cursor: add csClosed guard to fetchNextImpl to prevent NilAccessDefect

async_postgres/pg_client/cursor.nim

@@ -131,6 +131,8 @@ proc openCursorImpl(
 
 proc fetchNextImpl(cursor: Cursor): Future[seq[Row]] {.async.} =
   let conn = cursor.conn
+  if conn.state == csClosed:
+    raise newException(PgConnectionError, "Connection is closed")
   let rd = newRowData(int16(cursor.fields.len), cursor.colFormats, cursor.colTypeOids)
   rd.fields = cursor.fields
   var rowCount: int32 = 0
@@ -201,6 +203,9 @@ proc fetchNext*(cursor: Cursor): Future[seq[Row]] {.async.} =
   ## Fetch the next chunk of rows from the cursor.
   ## Returns an empty seq when the cursor is exhausted.
   ## On timeout, the connection is marked csClosed (protocol out of sync).
+  let conn = cursor.conn
+  if conn.state == csClosed:
+    raise newException(PgConnectionError, "Connection is closed")
   if cursor.bufferedCount > 0:
     result = newSeq[Row](cursor.bufferedCount)
     for i in 0 ..< cursor.bufferedCount:

tests/test_abandonment_e2e.nim

@@ -65,6 +65,21 @@ suite "E2E: Cursor lifecycle invariants":
 
     waitFor t()
 
+  test "fetchNext after conn.close() raises PgConnectionError, not SIGSEGV":
+    proc t() {.async.} =
+      let conn = await connect(plainConfig())
+      let cursor =
+        await conn.openCursor("SELECT i FROM generate_series(1, 100) i", chunkSize = 10)
+      await conn.close()
+      var raised = false
+      try:
+        discard await cursor.fetchNext()
+      except PgConnectionError:
+        raised = true
+      doAssert raised, "fetchNext after close must raise PgConnectionError"
+
+    waitFor t()
+
   test "openCursor with invalid SQL raises and leaves connection ready":
     # Recovery path: ErrorResponse during openCursor drains Sync back to
     # ReadyForQuery so the connection is reusable immediately.