add xrpc getBacklinks with new-style link source

constellation/src/server/mod.rs

···
 const DEFAULT_CURSOR_LIMIT: u64 = 16;
 const DEFAULT_CURSOR_LIMIT_MAX: u64 = 100;
 
+fn get_default_cursor_limit() -> u64 {
+    DEFAULT_CURSOR_LIMIT
+}
+
 const INDEX_BEGAN_AT_TS: u64 = 1738083600; // TODO: not this
 
 pub async fn serve<S, A>(store: S, addr: A, stay_alive: CancellationToken) -> anyhow::Result<()>
···
                 let store = store.clone();
                 move |accept, query| async {
                     block_in_place(|| count_distinct_dids(accept, query, store))
+                }
+            }),
+        )
+        .route(
+            "/xrpc/blue.microcosm.links.getBacklinks",
+            get({
+                let store = store.clone();
+                move |accept, query| async {
+                    block_in_place(|| get_backlinks(accept, query, store))
                 }
             }),
         )
···
 }
 
 #[derive(Clone, Deserialize)]
+struct GetBacklinksQuery {
+    /// The link target
+    ///
+    /// can be an AT-URI, plain DID, or regular URI
+    subject: String,
+    /// Filter links only from this link source
+    ///
+    /// eg.: `app.bsky.feed.like:subject.uri`
+    source: String,
+    cursor: Option<OpaqueApiCursor>,
+    /// Filter links only from these DIDs
+    ///
+    /// include multiple times to filter by multiple source DIDs
+    #[serde(default)]
+    did: Vec<String>,
+    /// Set the max number of links to return per page of results
+    #[serde(default = "get_default_cursor_limit")]
+    limit: u64,
+    // TODO: allow reverse (er, forward) order as well
+}
+#[derive(Template, Serialize)]
+#[template(path = "get-backlinks.html.j2")]
+struct GetBacklinksResponse {
+    total: u64,
+    records: Vec<RecordId>,
+    cursor: Option<OpaqueApiCursor>,
+    #[serde(skip_serializing)]
+    query: GetBacklinksQuery,
+    #[serde(skip_serializing)]
+    collection: String,
+    #[serde(skip_serializing)]
+    path: String,
+}
+fn get_backlinks(
+    accept: ExtractAccept,
+    query: axum_extra::extract::Query<GetBacklinksQuery>, // supports multiple param occurrences
+    store: impl LinkReader,
+) -> Result<impl IntoResponse, http::StatusCode> {
+    let until = query
+        .cursor
+        .clone()
+        .map(|oc| ApiCursor::try_from(oc).map_err(|_| http::StatusCode::BAD_REQUEST))
+        .transpose()?
+        .map(|c| c.next);
+
+    let limit = query.limit;
+    if limit > DEFAULT_CURSOR_LIMIT_MAX {
+        return Err(http::StatusCode::BAD_REQUEST);
+    }
+
+    let filter_dids: HashSet<Did> = HashSet::from_iter(
+        query
+            .did
+            .iter()
+            .map(|d| d.trim())
+            .filter(|d| !d.is_empty())
+            .map(|d| Did(d.to_string())),
+    );
+
+    let Some((collection, path)) = query.source.split_once(':') else {
+        return Err(http::StatusCode::BAD_REQUEST);
+    };
+    let path = format!(".{path}");
+
+    let paged = store
+        .get_links(
+            &query.subject,
+            collection,
+            &path,
+            limit,
+            until,
+            &filter_dids,
+        )
+        .map_err(|_| http::StatusCode::INTERNAL_SERVER_ERROR)?;
+
+    let cursor = paged.next.map(|next| {
+        ApiCursor {
+            version: paged.version,
+            next,
+        }
+        .into()
+    });
+
+    Ok(acceptable(
+        accept,
+        GetBacklinksResponse {
+            total: paged.total,
+            records: paged.items,
+            cursor,
+            query: (*query).clone(),
+            collection: collection.to_string(),
+            path,
+        },
+    ))
+}
+
+#[derive(Clone, Deserialize)]
 struct GetLinkItemsQuery {
     target: String,
     collection: String,
···
     ///
     /// deprecated: use `did`, which can be repeated multiple times
     from_dids: Option<String>, // comma separated: gross
-    #[serde(default = "get_default_limit")]
+    #[serde(default = "get_default_cursor_limit")]
     limit: u64,
     // TODO: allow reverse (er, forward) order as well
-}
-fn get_default_limit() -> u64 {
-    DEFAULT_CURSOR_LIMIT
 }
 #[derive(Template, Serialize)]
 #[template(path = "links.html.j2")]

