common: move lttng_payload[_view] to libcommon

[lttng-tools.git] / src / common / payload-view.h

diff --git a/src/common/payload-view.h b/src/common/payload-view.h
new file mode 100644 (file)
index 0000000..f125e88
--- /dev/null
+++ b/src/common/payload-view.h
@@ -0,0 +1,111 @@
+/*
+ * Copyright (C) 2020 Jérémie Galarneau <jeremie.galarneau@efficios.com>
+ *
+ * SPDX-License-Identifier: LGPL-2.1-only
+ *
+ */
+
+#ifndef LTTNG_PAYLOAD_VIEW_H
+#define LTTNG_PAYLOAD_VIEW_H
+
+#include <common/buffer-view.h>
+#include <common/dynamic-array.h>
+
+struct lttng_payload;
+
+/*
+ * An lttng_payload_view references a payload and allows code to share
+ * a `const` version of a subset of a payload.
+ *
+ * A payload view is invalidated whenever its source (a payload, or another
+ * payload view) is modified.
+ *
+ * While a payload view does not allow users to modify the underlying bytes
+ * of the payload, it can be used to 'pop' file descriptors using an iterator
+ * belonging to the top-level payload view.
+ *
+ * Hence, a payload view created from a        payload or a dynamic buffer contains
+ * an implicit file descriptor iterator. Any payload view created from another
+ * payload view will share the same underlying file descriptor iterator.
+ *
+ * The rationale for this is that a payload is never consumed directly, it
+ * must be consumed through a payload view.
+ *
+ * Typically, a payload view will be used to rebuild a previously serialized
+ * object hierarchy. Sharing an underlying iterator allows aggregate objects
+ * to provide a restricted view of the payload to their members, which will
+ * report the number of bytes consumed and `pop` the file descriptors they
+ * should own. In return, those objects can create an even narrower view for
+ * their children, allowing them to also consume file descriptors.
+ *
+ * Note that a payload view never assumes any ownership of the underlying
+ * payload.
+ */
+struct lttng_payload_view {
+       struct lttng_buffer_view buffer;
+       /* private */
+       const struct lttng_dynamic_array _fds;
+       struct {
+               size_t *p_fds_position;
+               size_t fds_position;
+       } _iterator;
+};
+
+/**
+ * Return a payload view referencing a subset of a payload.
+ *
+ * @payload    Source payload to reference
+ * @offset     Offset to apply to the payload's buffer
+ * @len                Length of the contents to reference. Passing -1 will
+ *             cause the view to reference the whole payload from the
+ *             offset provided.
+ */
+LTTNG_HIDDEN
+struct lttng_payload_view lttng_payload_view_from_payload(
+               const struct lttng_payload *payload, size_t offset,
+               ptrdiff_t len);
+
+/**
+ * Return a payload view referencing a subset of a payload referenced by
+ * another payload view.
+ *
+ * @view       Source payload view to reference
+ * @offset     Offset to apply to the payload view's buffer view
+ * @len                Length of the contents to reference. Passing -1 will
+ *             cause the payload view to reference the whole payload view's
+ *             buffer view from the offset provided.
+ */
+LTTNG_HIDDEN
+struct lttng_payload_view lttng_payload_view_from_view(
+               struct lttng_payload_view *view, size_t offset,
+               ptrdiff_t len);
+
+/**
+ * Return a payload view referencing a subset of a dynamic buffer.
+ *
+ * Meant as an adapter for code paths that need to create a payload view
+ * from an existing dynamic buffer.
+ *
+ * @src                Source dynamic buffer to reference
+ * @offset     Offset to apply to the payload's buffer
+ * @len                Length of the buffer contents to reference. Passing -1 will
+ *             cause the payload view to reference the whole payload from the
+ *             offset provided.
+ */
+LTTNG_HIDDEN
+struct lttng_payload_view lttng_payload_view_from_dynamic_buffer(
+               const struct lttng_dynamic_buffer *buffer, size_t offset,
+               ptrdiff_t len);
+
+/**
+ * Pop an fd from a payload view.
+ * No ownership of the file descriptor is assumed by the payload.
+ *
+ * @payload    Payload instance
+ *
+ * Returns a file descriptor on success, -1 on error.
+ */
+LTTNG_HIDDEN
+int lttng_payload_view_pop_fd(struct lttng_payload_view *payload_view);
+
+#endif /* LTTNG_PAYLOAD_VIEW_H */