Increase dialog exit status parsing safety in the locker

This also stops using -1 as auth failed, and moves all failure statuses
to positive numbers.  It looks like Exiting with -1 ends up setting the
status to 255 on exit, but then in the locker, it sees this as 255 and
not -1, since things get coerced into 32-bit integers.

util/src/dialog.rs

@@ -0,0 +1,68 @@
+use std::{error, fmt, process::ExitStatus, os::unix::prelude::ExitStatusExt};
+
+#[derive(Debug, Clone, Copy)]
+pub enum DialogExitStatusError {
+    NoStatus,
+    Crashed(i32),
+    InvalidStatus(i32),
+}
+
+impl fmt::Display for DialogExitStatusError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        use DialogExitStatusError::*;
+        match self {
+            InvalidStatus(other) => write!(f, "Unlock dialog exited with unexpected status {}", other),
+            Crashed(signum) => write!(f, "Unlock dialog crashed with signal {}", signum),
+            NoStatus => write!(f, "Unlock dialog exited unexpectedly"),
+        }
+    }
+}
+
+impl error::Error for DialogExitStatusError {}
+
+#[repr(i32)]
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub enum DialogExitStatus {
+    AuthSucceeded = 0,
+    AuthFailed = 1,
+    AuthCanceled = 2,
+    AuthTimedOut = 3,
+    OtherError = 4,
+}
+
+impl DialogExitStatus {
+    pub fn exit(&self) -> ! {
+        std::process::exit(*self as i32);
+    }
+}
+
+impl fmt::Display for DialogExitStatus {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        use DialogExitStatus::*;
+        match self {
+            AuthSucceeded => write!(f, "Authentication succeeded"),
+            AuthFailed => write!(f, "Authentication failed"),
+            AuthCanceled => write!(f, "Authentication canceled"),
+            AuthTimedOut => write!(f, "Authentication timed out"),
+            OtherError => write!(f, "An unexpected error occurred with the unlock dialog"),
+        }
+    }
+}
+
+impl TryFrom<ExitStatus> for DialogExitStatus {
+    type Error = DialogExitStatusError;
+    fn try_from(value: ExitStatus) -> Result<Self, Self::Error> {
+        match value.code() {
+            Some(0) => Ok(DialogExitStatus::AuthSucceeded),
+            Some(1) => Ok(DialogExitStatus::AuthFailed),
+            Some(2) => Ok(DialogExitStatus::AuthCanceled),
+            Some(3) => Ok(DialogExitStatus::AuthTimedOut),
+            Some(4) => Ok(DialogExitStatus::OtherError),
+            Some(other) => Err(DialogExitStatusError::InvalidStatus(other)),
+            None => match value.signal() {
+                Some(signum) => Err(DialogExitStatusError::Crashed(signum)),
+                None => Err(DialogExitStatusError::NoStatus),
+            },
+        }
+    }
+}

util/src/lib.rs

@@ -2,26 +2,11 @@ use std::{ffi::CStr, io};
 use xcb::x;
 
 pub mod desktop;
+pub mod dialog;
 pub mod settings;
 
 pub const BSCREENSAVER_WM_CLASS: &[u8] = b"bscreensaver\0Bscreensaver\0";
 
-#[repr(i32)]
-#[derive(Debug, Clone, Copy, PartialEq)]
-pub enum DialogExitStatus {
-    AuthFailed = -1,
-    AuthSucceeded = 0,
-    AuthCanceled = 1,
-    AuthTimedOut = 2,
-    OtherError = 3,
-}
-
-impl DialogExitStatus {
-    pub fn exit(&self) -> ! {
-        std::process::exit(*self as i32);
-    }
-}
-
 pub fn init_logging(env_name: &str) {
     env_logger::builder()
         .format_timestamp_millis()