HNSW: cursor fixes and thread safety.

hnsw.c

@@ -2237,36 +2237,58 @@ int hnsw_deserialize_index(HNSW *index) {
  *
  * The function returns NULL on out of memory. */
 hnswCursor *hnsw_cursor_init(HNSW *index) {
+    if (pthread_rwlock_wrlock(&index->global_lock) != 0) return NULL;
     hnswCursor *cursor = hmalloc(sizeof(*cursor));
-    if (cursor == NULL) return NULL;
+    if (cursor == NULL) {
+        pthread_rwlock_unlock(&index->global_lock);
+        return NULL;
+    }
+    cursor->index = index;
     cursor->next = index->cursors;
     cursor->current = index->head;
     index->cursors = cursor;
+    pthread_rwlock_unlock(&index->global_lock);
     return cursor;
 }
 
 /* Free the cursor. Can be called both at the end of the iteration, when
  * hnsw_cursor_next() returned NULL, or before. */
-void hnsw_cursor_free(HNSW *index, hnswCursor *cursor) {
+void hnsw_cursor_free(hnswCursor *cursor) {
-    hnswCursor *x = index->cursors;
+    pthread_rwlock_wrlock(&cursor->index->global_lock); // Best effort.
+    hnswCursor *x = cursor->index->cursors;
     hnswCursor *prev = NULL;
     while(x) {
         if (x == cursor) {
             if (prev)
                 prev->next = cursor->next;
             else
-                index->cursors = cursor->next;
+                cursor->index->cursors = cursor->next;
             hfree(cursor);
-            return;
+            break;
         }
         x = x->next;
     }
+    pthread_rwlock_unlock(&cursor->index->global_lock);
+}
+
+/* Acquire a lock to use the cursor. Returns 1 if the lock was acquired
+ * with success, otherwise zero is returned. The returned element is
+ * protected after calling hnsw_cursor_next() for all the time required to
+ * access it, then hnsw_cursor_release_lock() should be called in order
+ * to unlock the HNSW index. */
+int hnsw_cursor_acquire_lock(hnswCursor *cursor) {
+    return pthread_rwlock_rdlock(&cursor->index->global_lock) == 0;
+}
+
+/* Release the cursor lock, see hnsw_cursor_acquire_lock() top comment
+ * for more information. */
+void hnsw_cursor_release_lock(hnswCursor *cursor) {
+    pthread_rwlock_unlock(&cursor->index->global_lock);
 }
 
 /* Return the next element of the HNSW. See hnsw_cursor_init() for
  * the guarantees of the function. */
-hnswNode *hnsw_cursor_next(HNSW *index, hnswCursor *cursor) {
+hnswNode *hnsw_cursor_next(hnswCursor *cursor) {
-    (void)index; // Unused but future proof to have.
     hnswNode *ret = cursor->current;
     if (ret) cursor->current = ret->next;
     return ret;

hnsw.h

@@ -65,12 +65,15 @@ typedef struct hnswNode {
     hnswNodeLayer layers[];
 } hnswNode;
 
+struct HNSW;
+
 /* It is possible to navigate an HNSW with a cursor that guarantees
  * visiting all the elements that remain in the HNSW from the start to the
  * end of the process (but not the new ones, so that the process will
  * eventually finish). Check hnsw_cursor_init(), hnsw_cursor_next() and
  * hnsw_cursor_free(). */
 typedef struct hnswCursor {
+    struct HNSW *index; // Reference to the index of this cursor.
     hnswNode *current;  // Element to report when hnsw_cursor_next() is called.
     struct hnswCursor *next; // Next cursor active.
 } hnswCursor;
@@ -156,8 +159,10 @@ uint32_t hnsw_quants_bytes(HNSW *index);
 
 /* Cursors. */
 hnswCursor *hnsw_cursor_init(HNSW *index);
-void hnsw_cursor_free(HNSW *index, hnswCursor *cursor);
+void hnsw_cursor_free(hnswCursor *cursor);
-hnswNode *hnsw_cursor_next(HNSW *index, hnswCursor *cursor);
+hnswNode *hnsw_cursor_next(hnswCursor *cursor);
+int hnsw_cursor_acquire_lock(hnswCursor *cursor);
+void hnsw_cursor_release_lock(hnswCursor *cursor);
 
 /* Allocator selection. */
 void hnsw_set_allocator(void (*free_ptr)(void*), void *(*malloc_ptr)(size_t),