cli: Support for pass(1) as credential storage

`pass(1)`[1] is a small CLI-based password manager. When passing
`--pass-path edu/kit/uXXXX` to `KIT-ILIAS-Downloader`, it now attempts
to retrieve the password from `pass(1)`.

It is assumed that `pass(1)` is available in the `$PATH` of the process. If
that's not the case, it errors out with an error like this:

    Error: credentials input failed

    Caused by:
        pass not found in $PATH!

It's also taken care of the case where the value `--pass-path` is not
present in the store, the error will look like this:

    Error: credentials input failed

    Caused by:
        `pass` failed with non-zero exit code 1: Error: edu/kit/uXXXX is not in the password store.

Closes #32

[1] https://www.passwordstore.org/

src/cli.rs

@@ -1,6 +1,8 @@
 // SPDX-License-Identifier: GPL-3.0-or-later
 
+use std::io::{Error, ErrorKind};
 use std::path::PathBuf;
+use std::process::Command;
 use std::sync::atomic::{AtomicBool, AtomicUsize};
 
 use anyhow::anyhow;
@@ -72,6 +74,10 @@ pub struct Opt {
 	#[structopt(short = "P", long)]
 	pub password: Option<String>,
 
+	/// Path inside `pass(1)` to the password for shibboleth.
+	#[structopt(long)]
+	pub pass_path: Option<String>,
+
 	/// ILIAS page to download
 	#[structopt(long)]
 	pub sync_url: Option<String>,
@@ -170,6 +176,35 @@ pub fn ask_user_pass(opt: &Opt) -> Result<(String, String)> {
 				should_store = true;
 			}
 		}
+	} else if let Some(pass_path) = &opt.pass_path {
+		let pw_out = Command::new("pass")
+			.arg("show")
+			.arg(pass_path)
+			.output()
+			.map_err(|x| {
+				if x.kind() == ErrorKind::NotFound {
+					Error::new(ErrorKind::NotFound, "pass not found in $PATH!")
+				} else {
+					x
+				}
+			})?;
+		if !pw_out.status.success() {
+			return Err(Error::new(
+				ErrorKind::Other,
+				format!(
+					"`pass` failed with non-zero exit code {}: {}",
+					pw_out.status.code()
+						.expect("Failed retrieving pass exit code!"),
+					String::from_utf8(pw_out.stderr)
+						.expect("Failed decoding stderr of pass!")
+				)
+			))?
+		} else {
+			pass = String::from_utf8(pw_out.stdout).map(|x| {
+				x.trim_end().to_string()
+			}).expect("utf-8 decode of `pass(1)`-output failed");
+			should_store = false;
+		}
 	} else {
 		pass = rpassword::prompt_password("Password: ").context("password prompt")?;
 		should_store = true;