New RFC: file_create_exclusive

text/0000-file-create-exclusive.md

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+- Feature Name: file_create_exclusive
+- Start Date: 2015-10-30
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+Atomically create a file is it does not exist, or fail.
+
+# Motivation
+[motivation]: #motivation
+
+This exposes an OS operation available on at least Unix and Windows. It can be
+emulated as a non-atomic operation.
+
+My use case is to sequentially create files like `snapshot-1`, `snapshot-2`,
+etc. without overwriting an existing file. I believe a more common usage for
+this feature is to open a temporary file ensuring this does not conflict with
+another file.
+
+# Detailed design
+[design]: #detailed-design
+
+Add another function to `std::fs::OpenOptions`:
+
+    /// Sets the option for creating a file exclusively.
+    /// 
+    /// If this is set and "create" is set, the `open()` operation shall fail
+    /// if the file already exists.
+    /// 
+    /// The check for the existence of the file and the creation of the file
+    /// if it does not exist shall be atomic with respect to other threads
+    /// executing open() naming the same filename in the same directory with
+    /// "exclusive" and "create" options set.
+    fn exclusive(&mut self, excl: bool) -> &mut OpenOptions
+
+On Unix this shall set the [`O_EXCL` flag](http://linux.die.net/man/3/open).
+On Windows this shall use the [`CREATE_NEW` parameter](https://msdn.microsoft.com/en-us/library/windows/desktop/aa363858%28v=vs.85%29.aspx).
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+I don't see any. This functionality should eventually be exposed.
+
+# Alternatives
+[alternatives]: #alternatives
+
+I have no idea whether more extensive modifications to the file-open
+functionality are under way.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+I have looked over the code, and implementation looks straightforward, so I do
+not anticipate any issues. There may however be other opinions over how this
+functionality should be exposed.
+
+I am not certain what testing should be implemented.