Fixes to macOS watcher and ensuring that watch is set up before os.watch() returns (#398)

This PR contains multiple fixes.

1. macOS `FSEventsWatcher` was modified to use the new dispatch
queue-based API instead of the old, deprecated run-loop based API. This
made closing the watcher easier, without any class-level state to
maintain, making sure resource release is always done appropriately.
2. Invoking `watcher.close()` is now threadsafe and idempotent.
3. Invoking `os.watch` now requires the directory to exist, as a
sentinel file is written to that directory to make sure `os.watch` is
actually watching the directory before it returns. An exception is
thrown if the directory does not exist.

Tested with tests added to `os/watch/test/src/WatchTests.scala`.

These tests work fine in current `main` branch as well:
```
[238] + test.os.watch.WatchTests.emptyFolder.singleFileChangeManyTimes 4558ms  
[238] + test.os.watch.WatchTests.manyFiles.inManyFoldersSmall 2068ms  
[238] + test.os.watch.WatchTests.manyFiles.inManyFoldersMedium 3369ms  
[238] + test.os.watch.WatchTests.manyFiles.inManyFoldersLarge 4056ms  
[238] + test.os.watch.WatchTests.manyFiles.inManyFoldersLargest 15ms  
[238] + test.os.watch.WatchTests.manyFiles.inManyFoldersThreaded 28817ms  
[238] + test.os.watch.WatchTests.manyFiles.inManyFoldersThreadedSequential 26746ms  
[238] + test.os.watch.WatchTests.openClose.once 15ms
```

These tests reliably exhibit the [JVM
crash](com-lihaoyi/mill#5225) in `main`
branch:
```
arturaz@Arturass-MacBook-Air os-lib % ./mill -w 'os.watch.jvm[3.3.5].test.test' test.os.watch.WatchTests.emptyFolder.doesNotLeaveSentinel
[238/238] os.watch.jvm[3.3.5].test.test
[238] --- Running Tests test.os.watch.WatchTests.emptyFolder.doesNotLeaveSentinel ---
[238] #
[238] # A fatal error has been detected by the Java Runtime Environment:
[238] #
[238] #  SIGSEGV (0xb) at pc=0x000000018409957c, pid=41664, tid=3843
[238] #
[238] # JRE version: OpenJDK Runtime Environment Temurin-21.0.1+12 (21.0.1+12) (build 21.0.1+12-LTS)
[238] # Java VM: OpenJDK 64-Bit Server VM Temurin-21.0.1+12 (21.0.1+12-LTS, mixed mode, tiered, compressed oops, compressed class ptrs, g1 gc, bsd-aarch64)
[238] # Problematic frame:
[238] # C  [CoreFoundation+0x15057c]  __CFCheckCFInfoPACSignature+0x4
[238] #
[238] # No core dump will be written. Core dumps have been disabled. To enable core dumping, try "ulimit -c unlimited" before starting Java again
[238] #
[238] # An error report file with more information is saved as:
[238] # /Users/arturaz/work/os-lib/out/os/watch/jvm/3.3.5/test/test.dest/sandbox/hs_err_pid41664.log
[238] #
[238] # If you would like to submit a bug report, please visit:
[238] #   https://github.com/adoptium/adoptium-support/issues
[238] # The crash happened outside the Java Virtual Machine in native code.
[238] # See problematic frame for where to report the bug.
[238] #
```

These tests either fail in `main` branch because the behaviour there is
different:
```
[238] ----------- Running Tests test.os.watch.WatchTests.nonExistentFolder -----------
[238] X test.os.watch.WatchTests.nonExistentFolder 213ms 
[238]   utest.AssertionError: _root_.os.watch.watch(Seq(wd / "does-not-exist"), onEvent = _ => ())
[238]   wd: os.Path = /Users/arturaz/work/os-lib/out/os/watch/jvm/3.3.5/test/test.dest/sandbox/out/scratch/watch/WatchTests/tests/nonExistentFolder
[238]     utest.asserts.Util$.assertError(Util.scala:20)
[238]     utest.asserts.Asserts$.interceptImpl(Asserts.scala:49)
[238]     test.os.watch.WatchTests$.$init$$$anonfun$1$$anonfun$1$$anonfun$1(WatchTests.scala:60)
[238]     test.os.TestUtil$.prep$$anonfun$1(TestUtil.scala:92)
[238]     test.os.TestUtil$.mkDir(TestUtil.scala:64)
[238]     test.os.TestUtil$.prep(TestUtil.scala:93)
[238]     test.os.watch.WatchTests$.$init$$$anonfun$1$$anonfun$1(WatchTests.scala:60)
[238] Tests: 1, Passed: 0, Failed: 1
```

