add proc to check if directory can be read

[a-mr]

walkDir does not check whether opening directory succeeded or not, existsDir is not enough because it does not check permissions that the directory can be opened.
Partially addresses #10309 & forum https://forum.nim-lang.org/t/4546.
Manually tested on Linux & Windows.

[demotomohiro]

Calling checkDir before calling walkDir is LBYL (Look Before You Leap) as cumulonimbus explained here.

I think adding checkError parameter with default value false to walkDir (and it check errors only when checkError is true) would works without breaking change.
Or add walkDirSafe that is same to walkDir but it check errors.

[a-mr]

Alternatively, Nim can expose a function for zeroing the last error, say clearOsError, and use it in the old fashion:

clearOsError()
for sub in walkDir(dir):
  echo sub
if osLastError() != OSErrorCode(0):
  echo "status: ", osErrorMsg(osLastError())

[Araq]

• Add a .since: (1, 1) declaration.
• Indentation uses 2 spaces everywhere else.

[disruptek]

checkDir just doesn't seem like a good name. How does it differ from checking to see if the directory is traversable?

I'd rather these checks (LBYL) were opt-in versus opt-out.

[narimiran]

• Add a .since: (1, 1) declaration.
• Indentation uses 2 spaces everywhere else.

@a-mr, what's the status of this PR? Is it superseded by #13163? If so, can it be closed? If not, can you make the requested changes?

[alehander92]

ok guys, what is the usage of this? as @demotomohiro's comment seems very correct in that many usages of this (check if can be read, then read it .. oops now it cant be read as something changed) can lead to subtle bugs.
probably this is still useful for stuff like filesystem listings/file managers etc, but still a note in the docs might be good

[a-mr]

Well, I don't understand TOCTOU w.r.t. to directories. Both wiki page and CERT C coding standards only mention TOCTOU of file access.

But it's true that #13163 will supersede this one. checkDir can be easily implemented using #13163.
I close this unless someone thinks it can still be useful.