LibCore: Added FileWatcher, a binding for the watch_file syscall

This wrapper abstracts the watch_file setup and file handling, and
allows using the watch_file events as part of the event loop via the
Core::Notifier class.

Also renames the existing DirectoryWatcher class to BlockingFileWatcher,
and adds support for the Modified mode in this class.

7 files changed, 216 insertions, 119 deletions

diff --git a/AK/Debug.h.in b/AK/Debug.h.in
index 6a23a9935c..4b6ca779de 100644
--- a/AK/Debug.h.in
+++ b/AK/Debug.h.in
@@ -138,6 +138,10 @@
 #cmakedefine01 FILE_CONTENT_DEBUG
 #endif
 
+#ifndef FILE_WATCHER_DEBUG
+#cmakedefine01 FILE_WATCHER_DEBUG
+#endif
+
 #ifndef FILL_PATH_DEBUG
 #cmakedefine01 FILL_PATH_DEBUG
 #endif
diff --git a/Meta/CMake/all_the_debug_macros.cmake b/Meta/CMake/all_the_debug_macros.cmake
index 097ae7a555..d2fa65a8e2 100644
--- a/Meta/CMake/all_the_debug_macros.cmake
+++ b/Meta/CMake/all_the_debug_macros.cmake
@@ -165,6 +165,7 @@ set(CPP_DEBUG ON)
 set(DEBUG_SPAM ON)
 set(DEBUG_CPP_LANGUAGE_SERVER ON)
 set(DEBUG_AUTOCOMPLETE ON)
+set(FILE_WATCHER_DEBUG ON)
 
