From: normalperson@...
Date: 2014-08-29T08:03:39+00:00
Subject: [ruby-core:64636] [ruby-trunk - Feature #10181] New method	File.openat()

Issue #10181 has been updated by Eric Wong.


 Joel VanderWerf <joelvanderwerf@gmail.com> wrote:
 > On 08/28/2014 02:53 PM, Eric Wong wrote:
 > >I like this feature.
 > >
 > >If matz approves, I assume you also want to add other *at functions?
 > >e.g. fstatat, renameat, unlinkat, mkdirat, etc.
 > 
 > Hm, that suggests...
 > 
 > Dir.at(...).open(...)
 > Dir.at(...).fstat(...)
 
 How would that be implemented?
 
 I don't see it working...
 
 The reason for *at functions is the file descriptor points to the
 same file (directory) handle across multiple functions; in other words
 it's a way to avoid race conditions by creating a private reference
 to a container object (an FS directory)
 
 The file descriptor points to the same directory regardless of whether
 it's renamed (moved) or not.
 
 One can think of FS operations as operations on Ruby hashes.
 In your example, it might be like the following, assuming
 "fs" is a giant hash protected by OS-wide locks:
 
     # Dir.at(dirname).open("foo")
     fs[dirname]["foo"]  # open("/dirname/foo", ...)
                                # another thread may replace/remove
                                # root[dirname] here
     # Dir.at(dirname).open("bar")
     fs[dirname]["bar"]  # open("/dirname/bar", ...)
 
 We cannot guarantee Dir.at(dirname) / fs[dirname] returns
 the same value twice when called in succession.
 
 openat lets you work like this:
 
     dh = fs[dirname] # dh = opendir(dirname)
     dh["foo"] # openat(fileno(dh), "foo", ...)
     dh["bar"] # openat(fileno(dh), "bar", ...)
     ...
 
 Other threads can remove/replace/rename fs[dirname] with another
 directory, but the directory handle from the initial lookup
 remains valid to the thread which opened it.

----------------------------------------
Feature #10181: New method File.openat()
https://bugs.ruby-lang.org/issues/10181#change-48544

* Author: Technorama Ltd.
* Status: Open
* Priority: Normal
* Assignee: 
* Category: 
* Target version: 
----------------------------------------
The purpose of the openat() function is to enable opening files in directories other than the current working directory without exposure to race conditions. Any part of the path of a file could be changed in parallel to a call to open(), resulting in unspecified behavior. By opening a file descriptor for the target directory and using the openat() function it can be guaranteed that the opened file is located relative to the desired directory.

openat() is part of POSIX.1-2008.

Compatibility:
  Linux kernel >= 2.6.16
  FreeBSD >= 7.0
  OpenBSD >= 5.0
  NetBSD >= 6.1.4
  MacOS/X no

Pull request: https://github.com/ruby/ruby/pull/706



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