Ext2FS: Free Ext2FSInodes when the last user releases them.

The inode cache was keeping these alive forever. Added a cute little magic
trick to Retainable that calls T::one_retain_left() when the retain count
is decremented to 1.

AK/Retainable.h

@@ -17,6 +17,18 @@ constexpr auto call_will_be_destroyed_if_present(...) -> FalseType
     return { };
 }
 
+template<class T>
+constexpr auto call_one_retain_left_if_present(T* object) -> decltype(object->one_retain_left(), TrueType { })
+{
+    object->one_retain_left();
+    return { };
+}
+
+constexpr auto call_one_retain_left_if_present(...) -> FalseType
+{
+    return { };
+}
+
 template<typename T>
 class Retainable {
 public:
@@ -29,9 +41,12 @@ public:
     void release()
     {
         ASSERT(m_retain_count);
-        if (!--m_retain_count) {
+        --m_retain_count;
+        if (m_retain_count == 0) {
             call_will_be_destroyed_if_present(static_cast<T*>(this));
             delete static_cast<T*>(this);
+        } else if (m_retain_count == 1) {
+            call_one_retain_left_if_present(static_cast<T*>(this));
         }
     }
 

VirtualFileSystem/Ext2FileSystem.cpp

@@ -985,3 +985,8 @@ String Ext2FSInode::reverse_lookup(InodeIdentifier child_id)
     }
     return { };
 }
+
+void Ext2FSInode::one_retain_left()
+{
+    fs().m_inode_cache.remove(index());
+}

VirtualFileSystem/Ext2FileSystem.h

@@ -21,6 +21,9 @@ public:
     size_t size() const { return m_raw_inode.i_size; }
     bool is_symlink() const { return isSymbolicLink(m_raw_inode.i_mode); }
 
+    // ^Inode (Retainable magic)
+    virtual void one_retain_left() override;
+
 private:
     // ^Inode
     virtual ssize_t read_bytes(Unix::off_t, size_t, byte* buffer, FileDescriptor*) override;

VirtualFileSystem/FileSystem.h

@@ -63,6 +63,8 @@ class Inode : public Retainable<Inode> {
 public:
     virtual ~Inode();
 
+    virtual void one_retain_left() { }
+
     FS& fs() { return m_fs; }
     const FS& fs() const { return m_fs; }
     unsigned fsid() const;