Allow executing program to retrieve credentials

* config / authentication: permit to call external program for getting credentials

You can add into your ncspot/config.toml like this:

```
creds_username = "mylogin"
creds_passeval = "pass my_pass_path_to_spot_password"
```

Or using any password manager who send your password.to stdout.
If a newline is detected it will be automatically removed

* Move credential commands into separate structure

Also add an option for a username command

* Document credential commands

---------

Co-authored-by: Henrik Friedrichsen <henrik@affekt.org>

src/authentication.rs

@@ -1,3 +1,5 @@
+use std::process::Command;
+
 use cursive::traits::Resizable;
 use cursive::view::Nameable;
 use cursive::views::*;
@@ -60,6 +62,38 @@ pub fn create_credentials() -> Result<RespotCredentials, String> {
         .unwrap_or_else(|| Err("Didn't obtain any credentials".to_string()))
 }
 
+pub fn credentials_eval(
+    username_cmd: &str,
+    password_cmd: &str,
+) -> Result<RespotCredentials, String> {
+    fn eval(cmd: &str) -> Result<Vec<u8>, String> {
+        println!("Executing \"{}\"", cmd);
+        let mut result = Command::new("sh")
+            .args(["-c", cmd])
+            .output()
+            .map_err(|e| e.to_string())?
+            .stdout;
+        if let Some(&last_byte) = result.last() {
+            if last_byte == 10 {
+                result.pop();
+            }
+        }
+
+        Ok(result)
+    }
+
+    println!("Retrieving username");
+    let username = String::from_utf8_lossy(&eval(username_cmd)?).into();
+    println!("Retrieving password");
+    let password = eval(password_cmd)?;
+
+    Ok(RespotCredentials {
+        username,
+        auth_type: AuthenticationType::AUTHENTICATION_USER_PASS,
+        auth_data: password,
+    })
+}
+
 #[derive(Serialize, Deserialize, Debug)]
 pub struct AuthResponse {
     pub credentials: RespotCredentials,

src/config.rs

@@ -93,6 +93,13 @@ pub struct ConfigValues {
     pub statusbar_format: Option<String>,
     pub library_tabs: Option<Vec<LibraryTab>>,
     pub hide_display_names: Option<bool>,
+    pub credentials: Option<Credentials>,
+}
+
+#[derive(Serialize, Deserialize, Debug, Default, Clone)]
+pub struct Credentials {
+    pub username_cmd: Option<String>,
+    pub password_cmd: Option<String>,
 }
 
 #[derive(Serialize, Deserialize, Debug, Default, Clone)]

src/main.rs

@@ -191,7 +191,17 @@ fn main() -> Result<(), String> {
                 info!("Using cached credentials");
                 c
             }
-            None => credentials_prompt(None)?,
+            None => {
+                info!("Attempting to resolve credentials via username/password commands");
+                let creds = cfg.values().credentials.clone().unwrap_or_default();
+
+                match (creds.username_cmd, creds.password_cmd) {
+                    (Some(username_cmd), Some(password_cmd)) => {
+                        authentication::credentials_eval(&username_cmd, &password_cmd)?
+                    }
+                    _ => credentials_prompt(None)?,
+                }
+            }
         }
     };