prop289: Keep track of the last seen cell digests

This makes tor remember the last seen digest of a cell if that cell is the
last one before a SENDME on the Exit side.

Closes #26839

Signed-off-by: David Goulet <dgoulet@torproject.org>

1 files changed, 6 insertions, 0 deletions

diff --git a/src/core/or/circuit_st.h b/src/core/or/circuit_st.h
index cc21cf62f7..5adb158935 100644
--- a/src/core/or/circuit_st.h
+++ b/src/core/or/circuit_st.h
@@ -104,6 +104,12 @@ struct circuit_t {
    * circuit-level sendme cells to indicate that we're willing to accept
    * more. */
   int deliver_window;
+  /** FIFO containing the digest of the cells that are just before a SENDME is
+   * sent by the client. It is done at the last cell before our package_window
+   * goes down to 0 which is when we expect a SENDME. The protocol doesn't
+   * allow more than 10 outstanding SENDMEs worth of data meaning this list
+   * should only contain at most 10 digests of 4 bytes each. */
+  smartlist_t *sendme_last_digests;
 
   /** Temporary field used during circuits_handle_oom. */
   uint32_t age_tmp;