`FFI::MemoryPointer#read_pointer` type size differs from CRuby / JRuby

[ankane]

Hi, wanted to share a small inconsistency with CRuby / JRuby FFI.

require "ffi"

ptr = FFI::MemoryPointer.new(:pointer)
p ptr.read_pointer.type_size
p ptr.read_pointer[0]

CRuby 3.3.5 / JRuby 10.0.0.0

1
#<FFI::Pointer address=0x0000000000000000>

TruffleRuby 24.2.0

nil
<internal:core> core/truffle/ffi/pointer.rb:128:in `[]': unknown type size (ArgumentError)

[andrykonchin]

Thank you for the report, we'll look into it.

[eregon]

Is this correct behavior for the FFI gem?
Note that if I add ptr.type_size in your snippet it's 8, which seems correct because a pointer is 8 bytes on 64-bit machines (and that Pointer object is "an array of pointers").
But read_pointer returns a pointer, so what should be its type_size?
Is 1 correct or it should be 8?
I suppose the idea is ptr.read_pointer could be a pointer to anything so it just defaults to being a pointer to "a byte"?

[ankane]

From my understanding, it could be a pointer to anything, so I could see an argument for nil or 1, but in general, I think it's better to match the FFI gem so downstream gems don't need to account for this difference.

[eregon]

Fix in #3851
Looking at the code it's clear the "default type_size" is 1.
The issue is we created a Pointer directly from Java and that doesn't call initialize.
We do it in 5 places but read_pointer is the only place where type_size can matter (the rest are internal usages of the pointer which don't use type_size).