AK+Everywhere: Make FixedArray OOM-safe

FixedArray now doesn't expose any infallible constructors anymore.
Rather, it exposes fallible methods. Therefore, it can be used for
OOM-safe code.
This commit also converts the rest of the system to use the new API.
However, as an example, VMObject can't take advantage of this yet,
as we would have to endow VMObject with a fallible static
construction method, which would require a very fundamental change
to VMObject's whole inheritance hierarchy.

AK/FixedArray.h

@@ -1,11 +1,13 @@
 /*
- * Copyright (c) 2018-2021, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2018-2022, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2022, the SerenityOS developers.
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
 #pragma once
 
+#include <AK/Error.h>
 #include <AK/Iterator.h>
 #include <AK/Span.h>
 #include <AK/kmalloc.h>
@@ -16,40 +18,60 @@ template<typename T>
 class FixedArray {
 public:
     FixedArray() = default;
-    explicit FixedArray(size_t size)
-        : m_size(size)
+
+    static ErrorOr<FixedArray<T>> try_create(size_t size)
     {
-        if (m_size != 0) {
-            m_elements = static_cast<T*>(kmalloc_array(m_size, sizeof(T)));
-            for (size_t i = 0; i < m_size; ++i)
-                new (&m_elements[i]) T();
-        }
+        if (size == 0)
+            return FixedArray<T>();
+        T* elements = static_cast<T*>(kmalloc_array(size, sizeof(T)));
+        if (!elements)
+            return Error::from_errno(ENOMEM);
+        for (size_t i = 0; i < size; ++i)
+            new (&elements[i]) T();
+        return FixedArray<T>(size, elements);
     }
+
+    static FixedArray<T> must_create_but_fixme_should_propagate_errors(size_t size)
+    {
+        return MUST(try_create(size));
+    }
+
+    ErrorOr<FixedArray<T>> try_clone() const
+    {
+        if (m_size == 0)
+            return FixedArray<T>();
+        T* elements = static_cast<T*>(kmalloc_array(m_size, sizeof(T)));
+        if (!elements)
+            return Error::from_errno(ENOMEM);
+        for (size_t i = 0; i < m_size; ++i)
+            new (&elements[i]) T(m_elements[i]);
+        return FixedArray<T>(m_size, elements);
+    }
+
+    FixedArray<T> must_clone_but_fixme_should_propagate_errors() const
+    {
+        return MUST(try_clone());
+    }
+
+    // NOTE: Nobody can ever use these functions, since it would be impossible to make them OOM-safe due to their signatures. We just explicitly delete them.
+    FixedArray(FixedArray<T> const&) = delete;
+    FixedArray<T>& operator=(FixedArray<T> const&) = delete;
+
+    FixedArray(FixedArray<T>&& other)
+        : m_size(other.m_size)
+        , m_elements(other.m_elements)
+    {
+        other.m_size = 0;
+        other.m_elements = nullptr;
+    }
+    // NOTE: Nobody uses this function, so we just explicitly delete it.
+    FixedArray<T>& operator=(FixedArray<T>&&) = delete;
+
     ~FixedArray()
     {
         clear();
     }
 
-    FixedArray(FixedArray const& other)
-        : m_size(other.m_size)
-    {
-        if (m_size != 0) {
-            m_elements = static_cast<T*>(kmalloc_array(m_size, sizeof(T)));
-            for (size_t i = 0; i < m_size; ++i)
-                new (&m_elements[i]) T(other[i]);
-        }
-    }
-
-    FixedArray& operator=(FixedArray const& other)
-    {
-        FixedArray array(other);
-        swap(array);
-        return *this;
-    }
-
-    FixedArray(FixedArray&&) = delete;
-    FixedArray& operator=(FixedArray&&) = delete;
-
     void clear()
     {
         if (!m_elements)
@@ -86,7 +108,7 @@ public:
         return false;
     }
 
-    void swap(FixedArray& other)
+    void swap(FixedArray<T>& other)
     {
         ::swap(m_elements, other.m_elements);
         ::swap(m_size, other.m_size);
@@ -105,6 +127,12 @@ public:
     Span<T> span() { return { data(), size() }; }
 
 private:
+    FixedArray(size_t size, T* elements)
+        : m_size(size)
+        , m_elements(elements)
+    {
+    }
+
     size_t m_size { 0 };
     T* m_elements { nullptr };
 };