libcr_ipc: Add waitgrouplukeshu/waitgroup

1 files changed, 65 insertions, 0 deletions

diff --git a/libcr_ipc/include/libcr_ipc/waitgroup.h b/libcr_ipc/include/libcr_ipc/waitgroup.h
new file mode 100644
index 0000000..32369cf
--- /dev/null
+++ b/libcr_ipc/include/libcr_ipc/waitgroup.h
@@ -0,0 +1,65 @@
+/* libcr_ipc/waitgroup.h - Go-like WaitGroups for libcr
+ *
+ * Copyright (C) 2025  Luke T. Shumaker <lukeshu@lukeshu.com>
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+#ifndef _LIBCR_IPC_WAITGROUP_H_
+#define _LIBCR_IPC_WAITGROUP_H_
+
+#include <stdbool.h>
+
+#include <libmisc/private.h>
+
+#include <libcr_ipc/_linkedlist_pub.h>
+
+/**
+ * A cr_waitgroup_t is a fair read/write mutex.
+ *
+ * A waiting writer blocks any new readers; this ensures that the
+ * writer is able to eventually get the lock.
+ *
+ * None of the methods have `_from_intrhandler` variants because (1)
+ * an interrupt handler can't block, so it shouldn't ever lock a mutex
+ * because that can block; and (2) if it can't lock a mutex in the
+ * first place, then it has no business unlocking one.
+ */
+typedef struct {
+	BEGIN_PRIVATE(LIBCR_IPC_WAITGROUP_H);
+	int                     count;
+	bool                    unpausing;
+	_cr_ipc_sll_root        waiters;
+	END_PRIVATE(LIBCR_IPC_WAITGROUP_H);
+} cr_waitgroup_t;
+
+/**
+ * Add delta (which may be negative) to the counter.  If the counter
+ * becomes zero, all cr_waitgroup_wait() are unblocked.  The counter
+ * must not become negative.
+ *
+ * @runs_in coroutine
+ * @cr_pauses never
+ * @cr_yields never
+ */
+void cr_waitgroup_add(cr_waitgroup_t *wg, int delta);
+
+/**
+ * Like cr_waitgroup_add(), but for use from an interrupt handler.
+ *
+ * @runs_in intrhandler
+ */
+void cr_waitgroup_add_from_intrhandler(cr_waitgroup_t *wg, int delta);
+
+#define cr_waitgroup_done(WG) cr_waitgroup_add(WG, -1)
+#define cr_waitgroup_done_from_intrhandler(WG) cr_waitgroup_add_from_intrhandler(WG, -1)
+
+/**
+ * Wait until the counter becomes zero.  If the counter is already zero, returns immediately.
+ *
+ * @runs_in coroutine
+ * @cr_pauses maybe
+ * @cr_yields maybe
+ */
+void cr_waitgroup_wait(cr_waitgroup_t *wg);
+
+#endif /* _LIBCR_IPC_WAITGROUP_H_ */