---------

Co-authored-by: Jakob Borg <[email protected]>

go.mod

@@ -32,7 +32,7 @@ require (
 	github.com/rabbitmq/amqp091-go v1.10.0
 	github.com/rcrowley/go-metrics v0.0.0-20201227073835-cf1acfcdf475
 	github.com/shirou/gopsutil/v4 v4.25.1
-	github.com/syncthing/notify v0.0.0-20210616190510-c6b7342338d2
+	github.com/syncthing/notify v0.0.0-20250207082249-f0fa8f99c2bc
 	github.com/syndtr/goleveldb v1.0.1-0.20220721030215-126854af5e6d
 	github.com/thejerf/suture/v4 v4.0.6
 	github.com/urfave/cli v1.22.16

go.sum

@@ -227,8 +227,8 @@ github.com/stretchr/testify v1.8.4/go.mod h1:sz/lmYIOXD/1dqDmKjjqLyZ2RngseejIcXl
 github.com/stretchr/testify v1.9.0/go.mod h1:r2ic/lqez/lEtzL7wO/rwa5dbSLXVDPFyf8C91i36aY=
 github.com/stretchr/testify v1.10.0 h1:Xv5erBjTwe/5IxqUQTdXv5kgmIvbHo3QQyRwhJsOfJA=
 github.com/stretchr/testify v1.10.0/go.mod h1:r2ic/lqez/lEtzL7wO/rwa5dbSLXVDPFyf8C91i36aY=
-github.com/syncthing/notify v0.0.0-20210616190510-c6b7342338d2 h1:F4snRP//nIuTTW9LYEzVH4HVwDG9T3M4t8y/2nqMbiY=
-github.com/syncthing/notify v0.0.0-20210616190510-c6b7342338d2/go.mod h1:J0q59IWjLtpRIJulohwqEZvjzwOfTEPp8SVhDJl+y0Y=
+github.com/syncthing/notify v0.0.0-20250207082249-f0fa8f99c2bc h1:xc3UfSFlH/X5hRw3h21RF6WXnRUYKmGRx06FEaVxfkM=
+github.com/syncthing/notify v0.0.0-20250207082249-f0fa8f99c2bc/go.mod h1:J0q59IWjLtpRIJulohwqEZvjzwOfTEPp8SVhDJl+y0Y=
 github.com/syndtr/goleveldb v1.0.1-0.20220721030215-126854af5e6d h1:vfofYNRScrDdvS342BElfbETmL1Aiz3i2t0zfRj16Hs=
 github.com/syndtr/goleveldb v1.0.1-0.20220721030215-126854af5e6d/go.mod h1:RRCYJbIwD5jmqPI9XoAFR0OcDxqUctll6zUj/+B4S48=
 github.com/thejerf/suture/v4 v4.0.6 h1:QsuCEsCqb03xF9tPAsWAj8QOAJBgQI1c0VqJNaingg8=

lib/fs/basicfs_watch.go

@@ -4,12 +4,11 @@
 // License, v. 2.0. If a copy of the MPL was not distributed with this file,
 // You can obtain one at http://mozilla.org/MPL/2.0/.
 
-//go:build !(solaris && !cgo) && !(darwin && !cgo) && !(darwin && kqueue) && !(android && amd64) && !ios
+//go:build !(solaris && !cgo) && !(darwin && !cgo) && !(darwin && kqueue) && !(android && amd64)
 // +build !solaris cgo
 // +build !darwin cgo
 // +build !darwin !kqueue
 // +build !android !amd64
-// +build !ios
 
 package fs
 

lib/fs/basicfs_watch_eventtypes_kqueue.go

@@ -4,8 +4,8 @@
 // License, v. 2.0. If a copy of the MPL was not distributed with this file,
 // You can obtain one at http://mozilla.org/MPL/2.0/.
 
-//go:build dragonfly || freebsd || netbsd || openbsd
-// +build dragonfly freebsd netbsd openbsd
+//go:build dragonfly || freebsd || netbsd || openbsd || ios || kqueue
+// +build dragonfly freebsd netbsd openbsd ios kqueue
 
 package fs
 

lib/fs/basicfs_watch_notkqueue.go

@@ -4,8 +4,8 @@
 // License, v. 2.0. If a copy of the MPL was not distributed with this file,
 // You can obtain one at http://mozilla.org/MPL/2.0/.
 
-//go:build !dragonfly && !freebsd && !netbsd && !openbsd
-// +build !dragonfly,!freebsd,!netbsd,!openbsd
+//go:build !dragonfly && !freebsd && !netbsd && !openbsd && !kqueue && !ios
+// +build !dragonfly,!freebsd,!netbsd,!openbsd,!kqueue,!ios
 
 package fs
 

lib/fs/basicfs_watch_unsupported.go

@@ -4,8 +4,8 @@
 // License, v. 2.0. If a copy of the MPL was not distributed with this file,
 // You can obtain one at http://mozilla.org/MPL/2.0/.
 
-//go:build (solaris && !cgo) || (darwin && !cgo) || (android && amd64) || ios || (darwin && kqueue)
-// +build solaris,!cgo darwin,!cgo android,amd64 ios darwin,kqueue
+//go:build (solaris && !cgo) || (darwin && !cgo) || (android && amd64) || (darwin && kqueue)
+// +build solaris,!cgo darwin,!cgo android,amd64 darwin,kqueue
 
 package fs