gatewayd: mm, patch_post and username

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

crates/gatewayd/src/mm/rest.rs

@@ -30,6 +30,19 @@ struct Channel {
     id: String,
 }
 
+#[derive(Deserialize)]
+struct User {
+    username: String,
+}
+
+/// A Mattermost username: 1 to 64 characters of `a-z`, `0-9`, `.`, `-` and `_`.
+fn valid_username(name: &str) -> bool {
+    (1..=64).contains(&name.len())
+        && name.bytes().all(|b| {
+            b.is_ascii_lowercase() || b.is_ascii_digit() || matches!(b, b'.' | b'-' | b'_')
+        })
+}
+
 impl Client {
     /// `timeout` bounds connecting and each read: no bytes for that long is an error.
     pub fn new(connector: Connector, token: Secret, timeout: Duration) -> Client {
@@ -179,6 +192,42 @@ impl Client {
         }
         Ok(channel.id)
     }
+
+    /// `PUT /api/v4/posts/{post}/patch`: replace the post's text.
+    pub fn patch_post(&self, post: &str, message: &str) -> Result<(), MmError> {
+        // A post that is not `valid_id` is Json, and nothing is sent. Otherwise PUT
+        // /api/v4/posts/<post>/patch with {"message": message}, through `call`; the answer is a
+        // Post, `check`ed and dropped.
+        if !valid_id(post) {
+            return Err(MmError::Json(
+                "a post path must be a Mattermost id".to_string(),
+            ));
+        }
+        let payload = serde_json::json!({ "message": message }).to_string();
+        let body = self.call(
+            "PUT",
+            &format!("/api/v4/posts/{post}/patch"),
+            Some(payload.as_bytes()),
+        )?;
+        json::<Post>(&body)?.check().map(|_| ())
+    }
+
+    /// `GET /api/v4/users/{user}`: the user's username.
+    pub fn username(&self, user: &str) -> Result<String, MmError> {
+        // A user that is not `valid_id` is Json. Otherwise GET /api/v4/users/<user>; a username
+        // that is not `valid_username` is Json.
+        if !valid_id(user) {
+            return Err(MmError::Json(
+                "a user path must be a Mattermost id".to_string(),
+            ));
+        }
+        let body = self.call("GET", &format!("/api/v4/users/{user}"), None)?;
+        let user: User = json(&body)?;
+        if !valid_username(&user.username) {
+            return Err(MmError::Json("the user has no valid username".to_string()));
+        }
+        Ok(user.username)
+    }
 }
 
 fn status_error(status: u16, body: &[u8]) -> MmError {