api: add type casts

2 files changed, 47 insertions, 6 deletions

diff --git a/src/api/mod.rs b/src/api/mod.rs
index ede1cfa..9bb2bed 100644
--- a/src/api/mod.rs
+++ b/src/api/mod.rs
@@ -212,7 +212,8 @@ pub(crate) fn store_status(error: StoreError, context: &str) -> ApiError {
         | StoreError::NotAList
         | StoreError::PastTheEnd { .. }
         | StoreError::NotACounter
-        | StoreError::Retyping(_) => StatusCode::CONFLICT,
+        | StoreError::Retyping(_)
+        | StoreError::NotCastable { .. } => 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 e6921cd..48bdafc 100644
--- a/src/api/store.rs
+++ b/src/api/store.rs
@@ -83,6 +83,12 @@ pub(crate) struct PostQuery {
     /// Adds this to the counter at the path instead of inserting the body
     /// (`store::increment_path`).
     increment: Option<i64>,
+    /// Casts what is at the path to this type instead of inserting the body
+    /// (`store::cast_path`; `typed::Kind::name` names it).
+    cast: Option<String>,
+    /// With `cast`, how timestamps are read from and written as strings (`rfc3339` by
+    /// default; `typed::TimestampFormat`).
+    format: Option<String>,
 }
 
 #[derive(Debug, Deserialize)]
@@ -342,7 +348,11 @@ pub(crate) async fn put_store_entry(
         .await?;
     let read = read.ok_or(ApiError(StatusCode::INTERNAL_SERVER_ERROR, None))?;
 
-    let mut response = with_heads(read.respond()?, &heads);
+    Ok(with_retyped(with_heads(read.respond()?, &heads), &retyped))
+}
+
+/// `response` saying where a write changed types (`RETYPED_HEADER`), if it did.
+fn with_retyped(mut response: Response, retyped: &[store::Retyped]) -> Response {
     if !retyped.is_empty() {
         let mut listed: Vec<String> = retyped
             .iter()
@@ -356,7 +366,7 @@ pub(crate) async fn put_store_entry(
             response.headers_mut().insert(RETYPED_HEADER, value);
         }
     }
-    Ok(response)
+    response
 }
 
 fn etag_for(bytes: &[u8]) -> String {
@@ -619,7 +629,9 @@ fn body_value(headers: &HeaderMap, body: &Bytes) -> Result<WriteBody, ApiError>
 /// appends); `201` with where it went (`{"index": n}`). Not a list there, or an index
 /// 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`.
+/// counter there: `409`. With `?cast=<type>`, casts what is at `key` to that type
+/// instead (`store::cast_path`), answering with it then, as `PUT` does; no cast
+/// possible: `409`.
 pub(crate) async fn post_store_entry(
     State(state): State<AppState>,
     Extension(user): Extension<AuthenticatedUser>,
@@ -633,8 +645,15 @@ pub(crate) async fn post_store_entry(
         return Err(StatusCode::SERVICE_UNAVAILABLE.into());
     }
     let path = parse_write_path(&state, &user, &key, "POST")?;
-    // An increment names an item (`-1`: the last); an insert where it goes (`-1`: the end).
-    let insertion = query.increment.is_none();
+    if query.increment.is_some() && query.cast.is_some() {
+        return Err(ApiError(
+            StatusCode::BAD_REQUEST,
+            Some("increment or cast, not both".to_owned()),
+        ));
+    }
+    // An increment or a cast names an item (`-1`: the last); an insert where it goes
+    // (`-1`: the end).
+    let insertion = query.increment.is_none() && query.cast.is_none();
     if let Some(redirect) =
         to_positive(&state, &path, insertion, None, raw_query.as_deref()).await?
     {
@@ -663,6 +682,27 @@ pub(crate) async fn post_store_entry(
             &heads,
         ));
     }
+    if let Some(name) = &query.cast {
+        let bad = |message: String| ApiError(StatusCode::BAD_REQUEST, Some(message));
+        let to = Kind::named(name).ok_or_else(|| bad(format!("no type {name:?}")))?;
+        let timestamps = match &query.format {
+            None => TimestampFormat::default(),
+            Some(format) => TimestampFormat::named(format)
+                .ok_or_else(|| bad(format!("no timestamp format {format:?}")))?,
+        };
+        let accept = headers.get(ACCEPT).cloned();
+        // A value that can't be shown as asked is refused, and the cast with it.
+        let ((retyped, read), heads) = state
+            .mutate_doc_for(Some(user.0.clone()), move |doc| {
+                let retyped = store::cast_path(doc, &path, to, timestamps)
+                    .map_err(|e| store_status(e, "Failed to cast a store value"))?;
+                let read = read_shown(&*doc, &path, accept.as_ref(), false)?;
+                Ok::<_, ApiError>((retyped, read))
+            })
+            .await?;
+        let read = read.ok_or(ApiError(StatusCode::INTERNAL_SERVER_ERROR, None))?;
+        return Ok(with_retyped(with_heads(read.respond()?, &heads), &retyped));
+    }
     let index = path.last().and_then(|leaf| store::list_index(leaf));
     let value = body_value(&headers, &body)?;
     let inserted = state