automatically delete empty passwords from the key store

The frontend uses this to invalidate the GitHub token, even though
it can also deal with `null`. Let's keep the keystore clean and
only keep entries that contain an actual password.

Note that the consumers, i.e. the frontend, handle an empty password
for the short time it's in memory.

crates/gitbutler-core/src/secret.rs

@@ -8,7 +8,13 @@ use std::sync::Mutex;
 
 /// Persist `secret` so that it can be retrieved by the given `handle`.
 pub fn persist(handle: &str, secret: &Sensitive<String>) -> Result<()> {
-    Ok(entry_for(handle)?.set_password(&secret.0)?)
+    let entry = entry_for(handle)?;
+    if secret.0.is_empty() {
+        entry.delete_password()?;
+    } else {
+        entry.set_password(&secret.0)?;
+    }
+    Ok(())
 }
 
 /// Obtain the previously [stored](persist()) secret known as `handle`.

crates/gitbutler-core/tests/secret/mod.rs

@@ -28,5 +28,22 @@ fn store_and_retrieve() -> anyhow::Result<()> {
     Ok(())
 }
 
+#[test]
+#[serial]
+fn store_empty_equals_deletion() -> anyhow::Result<()> {
+    credentials::setup();
+    secret::persist("new", &Sensitive("secret".into()))?;
+    assert_eq!(credentials::count(), 1);
+
+    secret::persist("new", &Sensitive("".into()))?;
+    assert_eq!(
+        secret::retrieve("new")?.map(|s| s.0),
+        None,
+        "empty passwords are automatically deleted"
+    );
+    assert_eq!(credentials::count(), 0);
+    Ok(())
+}
+
 pub(crate) mod credentials;
 mod users;