LibCore: Improve support for the macOS file watcher with actual files

When asked to monitor a file (not a directory), we often need to instead
monitor the parent directory to receive FS events. For example, when a
symbolic link is deleted/created, we don't receive any events unless we
are watching the parent.

Tests/LibCore/TestLibCoreFileWatcher.cpp

@@ -4,9 +4,13 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <AK/LexicalPath.h>
 #include <LibCore/EventLoop.h>
+#include <LibCore/File.h>
 #include <LibCore/FileWatcher.h>
+#include <LibCore/System.h>
 #include <LibCore/Timer.h>
+#include <LibFileSystem/FileSystem.h>
 #include <LibTest/TestCase.h>
 #include <fcntl.h>
 #include <unistd.h>
@@ -61,3 +65,45 @@ TEST_CASE(file_watcher_child_events)
 
     event_loop.exec();
 }
+
+TEST_CASE(contents_changed)
+{
+    auto event_loop = Core::EventLoop();
+
+    auto temp_path = MUST(FileSystem::real_path("/tmp"sv));
+    auto test_path = LexicalPath::join(temp_path, "testfile"sv);
+
+    auto write_file = [&](auto contents) {
+        auto file = MUST(Core::File::open(test_path.string(), Core::File::OpenMode::Write));
+        MUST(file->write_until_depleted(contents));
+    };
+
+    write_file("line1\n"sv);
+
+    auto file_watcher = MUST(Core::FileWatcher::create());
+    MUST(file_watcher->add_watch(test_path.string(), Core::FileWatcherEvent::Type::ContentModified));
+
+    int event_count = 0;
+    file_watcher->on_change = [&](Core::FileWatcherEvent const& event) {
+        EXPECT_EQ(event.event_path, test_path.string());
+        EXPECT(has_flag(event.type, Core::FileWatcherEvent::Type::ContentModified));
+
+        if (++event_count == 2) {
+            MUST(Core::System::unlink(test_path.string()));
+            event_loop.quit(0);
+        }
+    };
+
+    auto timer1 = Core::Timer::create_single_shot(500, [&] { write_file("line2\n"sv); });
+    timer1->start();
+
+    auto timer2 = Core::Timer::create_single_shot(1000, [&] { write_file("line3\n"sv); });
+    timer2->start();
+
+    auto catchall_timer = Core::Timer::create_single_shot(2000, [&] {
+        VERIFY_NOT_REACHED();
+    });
+    catchall_timer->start();
+
+    event_loop.exec();
+}

Userland/Libraries/LibCore/FileWatcherMacOS.mm

@@ -26,6 +26,7 @@ namespace Core {
 struct MonitoredPath {
     ByteString path;
     FileWatcherEvent::Type event_mask { FileWatcherEvent::Type::Invalid };
+    bool is_directory { false };
 };
 
 static void on_file_system_event(ConstFSEventStreamRef, void*, size_t, void*, FSEventStreamEventFlags const[], FSEventStreamEventId const[]);
@@ -36,6 +37,13 @@ static ErrorOr<ino_t> inode_id_from_path(StringView path)
     return stat.st_ino;
 }
 
+static ErrorOr<bool> is_directory(StringView path)
+{
+    // We cannot use FileSystem::is_directory as LibFileSystem depends on LibCore.
+    auto stat = TRY(System::stat(path));
+    return S_ISDIR(stat.st_mode);
+}
+
 class FileWatcherMacOS final : public FileWatcher {
     AK_MAKE_NONCOPYABLE(FileWatcherMacOS);
 
@@ -64,6 +72,10 @@ public:
 
     ErrorOr<bool> add_watch(ByteString path, FileWatcherEvent::Type event_mask)
     {
+        auto path_is_directory = TRY(is_directory(path));
+        if (!path_is_directory)
+            path = LexicalPath { move(path) }.parent().string();
+
         if (m_path_to_inode_id.contains(path)) {
             dbgln_if(FILE_WATCHER_DEBUG, "add_watch: path '{}' is already being watched", path);
             return false;
@@ -71,7 +83,7 @@ public:
 
         auto inode_id = TRY(inode_id_from_path(path));
         TRY(m_path_to_inode_id.try_set(path, inode_id));
-        TRY(m_inode_id_to_path.try_set(inode_id, { path, event_mask }));
+        TRY(m_inode_id_to_path.try_set(inode_id, { path, event_mask, path_is_directory }));
 
         TRY(refresh_monitored_paths());
 
@@ -81,6 +93,9 @@ public:
 
     ErrorOr<bool> remove_watch(ByteString path)
     {
+        if (!TRY(is_directory(path)))
+            path = LexicalPath { move(path) }.parent().string();
+
         auto it = m_path_to_inode_id.find(path);
         if (it == m_path_to_inode_id.end()) {
             dbgln_if(FILE_WATCHER_DEBUG, "remove_watch: path '{}' is not being watched", path);
@@ -109,7 +124,8 @@ public:
 
         return MonitoredPath {
             LexicalPath::join(it->value.path, lexical_path.basename()).string(),
-            it->value.event_mask
+            it->value.event_mask,
+            it->value.is_directory
         };
     }
 
@@ -225,10 +241,16 @@ void on_file_system_event(ConstFSEventStreamRef, void* user_data, size_t event_s
         event.event_path = move(monitored_path.path);
 
         auto flags = event_flags[i];
-        if ((flags & kFSEventStreamEventFlagItemCreated) != 0)
-            event.type |= FileWatcherEvent::Type::ChildCreated;
-        if ((flags & kFSEventStreamEventFlagItemRemoved) != 0)
-            event.type |= FileWatcherEvent::Type::ChildDeleted;
+        if ((flags & kFSEventStreamEventFlagItemCreated) != 0) {
+            if (monitored_path.is_directory)
+                event.type |= FileWatcherEvent::Type::ChildCreated;
+        }
+        if ((flags & kFSEventStreamEventFlagItemRemoved) != 0) {
+            if (monitored_path.is_directory)
+                event.type |= FileWatcherEvent::Type::ChildDeleted;
+            else
+                event.type |= FileWatcherEvent::Type::Deleted;
+        }
         if ((flags & kFSEventStreamEventFlagItemModified) != 0)
             event.type |= FileWatcherEvent::Type::ContentModified;
         if ((flags & kFSEventStreamEventFlagItemInodeMetaMod) != 0)