fix(token-storage): implement deletion of tokens

Previously this case was entirely uncovered.
Interesting note: when a token is revoked, existing auth-tokens will
still work. However, you may not refresh them in case permissions
have been revoked. It's good as there is only one code-path to deal
with (and we verified it to be working), and bad for the user as
malicious software can keep using an account for certain time until
the token expires.

Fixes #79

src/rust/cli/cmn.rs

@@ -173,15 +173,29 @@ impl TokenStorage for JsonTokenStorage {
 
     // NOTE: logging might be interesting, currently we swallow all errors
     fn set(&mut self, scope_hash: u64, _: &Vec<&str>, token: Option<Token>) -> Option<io::Error> {
-        let json_token = json::encode(&token).unwrap();
-        match fs::OpenOptions::new().create(true).write(true).open(&self.path(scope_hash)) {
-            Ok(mut f) => {
-                match f.write(json_token.as_bytes()) {
-                    Ok(_) => None,
-                    Err(io_err) => Some(io_err),
+        match token {
+            None => {
+                match fs::remove_file(self.path(scope_hash)) {
+                    Err(err) => 
+                        match err.kind() {
+                            io::ErrorKind::NotFound => None,
+                            _ => Some(err)
+                        },
+                    Ok(_) => None
                 }
-            },
-            Err(io_err) => Some(io_err)
+            }
+            Some(token) => {
+                let json_token = json::encode(&token).unwrap();
+                match fs::OpenOptions::new().create(true).write(true).open(&self.path(scope_hash)) {
+                    Ok(mut f) => {
+                        match f.write(json_token.as_bytes()) {
+                            Ok(_) => None,
+                            Err(io_err) => Some(io_err),
+                        }
+                    },
+                    Err(io_err) => Some(io_err)
+                }
+            }
         }
     }