 # False positive: DEBUG is a flag but it works differently.
 # set(DEBUG ON)
diff --git a/Userland/Libraries/LibCore/CMakeLists.txt b/Userland/Libraries/LibCore/CMakeLists.txt
index ee6c046bb9..2705753b34 100644
--- a/Userland/Libraries/LibCore/CMakeLists.txt
+++ b/Userland/Libraries/LibCore/CMakeLists.txt
@@ -5,11 +5,11 @@ set(SOURCES
     ConfigFile.cpp
     Command.cpp
     DateTime.cpp
-    DirectoryWatcher.cpp
     DirIterator.cpp
     ElapsedTimer.cpp
     Event.cpp
     EventLoop.cpp
+    FileWatcher.cpp
     File.cpp
     GetPassword.cpp
     Gzip.cpp
diff --git a/Userland/Libraries/LibCore/DirectoryWatcher.cpp b/Userland/Libraries/LibCore/DirectoryWatcher.cpp
deleted file mode 100644
index a363c2461d..0000000000
--- a/Userland/Libraries/LibCore/DirectoryWatcher.cpp
+++ /dev/null
@@ -1,98 +0,0 @@
-/*
- * Copyright (c) 2020, Itamar S. <itamar8910@gmail.com>
- * All rights reserved.
- *
- * Redistribution and use in source and binary forms, with or without
- * modification, are permitted provided that the following conditions are met:
- *
- * 1. Redistributions of source code must retain the above copyright notice, this
- *    list of conditions and the following disclaimer.
- *
- * 2. Redistributions in binary form must reproduce the above copyright notice,
- *    this list of conditions and the following disclaimer in the documentation
- *    and/or other materials provided with the distribution.
- *
- * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
- * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
- * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
- * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
- * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
- * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
- * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
- * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
- * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
- * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
- */
-
-#include "DirectoryWatcher.h"
-#include <AK/LexicalPath.h>
-#include <AK/Optional.h>
-#include <LibCore/DirIterator.h>
-#include <fcntl.h>
-#include <sys/stat.h>
-
-namespace Core {
-
-// Only supported in serenity mode because we use `watch_file`
-#ifdef __serenity__
-
-DirectoryWatcher::DirectoryWatcher(const String& path)
-    : m_path(path)
-{
-    m_watcher_fd = watch_file(path.characters(), path.length());
-    ASSERT(m_watcher_fd != -1);
-}
-
-DirectoryWatcher::~DirectoryWatcher()
-{
-    close(m_watcher_fd);
-}
-
-Optional<DirectoryWatcher::Event> DirectoryWatcher::wait_for_event()
-{
-    InodeWatcherEvent event {};
-    int rc = read(m_watcher_fd, &event, sizeof(event));
-    if (rc <= 0)
-        return {};
-
-    Event result;
-    if (event.type == InodeWatcherEvent::Type::ChildAdded)
-        result.type = Event::Type::ChildAdded;
-    else if (event.type == InodeWatcherEvent::Type::ChildRemoved)
-        result.type = Event::Type::ChildRemoved;
-    else
-        return {};
-
-    auto child_path = get_child_with_inode_index(event.inode_index);
-    if (!LexicalPath(child_path).is_valid())
-        return {};
-
-    result.child_path = child_path;
-    return result;
-}
-
-String DirectoryWatcher::get_child_with_inode_index(unsigned child_inode_index) const
-{
-    DirIterator iterator(m_path, Core::DirIterator::SkipDots);
-    if (iterator.has_error()) {
-        return {};
-    }
-
-    while (iterator.has_next()) {
-        auto child_full_path = String::formatted("{}/{}", m_path, iterator.next_path());
-        struct stat st;
-
-        if (lstat(child_full_path.characters(), &st)) {
-            return {};
-        }
-
-        if (st.st_ino == child_inode_index) {
-            return child_full_path;
-        }
-    }
-    return {};
-}
-
-#endif
-
-}
diff --git a/Userland/Libraries/LibCore/FileWatcher.cpp b/Userland/Libraries/LibCore/FileWatcher.cpp
new file mode 100644
index 0000000000..19bb73eb8a
--- /dev/null
+++ b/Userland/Libraries/LibCore/FileWatcher.cpp
@@ -0,0 +1,171 @@
+/*
+ * Copyright (c) 2020, Itamar S. <itamar8910@gmail.com>
+ * Copyright (c) 2021, the SerenityOS developers.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "FileWatcher.h"
+#include <AK/Debug.h>
+#include <AK/Function.h>
+#include <AK/LexicalPath.h>
+#include <AK/Noncopyable.h>
+#include <AK/NonnullRefPtr.h>
+#include <AK/RefCounted.h>
+#include <AK/Result.h>
+#include <AK/String.h>
+#include <Kernel/API/InodeWatcherEvent.h>
+#include <LibCore/DirIterator.h>
+#include <LibCore/Notifier.h>
+#include <fcntl.h>
+#include <string.h>
+#include <sys/stat.h>
+
+namespace Core {
+
+// Only supported in serenity mode because we use `watch_file`
+#ifdef __serenity__
+
+static String get_child_path_from_inode_index(const String& path, unsigned child_inode_index)
+{
+    DirIterator iterator(path, Core::DirIterator::SkipDots);
+    if (iterator.has_error()) {
+        return {};
+    }
+
+    while (iterator.has_next()) {
+        auto child_full_path = String::formatted("{}/{}", path, iterator.next_path());
+        struct stat st;
+
+        if (lstat(child_full_path.characters(), &st)) {
+            return {};
+        }
+
+        if (st.st_ino == child_inode_index) {
+            return child_full_path;
+        }
+    }
+    return {};
+}
+
+BlockingFileWatcher::BlockingFileWatcher(const String& path)
+    : m_path(path)
+{
+    m_watcher_fd = watch_file(path.characters(), path.length());
+    ASSERT(m_watcher_fd != -1);
+}
+
+BlockingFileWatcher::~BlockingFileWatcher()
+{
+    close(m_watcher_fd);
+}
+
+Optional<FileWatcherEvent> BlockingFileWatcher::wait_for_event()
+{
+    InodeWatcherEvent event {};
+    int rc = read(m_watcher_fd, &event, sizeof(event));
+    if (rc <= 0)
+        return {};
+
+    FileWatcherEvent result;
+    if (event.type == InodeWatcherEvent::Type::ChildAdded)
+        result.type = FileWatcherEvent::Type::ChildAdded;
+    else if (event.type == InodeWatcherEvent::Type::ChildRemoved)
+        result.type = FileWatcherEvent::Type::ChildRemoved;
+    else if (event.type == InodeWatcherEvent::Type::Modified)
+        result.type = FileWatcherEvent::Type::Modified;
+    else
+        return {};
+
+    if (result.type == FileWatcherEvent::Type::ChildAdded || result.type == FileWatcherEvent::Type::ChildRemoved) {
+        auto child_path = get_child_path_from_inode_index(m_path, event.inode_index);
+        if (!LexicalPath(child_path).is_valid())
+            return {};
+
+        result.child_path = child_path;
+    }
+
+    return result;
+}
+
+Result<NonnullRefPtr<FileWatcher>, String> FileWatcher::watch(const String& path)
+{
+    auto watch_fd = watch_file(path.characters(), path.length());
+    if (watch_fd < 0) {
+        return String::formatted("Could not watch file '{}' : {}", path.characters(), strerror(errno));
+    }
+
+    fcntl(watch_fd, F_SETFD, FD_CLOEXEC);
+    if (watch_fd < 0) {
+        return String::formatted("Could not watch file '{}' : {}", path.characters(), strerror(errno));
+    }
+
+    dbgln_if(FILE_WATCHER_DEBUG, "Started watcher for file '{}'", path.characters());
+    auto notifier = Notifier::construct(watch_fd, Notifier::Event::Read);
+    return adopt(*new FileWatcher(move(notifier), move(path)));
+}
+
+FileWatcher::FileWatcher(NonnullRefPtr<Notifier> notifier, const String& path)
+    : m_notifier(move(notifier))
+    , m_path(path)
+{
+    m_notifier->on_ready_to_read = [this] {
+        InodeWatcherEvent event {};
+        int rc = read(m_notifier->fd(), &event, sizeof(event));
+        if (rc <= 0)
+            return;
+
+        FileWatcherEvent result;
+        if (event.type == InodeWatcherEvent::Type::ChildAdded) {
+            result.type = FileWatcherEvent::Type::ChildAdded;
+        } else if (event.type == InodeWatcherEvent::Type::ChildRemoved) {
+            result.type = FileWatcherEvent::Type::ChildRemoved;
+        } else if (event.type == InodeWatcherEvent::Type::Modified) {
+            result.type = FileWatcherEvent::Type::Modified;
+        } else {
+            warnln("Unknown event type {} returned by the watch_file descriptor for {}", (unsigned)event.type, m_path.characters());
+            return;
+        }
+
+        if (result.type == FileWatcherEvent::Type::ChildAdded || result.type == FileWatcherEvent::Type::ChildRemoved) {
+            auto child_path = get_child_path_from_inode_index(m_path, event.inode_index);
+            if (!LexicalPath(child_path).is_valid())
+                return;
+
+            result.child_path = child_path;
+        }
+
+        on_change(result);
+    };
+}
+
+FileWatcher::~FileWatcher()
+{
+    m_notifier->on_ready_to_read = nullptr;
+    close(m_notifier->fd());
+    dbgln_if(FILE_WATCHER_DEBUG, "Ended watcher for file '{}'", m_path.characters());
+}
+
+#endif
+
+}
diff --git a/Userland/Libraries/LibCore/DirectoryWatcher.h b/Userland/Libraries/LibCore/FileWatcher.h
index 63be2c1aa7..8a7df3acc9 100644
--- a/Userland/Libraries/LibCore/DirectoryWatcher.h
+++ b/Userland/Libraries/LibCore/FileWatcher.h
@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2020, Itamar S. <itamar8910@gmail.com>
+ * Copyright (c) 2021, the SerenityOS developers.
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -28,34 +29,52 @@
 
