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/test/sys/test_signal.rs b/test/sys/test_signal.rs
index c8c13e52..741d97b9 100644
--- a/test/sys/test_signal.rs
+++ b/test/sys/test_signal.rs
@@ -1,5 +1,5 @@
 #[cfg(not(target_os = "redox"))]
-use nix::Error;
+use nix::{errno::Errno, Error};
 use nix::sys::signal::*;
 use nix::unistd::*;
 use std::convert::TryFrom;
@@ -92,7 +92,7 @@ fn test_signal_sigaction() {
     let _m = crate::SIGNAL_MTX.lock().expect("Mutex got poisoned by another test");
 
     let action_handler = SigHandler::SigAction(test_sigaction_action);
-    assert_eq!(unsafe { signal(Signal::SIGINT, action_handler) }.unwrap_err(), Error::UnsupportedOperation);
+    assert_eq!(unsafe { signal(Signal::SIGINT, action_handler) }.unwrap_err(), Error(Errno::ENOTSUP));
 }
 
 #[test]