1 files changed, 83 insertions, 0 deletions
diff --git a/libcr_ipc/rpc.c b/libcr_ipc/rpc.c
new file mode 100644
index 0000000..6d9422f
--- /dev/null
+++ b/libcr_ipc/rpc.c
@@ -0,0 +1,83 @@
+/* libcr_ipc/rpc.c - Simple request/response system for libcr
+ *
+ * Copyright (C) 2024-2025  Luke T. Shumaker <lukeshu@lukeshu.com>
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+#include <string.h>  /* for memcpy() */
+
+#include <libcr/coroutine.h> /* for cid_t, cr_* */
+
+#include <libcr_ipc/rpc.h>
+
+struct cr_rpc_requester {
+	lm_sll_node;
+	cid_t            cid;
+	void            *req_ptr;  /* where to read req from */
+	void            *resp_ptr; /* where to write resp to */
+};
+
+struct cr_rpc_responder {
+	lm_sll_node;
+	/*                       before enqueued    | after dequeued      */
+	/*                       -------------------+-------------------- */
+	cid_t            cid; /* responder cid      | requester cid       */
+	void            *ptr; /* where to write req | where to write resp */
+};
+
+void _cr_rpc_send_req(struct _cr_rpc *ch, void *req_ptr, size_t req_size, void *resp_ptr) {
+	assert(ch);
+	assert(req_ptr);
+	assert(resp_ptr);
+
+	if (ch->waiters.front && ch->waiter_typ != _CR_RPC_REQUESTER) { /* fast-path (still blocks) */
+		struct cr_rpc_responder *responder =
+			lm_sll_node_cast(struct cr_rpc_responder, ch->waiters.front);
+		lm_sll_pop_from_front(&ch->waiters);
+		/* Copy the req to the responder's stack.  */
+		memcpy(responder->ptr, req_ptr, req_size);
+		/* Notify the responder that we have done so.  */
+		cr_unpause(responder->cid);
+		responder->cid = cr_getcid();
+		responder->ptr = resp_ptr;
+		/* Wait for the responder to set `*resp_ptr`.  */
+		cr_pause_and_yield();
+	} else { /* blocking slow-path */
+		struct cr_rpc_requester self = {
+			.cid      = cr_getcid(),
+			.req_ptr  = req_ptr,
+			.resp_ptr = resp_ptr,
+		};
+		lm_sll_push_to_rear(&ch->waiters, &self);
+		/* Wait for a responder to both copy our req and sed
+		 * `*resp_ptr`.  */
+		cr_pause_and_yield();
+	}
+}
+
+void _cr_rpc_recv_req(struct _cr_rpc *ch, void *req_ptr, size_t req_size, void **ret_resp_ptr, cid_t *ret_requester) {
+	assert(ch);
+	assert(req_ptr);
+	assert(ret_resp_ptr);
+	assert(ret_requester);
+
+	if (ch->waiters.front && ch->waiter_typ != _CR_RPC_RESPONDER) { /* non-blocking fast-path */
+		struct cr_rpc_requester *requester =
+			lm_sll_node_cast(struct cr_rpc_requester, ch->waiters.front);
+		lm_sll_pop_from_front(&ch->waiters);
+
+		memcpy(req_ptr, requester->req_ptr, req_size);
+		*ret_requester = requester->cid;
+		*ret_resp_ptr = requester->resp_ptr;
+	} else { /* blocking slow-path */
+		struct cr_rpc_responder self = {
+			.cid = cr_getcid(),
+			.ptr = req_ptr,
+		};
+		lm_sll_push_to_rear(&ch->waiters, &self);
+		ch->waiter_typ = _CR_RPC_RESPONDER;
+		cr_pause_and_yield();
+		*ret_requester = self.cid;
+		*ret_resp_ptr = self.ptr;
+	}
+}