 #include <AK/Function.h>
 #include <AK/Noncopyable.h>
+#include <AK/NonnullRefPtr.h>
+#include <AK/RefCounted.h>
+#include <AK/Result.h>
 #include <AK/String.h>
-#include <Kernel/API/InodeWatcherEvent.h>
+#include <LibCore/Notifier.h>
 
 namespace Core {
 
-class DirectoryWatcher {
-    AK_MAKE_NONCOPYABLE(DirectoryWatcher);
+struct FileWatcherEvent {
+    enum class Type {
+        Modified,
+        ChildAdded,
+        ChildRemoved,
+    };
+    Type type;
+    String child_path;
+};
+
+class BlockingFileWatcher {
+    AK_MAKE_NONCOPYABLE(BlockingFileWatcher);
 
 public:
-    explicit DirectoryWatcher(const String& path);
-    ~DirectoryWatcher();
-
-    struct Event {
-        enum class Type {
-            ChildAdded,
-            ChildRemoved,
-        };
-        Type type;
-        String child_path;
-    };
+    explicit BlockingFileWatcher(const String& path);
+    ~BlockingFileWatcher();
 
-    Optional<Event> wait_for_event();
+    Optional<FileWatcherEvent> wait_for_event();
 
 private:
-    String get_child_with_inode_index(unsigned) const;
-
     String m_path;
     int m_watcher_fd { -1 };
 };
 
+class FileWatcher : public RefCounted<FileWatcher> {
+    AK_MAKE_NONCOPYABLE(FileWatcher);
+
+public:
+    static Result<NonnullRefPtr<FileWatcher>, String> watch(const String& path);
+    ~FileWatcher();
+
+    Function<void(FileWatcherEvent)> on_change;
+
+private:
+    FileWatcher(NonnullRefPtr<Notifier>, const String& path);
+
+    NonnullRefPtr<Notifier> m_notifier;
+    String m_path;
+};
+
 }
diff --git a/Userland/Services/CrashDaemon/main.cpp b/Userland/Services/CrashDaemon/main.cpp
index 8dcd318566..b651675864 100644
--- a/Userland/Services/CrashDaemon/main.cpp
+++ b/Userland/Services/CrashDaemon/main.cpp
@@ -25,7 +25,7 @@
  */
 
 #include <AK/LexicalPath.h>
-#include <LibCore/DirectoryWatcher.h>
+#include <LibCore/FileWatcher.h>
 #include <LibCoreDump/Backtrace.h>
 #include <LibCoreDump/Reader.h>
 #include <serenity.h>
@@ -89,11 +89,11 @@ int main()
         return 1;
     }
 
-    Core::DirectoryWatcher watcher { "/tmp/coredump" };
+    Core::BlockingFileWatcher watcher { "/tmp/coredump" };
     while (true) {
         auto event = watcher.wait_for_event();
         ASSERT(event.has_value());
-        if (event.value().type != Core::DirectoryWatcher::Event::Type::ChildAdded)
+        if (event.value().type != Core::FileWatcherEvent::Type::ChildAdded)
             continue;
         auto coredump_path = event.value().child_path;
         dbgln("New coredump file: {}", coredump_path);