Opinions wanted: Special Handling of /proc FileSystem on Linux?


(Carl Brown1) #1

Hi,

I run the Austin Swift meetup group, and a question came up about the /proc
filesystem on Linux:

The question involved the inability to get any useful information from Data
(contentsOf: URL(fileURLWithPath: "/proc/sys/fs/file-max")).

It turns out that Data(contentsOf:) does a stat() on the inode of the file
you give it, and then runs alloc()/read() for the specified number of
bytes. This works fine on normal files on normal file systems. But all the
files in /proc have a 0 length specified by their inodes, and you have to
actually read them until EOF to find out how much is there.

So the question is: Is that something the *Programmer* should be
responsible for knowing, or should *Foundation* be responsible for handling
that correctly?

I can see a case that we'd want to avoid embedding things this OS-specific
in Foundation, but on the other hand, I know that this is very confusing
for Darwin developers moving to Linux and expecting Data(contentsOf:) to
"Just work."

So I thought I'd ask the community and see if there was a consensus.

Thanks,

-Carl

···

--
Carl Brown
Swift@IBM
Carl.Brown1@IBM.com


(Philippe Hausler) #2

IMHO it seems reasonable for Foundation to return the contents of procfs items when constructing Data. That being said - I would probably favor a more structured parsing for actually using that resultant Data than just a blob. Those structured elements are probably not very portable so perhaps they belong in something above Foundation.

I don’t know of any linux variants off hand that don’t have /proc, but it might be interesting to somehow detect if that is truly a procfs element. I think it would perhaps be a failure if just the path /proc is special cased; Android, Solaris and Plan9 also share this same concept iirc (there are likely others).

···

On Jan 12, 2017, at 11:40 AM, Carl Brown1 via swift-corelibs-dev <swift-corelibs-dev@swift.org> wrote:

Hi,

I run the Austin Swift meetup group, and a question came up about the /proc filesystem on Linux:

The question involved the inability to get any useful information from Data(contentsOf: URL(fileURLWithPath: "/proc/sys/fs/file-max")).

It turns out that Data(contentsOf:) does a stat() on the inode of the file you give it, and then runs alloc()/read() for the specified number of bytes. This works fine on normal files on normal file systems. But all the files in /proc have a 0 length specified by their inodes, and you have to actually read them until EOF to find out how much is there.

So the question is: Is that something the *Programmer* should be responsible for knowing, or should *Foundation* be responsible for handling that correctly?

I can see a case that we'd want to avoid embedding things this OS-specific in Foundation, but on the other hand, I know that this is very confusing for Darwin developers moving to Linux and expecting Data(contentsOf:) to "Just work."

So I thought I'd ask the community and see if there was a consensus.

Thanks,

-Carl

--
Carl Brown
Swift@IBM
Carl.Brown1@IBM.com

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