rdma: add terminal notification callback to the RC server ABI

RC sessions that never reach a completion (expired, abandoned, or canceled before READY) currently vanish inside the reaper without any trace on the operational surface. Add a terminal notification callback to the C ABI so the server reports the final outcome of every session exactly once, fired from the reaper with no server lock held.

cuwrapper/rc/rc_server_abi.cpp

@@ -101,6 +101,11 @@ struct rc_server {
    * races an in-flight sink pointer swap. */
   rc_log_fn log_fn = nullptr;
   void *log_ctx = nullptr;
+  /* Terminal notification sink: same lifetime contract as log_fn
+   * (installed once at init, cleared by destroy after the reaper
+   * joined). Fired by reapSession with no lock held. */
+  rc_terminal_fn term_fn = nullptr;
+  void *term_ctx = nullptr;
   std::atomic<uint64_t> epoch_counter{1};
   /* resource accounting (global buckets; per-principal map). */
   std::mutex acct_mtx;
@@ -253,6 +258,13 @@ void reapSession(rc_server *srv, RcSession *s) {
    * is actually gone: a surviving QP still holds the device. */
   if (destroyed) hipObj::v2::releaseDevice(srv->device);
   limitsRelease(srv, s->principal, s->staging_len);
+  /* Terminal notification: the sink runs after every server-side
+   * bookkeeping above so it observes the session as fully gone,
+   * and no lock is held here per the callback contract. */
+  rc_terminal_fn tfn = srv->term_fn;
+  if (tfn)
+    tfn(srv->term_ctx, s->core.id.c_str(), s->last_outcome,
+        (uint64_t)s->staging_len);
 }
 
 /* Runs the reap pass: sessions marked reap_pending (or in the
@@ -301,6 +313,13 @@ void rc_server_set_log_sink(rc_server *srv, rc_log_fn fn, void *ctx) {
   srv->log_ctx = ctx;
 }
 
+void rc_server_set_terminal_notify(rc_server *srv, rc_terminal_fn fn,
+                                   void *ctx) {
+  if (!srv) return;
+  srv->term_fn = fn;
+  srv->term_ctx = ctx;
+}
+
 int rc_server_init(const rc_device_opts *opts, rc_server **out) {
   if (!opts || !out) return RC_E_ARG;
   if (!hipObj::ibv.ensureLoaded()) {

cuwrapper/rc/rc_server_abi.h

@@ -52,6 +52,21 @@ typedef struct rc_server rc_server;
 
 void rc_server_set_log_sink(rc_server *srv, rc_log_fn fn, void *ctx);
 
+/* Terminal session notification. Fired exactly once per session
+ * when the reaper destroys it (expiry, CANCEL, or destroy),
+ * carrying the last outcome observed for the session. `id` is
+ * only valid for the duration of the call. Called with no server
+ * lock held; the sink must return promptly and must not call back
+ * into the server. Sessions that end inside a READY/FinishPut
+ * handler still fire this notification after the handler's own
+ * terminal bookkeeping, so the sink can treat it as the single
+ * authoritative "the session is gone" signal. */
+typedef void (*rc_terminal_fn)(void *ctx, const char *id, int outcome,
+                               uint64_t bytes);
+
+void rc_server_set_terminal_notify(rc_server *srv, rc_terminal_fn fn,
+                                   void *ctx);
+
 /* Device selection: matching GID prefix when gid_hint is set,
  * otherwise the first verbs device. */
 typedef struct {