2 files changed, 29 insertions, 5 deletions
diff --git a/src/api/mod.rs b/src/api/mod.rs
index 8ce855e..dfefcc8 100644
--- a/src/api/mod.rs
+++ b/src/api/mod.rs
@@ -201,9 +201,10 @@ impl IntoResponse for ApiError {
 /// client's; others are logged with `context`, and are a bare `500`.
 pub(crate) fn store_status(error: StoreError, context: &str) -> ApiError {
     let status = match error {
-        StoreError::Conflict | StoreError::NotAList | StoreError::PastTheEnd { .. } => {
-            StatusCode::CONFLICT
-        }
+        StoreError::Conflict
+        | StoreError::NotAList
+        | StoreError::PastTheEnd { .. }
+        | StoreError::NotACounter => StatusCode::CONFLICT,
         StoreError::NotFound | StoreError::UnknownHeads => StatusCode::NOT_FOUND,
         StoreError::Rejoining => StatusCode::SERVICE_UNAVAILABLE,
         other => {
diff --git a/src/api/store.rs b/src/api/store.rs
index 8b09487..7039008 100644
--- a/src/api/store.rs
+++ b/src/api/store.rs
@@ -78,6 +78,13 @@ macro_rules! read_at {
 }
 
 #[derive(Debug, Deserialize)]
+pub(crate) struct PostQuery {
+    /// Adds this to the counter at the path instead of inserting the body
+    /// (`store::increment_path`).
+    increment: Option<i64>,
+}
+
+#[derive(Debug, Deserialize)]
 pub(crate) struct DeleteQuery {
     #[serde(default)]
     prune: bool,
@@ -355,11 +362,14 @@ fn body_value(headers: &HeaderMap, body: &Bytes) -> Result<JsonValue, StatusCode
 /// `POST /store/{key}`: inserts the body into a list, `key` ending in the index it
 /// goes to (`store::insert_path`: the items from there on move up; the length, or `-1`,
 /// appends); `201` with where it went (`{"index": n}`). Not a list there, or an index
-/// past its end: `409`, saying which.
+/// past its end: `409`, saying which. With `?increment=n`, adds `n` to the counter at
+/// `key` instead (`store::increment_path`), answering with its value then; not a
+/// counter there: `409`.
 pub(crate) async fn post_store_entry(
     State(state): State<AppState>,
     Extension(user): Extension<AuthenticatedUser>,
     Path(key): Path<String>,
+    Query(query): Query<PostQuery>,
     RawQuery(raw_query): RawQuery,
     headers: HeaderMap,
     body: Bytes,
@@ -368,9 +378,22 @@ pub(crate) async fn post_store_entry(
         return Err(StatusCode::SERVICE_UNAVAILABLE.into());
     }
     let path = parse_write_path(&state, &user, &key, "POST")?;
-    if let Some(redirect) = to_positive(&state, &path, true, None, raw_query.as_deref()).await? {
+    // An increment names an item (`-1`: the last); an insert where it goes (`-1`: the end).
+    let insertion = query.increment.is_none();
+    if let Some(redirect) =
+        to_positive(&state, &path, insertion, None, raw_query.as_deref()).await?
+    {
         return Ok(redirect);
     }
+    if let Some(by) = query.increment {
+        let value = state
+            .mutate_doc_for(Some(user.0.clone()), move |doc| {
+                store::increment_path(doc, &path, by)
+            })
+            .await
+            .map_err(|e| store_status(e, "Failed to increment a counter"))?;
+        return response_for_value(&JsonValue::from(value), headers.get(ACCEPT));
+    }
     let index = path.last().and_then(|leaf| store::list_index(leaf));
     let value = body_value(&headers, &body)?;
     let inserted = state