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, 3 insertions, 3 deletions

diff --git a/test/sys/test_termios.rs b/test/sys/test_termios.rs
index 00aeb2fc..ce436a13 100644
--- a/test/sys/test_termios.rs
+++ b/test/sys/test_termios.rs
@@ -32,14 +32,14 @@ fn test_tcgetattr_pty() {
 fn test_tcgetattr_enotty() {
     let file = tempfile().unwrap();
     assert_eq!(termios::tcgetattr(file.as_raw_fd()).err(),
-               Some(Error::Sys(Errno::ENOTTY)));
+               Some(Error(Errno::ENOTTY)));
 }
 
 // Test tcgetattr on an invalid file descriptor
 #[test]
 fn test_tcgetattr_ebadf() {
     assert_eq!(termios::tcgetattr(-1).err(),
-               Some(Error::Sys(Errno::EBADF)));
+               Some(Error(Errno::EBADF)));
 }
 
 // Test modifying output flags
@@ -126,5 +126,5 @@ fn test_local_flags() {
     let read = read(pty.master, &mut buf).unwrap_err();
     close(pty.master).unwrap();
     close(pty.slave).unwrap();
-    assert_eq!(read, Error::Sys(Errno::EAGAIN));
+    assert_eq!(read, Error(Errno::EAGAIN));
 }