Kernel+FileManager: Disallow watch_file() in unsupported file systems

Currently only Ext2FS and TmpFS supports InodeWatchers. We now fail
with ENOTSUPP if watch_file() is called on e.g ProcFS.

This fixes an issue with FileManager chewing up all the CPU when /proc
was opened. Watchers don't keep the watched Inode open, and when they
close, the watcher FD will EOF.

Since nothing else kept /proc open in FileManager, the watchers created
for it would EOF immediately, causing a refresh over and over.

Fixes #879.

Kernel/FileSystem/Ext2FileSystem.h

@@ -74,6 +74,8 @@ public:
 
     virtual KResult prepare_to_unmount() const override;
 
+    virtual bool supports_watchers() const override { return true; }
+
 private:
     typedef unsigned BlockIndex;
     typedef unsigned GroupIndex;

Kernel/FileSystem/FileSystem.h

@@ -37,6 +37,7 @@ public:
     virtual bool initialize() = 0;
     virtual const char* class_name() const = 0;
     virtual InodeIdentifier root_inode() const = 0;
+    virtual bool supports_watchers() const { return false; }
 
     bool is_readonly() const { return m_readonly; }
 

Kernel/FileSystem/TmpFS.h

@@ -17,6 +17,8 @@ public:
 
     virtual const char* class_name() const override { return "TmpFS"; }
 
+    virtual bool supports_watchers() const override { return true; }
+
     virtual InodeIdentifier root_inode() const override;
     virtual RefPtr<Inode> get_inode(InodeIdentifier) const override;
 

Kernel/Process.cpp

@@ -3230,6 +3230,9 @@ int Process::sys$watch_file(const char* path, int path_length)
     auto& custody = custody_or_error.value();
     auto& inode = custody->inode();
 
+    if (!inode.fs().supports_watchers())
+        return -ENOTSUP;
+
     int fd = alloc_fd();
     if (fd < 0)
         return fd;

Libraries/LibC/string.cpp

@@ -406,6 +406,8 @@ const char* const sys_errlist[] = {
     "Wrong protocol type",
     "Operation in progress",
     "No such thread",
+    "Protocol error",
+    "Not supported",
     "The highest errno +1 :^)",
 };
 

Libraries/LibGUI/GDirectoryModel.cpp

@@ -341,21 +341,23 @@ void GDirectoryModel::open(const StringView& a_path)
     if (!dirp)
         return;
     closedir(dirp);
-    if (m_notifier)
+    if (m_notifier) {
         close(m_notifier->fd());
+        m_notifier = nullptr;
+    }
     m_path = path;
     int watch_fd = watch_file(path.characters(), path.length());
     if (watch_fd < 0) {
         perror("watch_file");
-        ASSERT_NOT_REACHED();
+    } else {
+        m_notifier = CNotifier::construct(watch_fd, CNotifier::Event::Read);
+        m_notifier->on_ready_to_read = [this] {
+            update();
+            char buffer[32];
+            int rc = read(m_notifier->fd(), buffer, sizeof(buffer));
+            ASSERT(rc >= 0);
+        };
     }
-    m_notifier = CNotifier::construct(watch_fd, CNotifier::Event::Read);
-    m_notifier->on_ready_to_read = [this] {
-        update();
-        char buffer[32];
-        int rc = read(m_notifier->fd(), buffer, sizeof(buffer));
-        ASSERT(rc >= 0);
-    };
     if (on_path_change)
         on_path_change();
     update();