constellation/templates/get-backlinks.html.j2

···
+{% extends "base.html.j2" %}
+{% import "try-it-macros.html.j2" as try_it %}
+
+{% block title %}Links{% endblock %}
+{% block description %}All {{ query.source }} records with links to {{ query.subject }}{% endblock %}
+
+{% block content %}
+
+  {% call try_it::get_backlinks(query.subject, query.source, query.did, query.limit) %}
+
+  <h2>
+    Links to <code>{{ query.subject }}</code>
+    {% if let Some(browseable_uri) = query.subject|to_browseable %}
+      <small style="font-weight: normal; font-size: 1rem"><a href="{{ browseable_uri }}">browse record</a></small>
+    {% endif %}
+  </h2>
+
+  <p><strong>{{ total|human_number }} links</strong> from <code>{{ query.source }}</code>.</p>
+
+  <ul>
+    <li>See distinct linking DIDs at <code>/links/distinct-dids</code>: <a href="/links/distinct-dids?target={{ query.subject|urlencode }}&collection={{ collection|urlencode }}&path={{ path|urlencode }}">/links/distinct-dids?target={{ query.subject }}&collection={{ collection }}&path={{ path }}</a></li>
+    <li>See all links to this target at <code>/links/all</code>: <a href="/links/all?target={{ query.subject|urlencode }}">/links/all?target={{ query.subject }}</a></li>
+  </ul>
+
+  <h3>Links, most recent first:</h3>
+
+  {% for record in records %}
+    <pre style="display: block; margin: 1em 2em" class="code"><strong>DID</strong>:        {{ record.did().0 }} (<a href="/links/all?target={{ record.did().0|urlencode }}">DID links</a>)
+<strong>Collection</strong>: {{ record.collection }}
+<strong>RKey</strong>:       {{ record.rkey }}
+-> <a href="https://pdsls.dev/at://{{ record.did().0 }}/{{ record.collection }}/{{ record.rkey }}">browse record</a></pre>
+  {% endfor %}
+
+  {% if let Some(c) = cursor %}
+    <form method="get" action="/xrpc/blue.microcosm.links.getBacklinks">
+      <input type="hidden" name="subject" value="{{ query.subject }}" />
+      <input type="hidden" name="source" value="{{ query.source }}" />
+      <input type="hidden" name="limit" value="{{ query.limit }}" />
+      {% for did in query.did %}
+        <input type="hidden" name="did" value="{{ did }}" />
+      {% endfor %}
+      <input type="hidden" name="cursor" value={{ c|json|safe }} />
+      <button type="submit">next page&hellip;</button>
+    </form>
+  {% else %}
+    <button disabled><em>end of results</em></button>
+  {% endif %}
+
+  <details>
+    <summary>Raw JSON response</summary>
+    <pre class="code">{{ self|tojson }}</pre>
+  </details>
+
+{% endblock %}

constellation/templates/hello.html.j2

···
 
   <h2>API Endpoints</h2>
 
+  <h3 class="route"><code>GET /xrpc/blue.microcosm.links.getBacklinks</code></h3>
+
+  <p>A list of records linking to any record, identity, or uri.</p>
+
+  <h4>Query parameters:</h4>
+
+  <ul>
+    <li><p><code>subject</code>: required, must url-encode. Example: <code>at://did:plc:vc7f4oafdgxsihk4cry2xpze/app.bsky.feed.post/3lgwdn7vd722r</code></p></li>
+    <li><p><code>source</code>: required. Example: <code>app.bsky.feed.like:subject.uri</code></p></li>
+    <li><p><code>did</code>: optional, filter links to those from specific users. Include multiple times to filter by multiple users. Example: <code>did=did:plc:vc7f4oafdgxsihk4cry2xpze&did=did:plc:vc7f4oafdgxsihk4cry2xpze</code></p></li>
+    <li><p><code>limit</code>: optional. Default: <code>16</code>. Maximum: <code>100</code></p></li>
+  </ul>
+
+  <p style="margin-bottom: 0"><strong>Try it:</strong></p>
+  {% call try_it::get_backlinks("at://did:plc:a4pqq234yw7fqbddawjo7y35/app.bsky.feed.post/3m237ilwc372e", "app.bsky.feed.like:subject.uri", [""], 16) %}
+
+
   <h3 class="route"><code>GET /links</code></h3>
 
   <p>A list of records linking to a target.</p>
+
+  <p>[DEPRECATED]: use <code>GET /xrpc/blue.microcosm.links.getBacklinks</code>. New apps should avoid it, but this endpoint <strong>will</strong> remain supported for the forseeable future.</p>
 
   <h4>Query parameters:</h4>
 

constellation/templates/try-it-macros.html.j2

···
+{% macro get_backlinks(subject, source, dids, limit) %}
+  <form method="get" action="/xrpc/blue.microcosm.links.getBacklinks">
+    <pre class="code"><strong>GET</strong> /links
+  ?subject=    <input type="text" name="subject" value="{{ subject }}" placeholder="at-uri, did, uri..." />
+  &source=     <input type="text" name="source" value="{{ source }}" placeholder="app.bsky.feed.like:subject.uri" />
+  {%- for did in dids %}{% if !did.is_empty() %}
+  &did=        <input type="text" name="did" value="{{ did }}" placeholder="did:plc:..." />{% endif %}{% endfor %}
+  <span id="did-placeholder"></span>             <button id="add-did">+ did filter</button>
+  &limit=      <input type="number" name="limit" value="{{ limit }}" max="100" placeholder="100" /> <button type="submit">get links</button></pre>
+  </form>
+  <script>
+    const addDidButton = document.getElementById('add-did');
+    const didPlaceholder = document.getElementById('did-placeholder');
+    addDidButton.addEventListener('click', e => {
+      e.preventDefault();
+      const i = document.createElement('input');
+      i.placeholder = 'did:plc:...';
+      i.name = "did"
+      const p = addDidButton.parentNode;
+      p.insertBefore(document.createTextNode('&did=        '), didPlaceholder);
+      p.insertBefore(i, didPlaceholder);
+      p.insertBefore(document.createTextNode('\n  '), didPlaceholder);
+    });
+  </script>
+{% endmacro %}
+
 {% macro links(target, collection, path, dids, limit) %}
   <form method="get" action="/links">
     <pre class="code"><strong>GET</strong> /links
···
     });
   </script>
 {% endmacro %}
-
 
 {% macro dids(target, collection, path) %}
   <form method="get" action="/links/distinct-dids">