[Utility] Add a cross-platform filesystem watching utility

This adds a cross-plaform filesystem watching utility based on
FSEventStream (macOS) and inotify (Linux). The eventual goal is to add
APIs in libSwiftPM so long living clients like IDE can be notified of
changes in the manifest or the file structure of the package.

The inotify wrapper needs some work but that can happen in follow on
commits.

<rdar://problem/45795219> Add APIs for watching the filesystem for changes

Author: aciidgh

Sources/Utility/FSWatch.swift

@@ -1,5 +1,9 @@
 #include <fts.h>
 
+#if defined(__linux__)
+#include <sys/inotify.h>
+#endif
+
 #define STR_EXPAND(VALUE) #VALUE
 #define STR(VALUE) STR_EXPAND(VALUE)
 

Sources/clibc/include/clibc.h

@@ -0,0 +1,4 @@
+.DS_Store
+/.build
+/Packages
+/*.xcodeproj

Tests/ExtraTests/.gitignore

@@ -0,0 +1,15 @@
+// swift-tools-version:4.2
+
+import PackageDescription
+
+let package = Package(
+    name: "ExtraTests",
+    dependencies: [
+        .package(path: "../../"),
+    ],
+    targets: [
+        .testTarget(
+            name: "ExtraTests",
+            dependencies: ["Utility"]),
+    ]
+)

Tests/ExtraTests/Package.swift

@@ -0,0 +1,4 @@
+# ExtraTests
+
+This contains some extra tests that are not suitable for running on the official
+CI but can be run elsewhere.

Tests/ExtraTests/README.md

@@ -0,0 +1,60 @@
+import XCTest
+
+import Basic
+import Utility 
+
+class FSWatchTests: XCTestCase {
+
+    func testBasics() throws {
+        let tmpDir = try TemporaryDirectory(removeTreeOnDeinit: true)
+        let path = tmpDir.path
+
+        // Construct the paths that we need to watch.
+        let pathsToWatch = [
+            path.appending(component: "foo"),
+            path.appending(component: "bar"),
+        ]
+
+        // Create the paths.
+        for path in pathsToWatch {
+            try localFileSystem.createDirectory(path)
+        }
+
+        let condition = Condition()
+        let delegate = Delegate(condition)
+
+        let watcher = FSWatch(paths: pathsToWatch, delegate: delegate)
+        try watcher.start()
+
+        for file in ["a", "b", "c"] {
+            let filePath = path.appending(components: "foo", file)
+            try localFileSystem.writeFileContents(filePath, bytes: "")
+        }
+
+        condition.whileLocked {
+            condition.wait()
+        }
+
+        XCTAssertFalse(delegate.receivedEvents.isEmpty)
+    }
+}
+
+class Delegate: FSWatchDelegate {
+    var receivedEvents: [AbsolutePath] = []
+
+    let condition: Condition
+
+    init(_ condition: Condition) {
+        self.condition = condition
+    }
+
+    func pathsDidReceiveEvent(_ paths: [AbsolutePath]) {
+        receivedEvents += paths
+
+        print(paths)
+
+        condition.whileLocked {
+            condition.signal()
+        }
+    }
+}

Tests/ExtraTests/Tests/ExtraTests/FSWatchTests.swift

@@ -0,0 +1,18 @@
+#if !canImport(ObjectiveC)
+import XCTest
+
+extension FSWatchTests {
+    // DO NOT MODIFY: This is autogenerated, use: 
+    //   `swift test --generate-linuxmain`
+    // to regenerate.
+    static let __allTests__FSWatchTests = [
+        ("testBasics", testBasics),
+    ]
+}
+
+public func __allTests() -> [XCTestCaseEntry] {
+    return [
+        testCase(FSWatchTests.__allTests__FSWatchTests),
+    ]
+}
+#endif

Tests/ExtraTests/Tests/ExtraTests/XCTestManifests.swift

@@ -0,0 +1,8 @@
+import XCTest
+
+import ExtraTests
+
+var tests = [XCTestCaseEntry]()
+tests += ExtraTests.__allTests()
+
+XCTMain(tests)