systemd: Support Credentials

Add `mod credential` with `fn path` which implements a simple lookup of
systemd credentials. See <https://systemd.io/CREDENTIALS/>

crates/radicle-systemd/src/credential.rs

@@ -0,0 +1,46 @@
+use std::env::{var, VarError::*};
+use std::ffi::OsString;
+use std::fmt;
+use std::path::{is_separator, PathBuf};
+
+const CREDENTIALS_DIRECTORY: &str = "CREDENTIALS_DIRECTORY";
+
+/// Takes a systemd credential ID. If the environment variable
+/// `CREDENTIALS_DIRECTORY` is set and valid Unicode, and the file corresponding
+/// to the credential exists, returns the path of the file corresponding to the
+/// credential.
+///
+/// Absence of the environment variable and inexistence of the file are handled
+/// gracefully returning `Ok(None)`.
+pub fn path(id: &str) -> Result<Option<PathBuf>, PathError> {
+    use PathError::*;
+
+    if id.contains(is_separator) {
+        return Err(InvalidCredentialId { id: id.to_owned() });
+    }
+
+    let credential = match var(CREDENTIALS_DIRECTORY) {
+        Err(NotUnicode(os)) => return Err(EnvVarNotUnicode { os }),
+        Err(NotPresent) => return Ok(None),
+        Ok(env) => PathBuf::from(env).join(id),
+    };
+
+    Ok(credential.exists().then_some(credential))
+}
+
+/// The error returned by [`path`].
+#[derive(Debug)]
+pub enum PathError {
+    InvalidCredentialId { id: String },
+    EnvVarNotUnicode { os: OsString },
+}
+
+impl fmt::Display for PathError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        use PathError::*;
+        match self {
+		InvalidCredentialId { id } => write!(f, "The systemd credential ID '{id}' is invalid."),
+		EnvVarNotUnicode { os } => write!(f, "The value of environment variable '{CREDENTIALS_DIRECTORY}' is not valid Unicode (it lossily translates to '{}').", os.to_string_lossy()),
+	}
+    }
+}

crates/radicle-systemd/src/lib.rs

@@ -5,3 +5,5 @@ pub mod journal;
 
 #[cfg(all(feature = "listen", unix))]
 pub mod listen;
+
+pub mod credential;