feat(error): Add error-mapping from C calls

Implements an error enum with mappings from the low-level C calls and
appropriate error descriptions.

Cargo.toml

@@ -4,3 +4,5 @@ version = "0.1.0"
 authors = ["Vincent Ambo <tazjin@gmail.com>"]
 
 [dependencies]
+nix = "0.9"
+libc = "0.2"

src/error.rs

@@ -0,0 +1,97 @@
+use nix;
+use std::error;
+use std::fmt;
+
+/// This module implements a simple error type to match the errors that can be thrown from the C
+/// functions as well as some extra errors resulting from internal validations.
+///
+/// As this crate exposes an opinionated API to the POSIX queues certain errors have been
+/// ignored:
+///
+/// * ETIMEDOUT: The low-level timed functions are not exported and this error can not occur.
+/// * EAGAIN: Non-blocking queue calls are not supported.
+/// * EINVAL: Same reason as ETIMEDOUT
+/// * EMSGSIZE: The message size is immutable after queue creation and this crate checks it.
+/// * ENAMETOOLONG: This crate performs name validation
+///
+/// If an unexpected error is encountered it will be wrapped appropriately and should be reported
+/// as a bug on https://github.com/aprilabank/posix_mq.rs
+
+#[derive(Debug)]
+pub enum Error {
+    // These errors match what is described in the man pages (from mq_overview(7) onwards).
+    PermissionDenied(),
+    InvalidQueueDescriptor(),
+    QueueCallInterrupted(),
+    QueueAlreadyExists(),
+    QueueNotFound(),
+    InsufficientMemory(),
+    InsufficientSpace(),
+
+    // These two are (hopefully) unlikely in modern systems
+    ProcessFileDescriptorLimitReached(),
+    SystemFileDescriptorLimitReached(),
+
+    // If an unhandled / unknown / unexpected error occurs this error will be used.
+    // In those cases bug reports would be welcome!
+    UnknownForeignError(nix::Errno),
+
+    // Some other unexpected / unknown error occured. This is probably an error from
+    // the nix crate. Bug reports also welcome for this!
+    UnknownInternalError(Option<nix::Error>),
+}
+
+impl error::Error for Error {
+    fn description(&self) -> &str {
+        use Error::*;
+        match *self {
+            PermissionDenied() => "permission to the specified queue was denied",
+            InvalidQueueDescriptor() => "the internal queue descriptor was invalid",
+            QueueCallInterrupted() => "queue method interrupted by signal",
+            QueueAlreadyExists() => "the specified queue already exists",
+            QueueNotFound() => "the specified queue could not be found",
+            InsufficientMemory() => "insufficient memory to call queue method",
+            InsufficientSpace() => "insufficient space to call queue method",
+            ProcessFileDescriptorLimitReached() => "max. number of process file descriptors reached",
+            SystemFileDescriptorLimitReached() => "max. number of system file descriptors reached",
+            UnknownForeignError(_) => "unknown foreign error occured: please report a bug!",
+            UnknownInternalError(_) => "unknown internal error occured: please report a bug!",
+        }
+    }
+}
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        // Explicitly import this to gain access to Error::description()
+        use std::error::Error;
+        f.write_str(self.description())
+    }
+}
+
+/// This from implementation is used to translate errors from the lower-level
+/// C-calls into sensible Rust errors.
+impl From<nix::Error> for Error {
+    fn from(e: nix::Error) -> Self {
+        match e {
+            nix::Error::Sys(e) => match_errno(e),
+            _ => Error::UnknownInternalError(Some(e)),
+        }
+    }
+}
+
+fn match_errno(err: nix::Errno) -> Error {
+    use nix::errno::*;
+
+    match err {
+        EACCES => Error::PermissionDenied(),
+        EBADF  => Error::InvalidQueueDescriptor(),
+        EINTR  => Error::QueueCallInterrupted(),
+        EEXIST => Error::QueueAlreadyExists(),
+        EMFILE => Error::ProcessFileDescriptorLimitReached(),
+        ENFILE => Error::SystemFileDescriptorLimitReached(),
+        ENOENT => Error::QueueNotFound(),
+        ENOMEM => Error::InsufficientMemory(),
+        ENOSPC => Error::InsufficientSpace(),
+        _      => Error::UnknownForeignError(err),
+    }
+}