Overhaul Nix's error types

For many of Nix's consumers it be convenient to easily convert a Nix
error into a std::io::Error.  That's currently not possible because of
the InvalidPath, InvalidUtf8, and UnsupportedOperation types that have
no equivalent in std::io::Error.

However, very few of Nix's public APIs actually return those unusual
errors.  So a more useful API would be for Nix's standard error type to
implement Into<std::io::Error>.

This commit makes Error a simple NewType around Errno.  For most
functions it's a drop-in replacement.  There are only three exceptions:

* clearenv now returns a bespoke error type.  It was the only Nix
  function whose error couldn't be cleanly mapped onto an Errno.

* sys::signal::signal now returns Error(Errno::ENOTSUP) instead of
  Error::UnsupportedOperation when the user passes an incompatible
  argument to `handler`.

* When a NixPath exceeds PATH_MAX, it will now return
  Error(Errno::ENAMETOOLONG) instead of Error::InvalidPath.

In the latter two cases there is now some abiguity about whether the
error code was generated by Nix or by the OS.  But I think the ambiguity
is worth it for the sake of being able to implement Into<io::Error>.

This commit also introduces Error::Sys() as a migration aid.  Previously
that as an enum variant.  Now it's a function, but it will work in many
of the same contexts as the original.

Fixes #1155

1 files changed, 2 insertions, 2 deletions

diff --git a/src/sys/socket/mod.rs b/src/sys/socket/mod.rs
index 69070463..ff64b73f 100644
--- a/src/sys/socket/mod.rs
+++ b/src/sys/socket/mod.rs
@@ -1572,7 +1572,7 @@ pub fn recvfrom(sockfd: RawFd, buf: &mut [u8])
             &mut len as *mut socklen_t))? as usize;
 
         match sockaddr_storage_to_addr(&addr, len as usize) {
-            Err(Error::Sys(Errno::ENOTCONN)) => Ok((ret, None)),
+            Err(Error(Errno::ENOTCONN)) => Ok((ret, None)),
             Ok(addr) => Ok((ret, Some(addr))),
             Err(e) => Err(e)
         }
@@ -1708,7 +1708,7 @@ pub fn sockaddr_storage_to_addr(
 
     assert!(len <= mem::size_of::<sockaddr_un>());
     if len < mem::size_of_val(&addr.ss_family) {
-        return Err(Error::Sys(Errno::ENOTCONN));
+        return Err(Error::from(Errno::ENOTCONN));
     }
 
     match c_int::from(addr.ss_family) {