[POSIX, Mac] Refactor BSD-specific values out of POSIXFileSystem

[chrisd8088]

Along with #1104 (DONE), #1119 (DONE), and #1128, this PR is a prerequisite to the addition of a GVFS.Platform.Linux set of classes in #1125.

In particular, we convert POSIXFileSystem into an abstract class and move the majority of its implementation into MacFileSystem, due to the fact that many of the syscall function signatures and flag values, plus the layout of the struct stat fields, differ between BSD/Darwin and Linux. So we need to separate these out from the POSIX-level commonalities between the two.

The visibility of the members of the NativeFileReader subclass is also adjusted, and we remove some of its unused constants such as Create (for the O_CREAT flag, which isn't used here).

And we correct the return value of getuid(2) in POSIXPlatform to a 32-bit uint. Note that on both Linux and BSD/Darwin, the uid_t typedef can be checked with the following:

gcc -E -xc -include 'sys/types.h' - </dev/null | grep [^u]uid_t | grep ^typedef

/cc @jrbriggs

[kivikakk]

LGTM, thank you for this!

[chrisd8088]

Thanks @kivikakk -- and I think I managed to retain you as the primary author, since you did the work here!

[derrickstolee]

I noticed that WriteOnly and Create were dropped, but they were not used. @wilbaker @nickgra do you know why they were there at all?

[chrisd8088]

As a bit of context, these are mappings of the libc API flags O_WRONLY and O_CREAT, as defined in, e.g., FreeBSD's fcntl.h.

It looks to me as though there was some refactoring of native filesystem methods back in December which resulted in the creation of FastFetch.NativeUnixMethods as well as GVFS.Platform.Mac.MacFileSystem, which was then migrated GVFS.Platform.POSIX.POSIXFileSystem by @kivikakk recently so we could share the common POSIX bits with Linux.

My hunch would be that because some of these flags are used in the FastFetch.NativeUnixMethods code, they were included here by accident.

It's a good question, because it points out that if we want to use FastFetch on Linux, we'll need to refactor FastFetch.NativeUnixMethods, which is currently going to work on BSD/Darwin only as these flags like O_CREAT have different values on Linux, and the fields of struct stat are also very different. I've created an issue in our backlog for that!

[derrickstolee]

LGTM. Thanks!

[wilbaker]

Approved (with question on chmod)

[wilbaker]

Why does this not work on Linux? Is mode_t something other than ushort?

[chrisd8088]

Yes, it's a uint -- so the Chmod DLL import wrapper is different on Linux, and hence ChangeMode() ends up having to be in the OS-specific class rather than the base POSIX class, even though it looks the same. You can see these Linux variants in #1125.

[chrisd8088]

On Linux, it's a 32-bit int (assuming we're using fairly generic hardware and compiler, of course):

$ gcc -E -xc -include 'sys/types.h' - </dev/null | grep mode_t
typedef unsigned int __mode_t;
typedef __mode_t mode_t;

On Mac:

gcc -E -xc -include 'sys/types.h' - </dev/null | grep mode_t | grep ^typedef
typedef __uint16_t __darwin_mode_t;
typedef __darwin_mode_t mode_t;