or randomly crash:
```
[238] -------------------- Running Tests test.os.watch.WatchTests --------------------
[238] X test.os.watch.WatchTests.nonExistentFolder 211ms 
[238]   utest.AssertionError: _root_.os.watch.watch(Seq(wd / "does-not-exist"), onEvent = _ => ())
[238]   wd: os.Path = /Users/arturaz/work/os-lib/out/os/watch/jvm/3.3.5/test/test.dest/sandbox/out/scratch/watch/WatchTests/tests/nonExistentFolder
[238]     utest.asserts.Util$.assertError(Util.scala:20)
[238]     utest.asserts.Asserts$.interceptImpl(Asserts.scala:49)
[238]     test.os.watch.WatchTests$.$init$$$anonfun$1$$anonfun$1$$anonfun$1(WatchTests.scala:60)
[238]     test.os.TestUtil$.prep$$anonfun$1(TestUtil.scala:92)
[238]     test.os.TestUtil$.mkDir(TestUtil.scala:64)
[238]     test.os.TestUtil$.prep(TestUtil.scala:93)
[238]     test.os.watch.WatchTests$.$init$$$anonfun$1$$anonfun$1(WatchTests.scala:60)
[238] #
[238] # A fatal error has been detected by the Java Runtime Environment:
[238] #
[238] #  SIGSEGV (0xb) at pc=0x000000018409957c, pid=33912, tid=4355
[238] #
[238] # JRE version: OpenJDK Runtime Environment Temurin-21.0.1+12 (21.0.1+12) (build 21.0.1+12-LTS)
[238] # Java VM: OpenJDK 64-Bit Server VM Temurin-21.0.1+12 (21.0.1+12-LTS, mixed mode, tiered, compressed oops, compressed class ptrs, g1 gc, bsd-aarch64)
[238] # Problematic frame:
[238] # C  [CoreFoundation+0x15057c]  __CFCheckCFInfoPACSignature+0x4
[238] #
[238] # No core dump will be written. Core dumps have been disabled. To enable core dumping, try "ulimit -c unlimited" before starting Java again
[238] #
[238] # An error report file with more information is saved as:
[238] # /Users/arturaz/work/os-lib/out/os/watch/jvm/3.3.5/test/test.dest/sandbox/hs_err_pid33912.log
[238] [0.663s][warning][os] Loading hsdis library failed
[238] #
[238] # If you would like to submit a bug report, please visit:
[238] #   https://github.com/adoptium/adoptium-support/issues
[238] # The crash happened outside the Java Virtual Machine in native code.
[238] # See problematic frame for where to report the bug.
[238] #
```

This one randomly fails on `main` as well:
```
[238] X test.os.watch.WatchTests.openClose.manyTimes 4668ms 
[238]   java.util.concurrent.TimeoutException: no file system changes detected within 3 seconds
[238]     test.os.watch.WatchTests$.testOpenClose$1$$anonfun$1(WatchTests.scala:354)
[238]     scala.runtime.java8.JFunction1$mcVI$sp.apply(JFunction1$mcVI$sp.scala:18)
[238]     scala.collection.immutable.Range.foreach(Range.scala:192)
[238]     test.os.watch.WatchTests$.testOpenClose$1(WatchTests.scala:333)
[238]     test.os.watch.WatchTests$.$init$$$anonfun$1$$anonfun$5$$anonfun$2$$anonfun$1(WatchTests.scala:367)
[238]     scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
[238]     scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
[238]     test.os.TestUtil$.prep$$anonfun$1(TestUtil.scala:92)
[238]     test.os.TestUtil$.mkDir(TestUtil.scala:64)
[238]     test.os.TestUtil$.prep(TestUtil.scala:93)
[238]     test.os.watch.WatchTests$.$init$$$anonfun$1$$anonfun$5$$anonfun$2(WatchTests.scala:60)
```

This one fails because the behaviour has changed:
```
[238] X test.os.watch.WatchTests.closeIsSafeToInvokeMultipleTimes 210ms 
[238]   java.lang.AssertionError: assertion failed
[238]     scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:11)
[238]     os.watch.FSEventsWatcher.close(FSEventsWatcher.scala:101)
[238]     test.os.watch.WatchTests$.$init$$$anonfun$1$$anonfun$6$$anonfun$1(WatchTests.scala:379)
[238]     scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
[238]     scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
[238]     test.os.TestUtil$.mkDir(TestUtil.scala:64)
[238]     test.os.watch.WatchTests$.$init$$$anonfun$1$$anonfun$6(WatchTests.scala:60)
```

This should fix the issue witnessed in mill
(com-lihaoyi/mill#5225)

Author: arturaz

.gitignore

@@ -8,6 +8,8 @@ target/
 .sbtserver
 .scala-build/
 .bsp/
+.bloop
+.metals
 project/.sbtserver
 tags
 nohup.out

.vscode/settings.json

@@ -0,0 +1,5 @@
+{
+  "files.watcherExclude": {
+    "**/target": true
+  }
+}

os/test/src/TestUtil.scala

@@ -38,7 +38,8 @@ object TestUtil {
     str1Normalized == str2Normalized
   }
 
-  def prep[T](f: os.Path => T)(implicit tp: TestPath, fn: sourcecode.FullName) = {
+  /** Creates a temporary directory for the current test. */
+  def mkDir[T](f: os.Path => T)(implicit tp: TestPath, fn: sourcecode.FullName) = {
     val segments = Seq("out", "scratch") ++ fn.value.split('.').drop(2) ++ tp.value
 
     val directory = Paths.get(segments.mkString("/"))
@@ -58,6 +59,15 @@ object TestUtil {
       }
     )
 
+    val wd = os.Path(directory.toAbsolutePath)
+    os.makeDir.all.apply(wd)
+    f(wd)
+  }
+
+  /** Populates the directory with test resources. */
+  def populate[T](directory: Path)(implicit tp: TestPath, fn: sourcecode.FullName): Unit = {
+    if (os.exists(os.Path(directory))) os.remove.all(os.Path(directory))
+
     val original = Paths.get(sys.env("OS_TEST_RESOURCE_FOLDER"), "test")
     Files.walkFileTree(
       original,
@@ -73,8 +83,14 @@ object TestUtil {
         }
       }
     )
+  }
 
-    f(os.Path(directory.toAbsolutePath))
+  /** Creates a temporary directory for the current test and populates it with test resources. */
+  def prep[T](f: os.Path => T)(implicit tp: TestPath, fn: sourcecode.FullName) = {
+    mkDir { wd =>
+      populate(wd.toNIO)
+      f(wd)
+    }
   }
 
   def prepChecker[T](f: os.Path => T)(implicit tp: TestPath, fn: sourcecode.FullName): T =