FS subfiles are created with File::OpenSubFile, they have a start offset that must be added to all read/write operations.
The implementation in this commit is done using a new FileBackend that wraps the FS::File along with the start offset.
boost::static_pointer_cast for boost::intrusive_ptr (what SharedPtr is),
takes its parameter by const reference. Given that, it means that this
std::move doesn't actually do anything other than obscure what the
function's actual behavior is, so we can remove this. To clarify, this
would only do something if the parameter was either taking its argument
by value, by non-const ref, or by rvalue-reference.
This can just be a regular function, getting rid of the need to also
explicitly undef the define at the end of the file. Given FuncReturn()
was already converted into a function, it's #undef can also be removed.
Instead of using an unsigned int as a parameter and expecting a user to
always pass in the correct values, we can just convert the enum into an
enum class and use that type as the parameter type instead, which makes
the interface more type safe.
We also get rid of the bookkeeping "NUM_" element in the enum by just
using an unordered map. This function is generally low-frequency in
terms of calls (and I'd hope so, considering otherwise would mean we're
slamming the disk with IO all the time) so I'd consider this acceptable
in this case.
There were a few places where nested namespace specifiers weren't being
used where they could be within the service code. This amends that to
make the namespacing a tiny bit more compact.
While likely very uncommon, this sanitizes the input and does nothing in
the event of the length being equal to or less than zero, avoiding
constructing a std::string when there's no need to. It also avoids an
out-of-memory scenario, as a negative value would wrap around to its
equivalent unsigned representation in std::string's constructor.
e.g. If someone was silly and a length of -1 was specified, this would
make a string with a length of 0xFFFFFFFFFFFFFFFF on a 64-bit platform,
which will obviously eventually fail due to the allocation being way too
large.
Previously, these were sitting outside of the Kernel namespace, which
doesn't really make sense, given they're related to the Thread class
which is within the Kernel namespace.
