Kernel: Make it possible to look up FIFO's by ID.

This will be useful when implementing the /proc/PID/fd/N links where N is
a file descriptor referring to a FIFO.

Kernel/FileSystem/FIFO.cpp

@@ -1,15 +1,43 @@
 #include <Kernel/FileSystem/FIFO.h>
+#include <Kernel/Lock.h>
 #include <AK/StdLibExtras.h>
+#include <AK/HashTable.h>
 
 //#define FIFO_DEBUG
 
-Retained<FIFO> FIFO::create()
+Lockable<HashTable<FIFO*>>& all_fifos()
 {
-    return adopt(*new FIFO);
+    static Lockable<HashTable<FIFO*>>* s_table;
+    if (!s_table)
+        s_table = new Lockable<HashTable<FIFO*>>;
+    return *s_table;
 }
 
-FIFO::FIFO()
+RetainPtr<FIFO> FIFO::from_fifo_id(dword id)
 {
+    auto* ptr = reinterpret_cast<FIFO*>(id);
+    LOCKER(all_fifos().lock());
+    if (auto it = all_fifos().resource().find(ptr); it == all_fifos().resource().end())
+        return nullptr;
+    return ptr;
+}
+
+Retained<FIFO> FIFO::create(uid_t uid)
+{
+    return adopt(*new FIFO(uid));
+}
+
+FIFO::FIFO(uid_t uid)
+    : m_uid(uid)
+{
+    LOCKER(all_fifos().lock());
+    all_fifos().resource().set(this);
+}
+
+FIFO::~FIFO()
+{
+    LOCKER(all_fifos().lock());
+    all_fifos().resource().remove(this);
 }
 
 void FIFO::open(Direction direction)

Kernel/FileSystem/FIFO.h

@@ -11,7 +11,12 @@ public:
         Neither, Reader, Writer
     };
 
-    static Retained<FIFO> create();
+    static RetainPtr<FIFO> from_fifo_id(dword);
+
+    static Retained<FIFO> create(uid_t);
+    ~FIFO();
+
+    uid_t uid() const { return m_uid; }
 
     void open(Direction);
     void close(Direction);
@@ -23,9 +28,11 @@ public:
     bool can_write() const;
 
 private:
-    FIFO();
+    explicit FIFO(uid_t);
 
     unsigned m_writers { 0 };
     unsigned m_readers { 0 };
     DoubleBuffer m_buffer;
+
+    uid_t m_uid { 0 };
 };

Kernel/FileSystem/FileDescriptor.cpp

@@ -402,7 +402,7 @@ KResultOr<String> FileDescriptor::absolute_path()
     if (is_tty())
         return tty()->tty_name();
     if (is_fifo())
-        return String::format("fifo:%x", m_fifo.ptr());
+        return String::format("fifo:%u", m_fifo.ptr());
     if (is_device())
         return String::format("device:%u,%u (%s)", m_device->major(), m_device->minor(), m_device->class_name());
     if (is_socket())