Moved the check for formatting a directory

[DrDeano]

The original check for a directory was for the readAllAlloc so move the check from open to read. This in turn fixes the fmt step in the build script for directories.

[DrDeano]

Closes ZystemOS/pluto#165

[squeek502]

I believe the catch needs to be in both open and read rather than moved from one to the other. If I'm reading the code correctly, then on Windows, error.IsDir can only happen during open.

(note that previously, the fs.cwd().readAllAlloc was doing open/stat/read all at once)

EDIT: Would fs.cwd().openFile not returning error.IsDir on Linux be considered a bug, by the way? I'm presuming that's what's going on.

[andrewrk]

@DrDeano thank you for the patch, and apologies for the downstream breakage. This landed in edea7a4.

EDIT: Would fs.cwd().openFile not returning error.IsDir on Linux be considered a bug, by the way? I'm presuming that's what's going on.

On Linux, when opening a file with read-only permissions, there is no way to determine whether the file descriptor is a directory handle without an additional syscall. In the additional commits I added to this branch before merging, I modified the symlink loop detection to use the inode rather than the realpath. This means unconditionally doing a stat() for both directories and files, which also tells whether the file descriptor is a directory or not.

I got a bit carried away with the merge, but the end result should be that zig fmt is significantly faster with this merge than before. From the operating system's perspective, running zig fmt on an already formatted codebase now looks like (repeated for every file):

openat(4, "poly1305.zig", O_RDONLY|O_CLOEXEC) = 5
fstat(5, {st_mode=S_IFREG|0644, st_size=8356, ...}) = 0
read(5, "// Translated from monocypher wh"..., 8356) = 8356
close(5)                                = 0

and (repeated for every directory)

openat(3, "time", O_RDONLY|O_CLOEXEC|O_DIRECTORY) = 4
fstat(4, {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
getdents64(4, /* 3 entries */, 8192)    = 80

[andrewrk]

This auto generated graph from https://github.com/ziglang/gotta-go-fast/blob/master/records.csv is messy, but you can still see the dip at the end from the merge: