feat(NIP-01): Implement limit

This was quickly sneaked in by fiatjaf per my request[0], it makes many
queries more efficient and allows for paging when combined with until.

It is a bit weird to have multiple limits on each filter... for now we
just choose any or the last limit seen.

[0]: https://github.com/nostr-protocol/nips/commit/a4aea5337fe6b93e55f9dae1974ead962c1997e8

Signed-off-by: William Casarin <jb55@jb55.com>

src/db.rs

@@ -385,6 +385,7 @@ fn query_from_sub(sub: &Subscription) -> (String, Vec<Box<dyn ToSql>>) {
     // (sqli-safe), or a string that is filtered to only contain
     // hexadecimal characters.  Strings that require escaping (tag
     // names/values) use parameters.
+    let mut limit: Option<u32> = None;
     let mut query =
         "SELECT DISTINCT(e.content) FROM event e LEFT JOIN tag t ON e.id=t.event_id ".to_owned();
     // parameters
@@ -422,6 +423,9 @@ fn query_from_sub(sub: &Subscription) -> (String, Vec<Box<dyn ToSql>>) {
             let authors_clause = format!("({})", auth_searches.join(" OR "));
             filter_components.push(authors_clause);
         }
+        if let Some(lim) = f.limit {
+            limit = Some(lim)
+        }
         // Query for Kind
         if let Some(ks) = &f.kinds {
             // kind is number, no escaping needed
@@ -513,7 +517,13 @@ fn query_from_sub(sub: &Subscription) -> (String, Vec<Box<dyn ToSql>>) {
         query.push_str(") ");
     }
     // add order clause
-    query.push_str(" ORDER BY created_at ASC");
+    query.push_str(&format!(
+        " ORDER BY created_at {}",
+        limit.map_or("ASC", |_| "DESC")
+    ));
+    if let Some(lim) = limit {
+        query.push_str(&format!(" LIMIT {}", lim))
+    }
     debug!("query string: {}", query);
     (query, params)
 }

src/subscription.rs

@@ -31,6 +31,8 @@ pub struct ReqFilter {
     pub until: Option<u64>,
     /// List of author public keys
     pub authors: Option<Vec<String>>,
+    /// Limit number of results
+    pub limit: Option<u32>,
     /// Set of tags
     #[serde(skip)]
     pub tags: Option<HashMap<String, HashSet<String>>>,
@@ -54,6 +56,7 @@ impl<'de> Deserialize<'de> for ReqFilter {
             since: None,
             until: None,
             authors: None,
+            limit: None,
             tags: None,
         };
         let mut ts = None;
@@ -68,6 +71,8 @@ impl<'de> Deserialize<'de> for ReqFilter {
                 rf.since = Deserialize::deserialize(val).ok();
             } else if key == "until" {
                 rf.until = Deserialize::deserialize(val).ok();
+            } else if key == "limit" {
+                rf.limit = Deserialize::deserialize(val).ok();
             } else if key == "authors" {
                 rf.authors = Deserialize::deserialize(val).ok();
             } else if key.starts_with('#') && key.len() > 1 && val.is_array() {