sys: add a bunch of error helpers

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>

proxmox-sys/Cargo.toml

@@ -11,6 +11,7 @@ authors = [
 failure = "0.1"
 lazy_static = "1.4"
 libc = "0.2"
+nix = "0.16"
 proxmox-tools = { version = "0.1.1-dev.1", path = "../proxmox-tools" }
 
 # Docs should be able to reference the proxmox crate.

proxmox-sys/src/error.rs

@@ -0,0 +1,165 @@
+//! Helpers for `io::Error` and `nix::Error`.
+//!
+//! When dealing with low level functionality, the `nix` crate contains a lot of helpful
+//! functionality. Unfortunately it also contains its own error type which doesn't mix all too well
+//! with `std::io::Error`. Some of the common cases we want to deal with is checking for `EAGAIN`,
+//! `EWOULDBLOCK` or `ENOENT` specifically. To do this easily, we add the `SysError` trait (rather
+//! than a type), which is implemented for both these errors and allows checking for the most
+//! common errors in a unified way.
+//!
+//! Another problem with the `nix` error type is that it has no equivalent to `ErrorKind::Other`.
+//! Unfortunately this is more difficult to deal with, so for now, we consider `io::Error` to be
+//! the more general error (and require an `into_io_error` implementation in `SysError`).
+//!
+//! See the `SysError` and `SysResult` traits for examples.
+
+use std::io;
+
+use nix::errno::Errno;
+use nix::Error;
+
+/// Helper to convert non-system-errors into an `io::Error` or `io::ErrorKind::Other`.
+///
+/// A more convenient way is to use the `io_format_err!` macro.
+pub fn io_err_other<E: ToString>(e: E) -> io::Error {
+    io::Error::new(std::io::ErrorKind::Other, e.to_string())
+}
+
+/// This trait should be implemented for error types which can represent system errors. Note that
+/// it is discouraged to try to map non-system errors to with this trait, since users of this trait
+/// assume there to be a relation between the error code and a previous system call. For instance,
+/// `error.is_errno(Errno::EAGAIN)` can be used when implementing a reactor to drive async I/O.
+///
+/// Usage examples:
+///
+/// ```
+/// # use failure::{bail, Error};
+/// use nix::{dir::Dir, fcntl::OFlag, sys::stat::Mode};
+///
+/// use proxmox_sys::error::SysError;
+///
+/// # fn test() -> Result<(), Error> {
+///
+/// match Dir::open(".", OFlag::O_RDONLY, Mode::empty()) {
+///     Ok(_dir) => {
+///         // Do something
+///     }
+///     Err(ref err) if err.not_found() => {
+///         // Handle ENOENT specially
+///     }
+///     Err(err) => bail!("failed to open directory: {}", err),
+/// }
+///
+/// # Ok(())
+/// # }
+/// ```
+pub trait SysError {
+    /// Check if this error is a specific error returned from a system call.
+    fn is_errno(&self, value: Errno) -> bool;
+
+    /// Convert this error into a `std::io::Error`. This must use the correct `std::io::ErrorKind`,
+    /// so that for example `ErrorKind::WouldBlock` means that the previous system call failed with
+    /// `EAGAIN`.
+    fn into_io_error(self) -> io::Error;
+
+    /// Convenience shortcut to check for `EAGAIN`.
+    #[inline]
+    fn would_block(&self) -> bool {
+        self.is_errno(Errno::EAGAIN)
+    }
+
+    /// Convenience shortcut to check for `ENOENT`.
+    #[inline]
+    fn not_found(&self) -> bool {
+        self.is_errno(Errno::ENOENT)
+    }
+
+    /// Convenience shortcut to check for `EEXIST`.
+    #[inline]
+    fn already_exists(&self) -> bool {
+        self.is_errno(Errno::EEXIST)
+    }
+}
+
+impl SysError for io::Error {
+    #[inline]
+    fn is_errno(&self, value: Errno) -> bool {
+        self.raw_os_error() == Some(value as i32)
+    }
+
+    #[inline]
+    fn into_io_error(self) -> io::Error {
+        self
+    }
+}
+
+impl SysError for nix::Error {
+    #[inline]
+    fn is_errno(&self, value: Errno) -> bool {
+        *self == Error::Sys(value)
+    }
+
+    #[inline]
+    fn into_io_error(self) -> io::Error {
+        match self {
+            Error::Sys(raw) => io::Error::from_raw_os_error(raw as _),
+            other => io::Error::new(io::ErrorKind::Other, other.to_string()),
+        }
+    }
+}
+
+/// Convenience helper to map a `Result<_, nix::Error>` to a `Result<_, std::io::Error>` quickly.
+///
+/// Usage example:
+///
+/// ```no_run
+/// # use std::os::unix::io::RawFd;
+/// # use failure::{bail, Error};
+///
+/// use proxmox_sys::error::SysResult;
+///
+/// struct MyReader(RawFd);
+///
+/// impl std::io::Read for MyReader {
+///     fn read(&mut self, buf: &mut [u8]) -> std::io::Result<usize> {
+///         nix::unistd::read(self.0, buf).into_io_result()
+///     }
+///
+///     // ... rest
+/// }
+/// ```
+pub trait SysResult {
+    type Ok;
+
+    fn into_io_result(self) -> io::Result<Self::Ok>;
+}
+
+impl<T> SysResult for Result<T, nix::Error> {
+    type Ok = T;
+
+    #[inline]
+    fn into_io_result(self) -> io::Result<T> {
+        self.map_err(|e| e.into_io_error())
+    }
+}
+
+macro_rules! other_error {
+    ($err:ty) => {
+        impl<T> SysResult for Result<T, $err> {
+            type Ok = T;
+
+            #[inline]
+            fn into_io_result(self) -> io::Result<T> {
+                self.map_err($crate::error::io_err_other)
+            }
+        }
+    };
+}
+
+other_error!(std::char::ParseCharError);
+other_error!(std::net::AddrParseError);
+other_error!(std::num::ParseFloatError);
+other_error!(std::num::ParseIntError);
+other_error!(std::str::ParseBoolError);
+other_error!(std::str::Utf8Error);
+other_error!(std::string::FromUtf8Error);

proxmox-sys/src/lib.rs

@@ -2,4 +2,5 @@
 
 pub mod macros;
 
+pub mod error;
 pub mod linux;