Kernel/FATFS: Implement add_child

Author: https://github.com/cqundefine
Commit: 3b39a2f71b
Pull-request: https://github.com/SerenityOS/serenity/pull/23907
Reviewed-by: https://github.com/Hendiadyoin1
Reviewed-by: https://github.com/implicitfield
Reviewed-by: https://github.com/nico
Reviewed-by: https://github.com/timschumi ✅

Kernel/FileSystem/FATFS/Inode.cpp

@@ -568,9 +568,40 @@ ErrorOr<NonnullRefPtr<Inode>> FATInode::create_child(StringView name, mode_t mod
     return TRY(FATInode::create(fs(), entry, entries[lfn_entries.size()], lfn_entries));
 }
 
-ErrorOr<void> FATInode::add_child(Inode&, StringView, mode_t)
+ErrorOr<void> FATInode::add_child(Inode& inode, StringView name, mode_t mode)
 {
-    return EROFS;
+    VERIFY(has_flag(m_entry.attributes, FATAttributes::Directory));
+    VERIFY(inode.fsid() == fsid());
+
+    // FIXME: There's a lot of similar code between this function and create_child, we should try to factor out some of the common code.
+
+    dbgln_if(FAT_DEBUG, "FATInode[{}]::add_child(): appending inode {} as \"{}\"", identifier(), inode.identifier(), name);
+
+    auto entry = bit_cast<FATInode*>(&inode)->m_entry;
+    create_83_filename_for(entry, name);
+
+    // TODO: We should set the hidden attribute if the file starts with a dot or read only (the same way Linux does this).
+    if (mode & S_IFDIR)
+        entry.attributes |= FATAttributes::Directory;
+
+    // FIXME: Set the dates
+
+    // FIXME: For some filenames lfn entries are not necessary
+    auto lfn_entries = TRY(create_lfn_entries(name, lfn_entry_checksum(entry)));
+
+    MutexLocker locker(m_inode_lock);
+
+    auto entries = TRY(allocate_entries(lfn_entries.size() + 1));
+
+    // FIXME: If we fail here we should clean up the entries we wrote
+    TRY(fs().write_block(entries[lfn_entries.size()].block, UserOrKernelBuffer::for_kernel_buffer(bit_cast<u8*>(&entry)), sizeof(FATEntry), entries[lfn_entries.size()].entry * sizeof(FATEntry)));
+
+    for (u32 i = 0; i < lfn_entries.size(); i++) {
+        auto location = entries[lfn_entries.size() - i - 1];
+        TRY(fs().write_block(location.block, UserOrKernelBuffer::for_kernel_buffer(bit_cast<u8*>(&lfn_entries[i])), sizeof(FATLongFileNameEntry), location.entry * sizeof(FATLongFileNameEntry)));
+    }
+
+    return {};
 }
 
 ErrorOr<void> FATInode::remove_child(StringView)