src/common/payload-view.hpp

   1 /*
   2  * Copyright (C) 2020 Jérémie Galarneau <jeremie.galarneau@efficios.com>
   3  *
   4  * SPDX-License-Identifier: LGPL-2.1-only
   5  *
   6  */
   7
   8 #ifndef LTTNG_PAYLOAD_VIEW_H
   9 #define LTTNG_PAYLOAD_VIEW_H
  10
  11 #include <common/buffer-view.hpp>
  12 #include <common/dynamic-array.hpp>
  13
  14 struct lttng_payload;
  15 struct fd_handle;
  16
  17 /*
  18  * An lttng_payload_view references a payload and allows code to share
  19  * a `const` version of a subset of a payload.
  20  *
  21  * A payload view is invalidated whenever its source (a payload, or another
  22  * payload view) is modified.
  23  *
  24  * While a payload view does not allow users to modify the underlying bytes
  25  * of the payload, it can be used to 'pop' file descriptor handles using an
  26  * iterator belonging to the top-level payload view.
  27  *
  28  * Hence, a payload view created from a payload or a dynamic buffer contains
  29  * an implicit file descriptor handle iterator. Any payload view created from
  30  * another payload view will share the same underlying file descriptor handle
  31  * iterator.
  32  *
  33  * The rationale for this is that a payload is never consumed directly, it must
  34  * be consumed through a payload view.
  35  *
  36  * Typically, a payload view will be used to rebuild a previously serialized
  37  * object hierarchy. Sharing an underlying iterator allows aggregate objects
  38  * to provide a restricted view of the payload to their members, which will
  39  * report the number of bytes consumed and `pop` the file descriptor handle they
  40  * should own. In return, those objects can create an even narrower view for
  41  * their children, allowing them to also consume file descriptor handles.
  42  *
  43  * Note that a payload view never assumes any ownership of the underlying
  44  * payload.
  45  */
  46 struct lttng_payload_view {
  47         struct lttng_buffer_view buffer;
  48         /* private */
  49
  50         const struct lttng_dynamic_pointer_array _fd_handles;
  51
  52         struct {
  53                 size_t *p_fd_handles_position;
  54                 size_t fd_handles_position;
  55         } _iterator;
  56 };
  57
  58 /**
  59  * Checks if a payload view's buffer is safe to access.
  60  *
  61  * After calling the payload view creation functions, callers should verify
  62  * if the resquested length (if any is explicitly provided) could be mapped
  63  * to a new view.
  64  *
  65  * @view        Payload to validate
  66  */
  67 bool lttng_payload_view_is_valid(const struct lttng_payload_view *view);
  68
  69 /**
  70  * Return a payload view referencing a subset of a payload.
  71  *
  72  * @payload     Source payload to reference
  73  * @offset      Offset to apply to the payload's buffer
  74  * @len         Length of the contents to reference. Passing -1 will
  75  *              cause the view to reference the whole payload from the
  76  *              offset provided.
  77  */
  78 struct lttng_payload_view
  79 lttng_payload_view_from_payload(const struct lttng_payload *payload, size_t offset, ptrdiff_t len);
  80
  81 /**
  82  * Return a payload view referencing a subset of a payload referenced by
  83  * another payload view.
  84  *
  85  * @view        Source payload view to reference
  86  * @offset      Offset to apply to the payload view's buffer view
  87  * @len         Length of the contents to reference. Passing -1 will
  88  *              cause the payload view to reference the whole payload view's
  89  *              buffer view from the offset provided.
  90  */
  91 struct lttng_payload_view
  92 lttng_payload_view_from_view(struct lttng_payload_view *view, size_t offset, ptrdiff_t len);
  93
  94 /**
  95  * Return a payload view referencing a subset of a dynamic buffer.
  96  *
  97  * Meant as an adapter for code paths that need to create a payload view
  98  * from an existing dynamic buffer.
  99  *
 100  * @src         Source dynamic buffer to reference
 101  * @offset      Offset to apply to the dynamic buffer
 102  * @len         Length of the buffer contents to reference. Passing -1 will
 103  *              cause the payload view to reference the whole payload from the
 104  *              offset provided.
 105  */
 106 struct lttng_payload_view lttng_payload_view_from_dynamic_buffer(
 107         const struct lttng_dynamic_buffer *buffer, size_t offset, ptrdiff_t len);
 108 /**
 109  *
 110  * Return a payload view referencing a subset of a dynamic buffer.
 111  *
 112  * Meant as an adapter for code paths that need to create a payload view
 113  * from an existing buffer view.
 114  *
 115  * @src         Source buffer view to reference
 116  * @offset      Offset to apply to the buffer view
 117  * @len         Length of the buffer contents to reference. Passing -1 will
 118  *              cause the payload view to reference the whole payload from the
 119  *              offset provided.
 120  */
 121 struct lttng_payload_view lttng_payload_view_from_buffer_view(const struct lttng_buffer_view *view,
 122                                                               size_t offset,
 123                                                               ptrdiff_t len);
 124
 125 /**
 126  * Return a payload view referencing a subset of the memory referenced by a raw
 127  * pointer.
 128  *
 129  * @src         Source buffer to reference
 130  * @offset      Offset to apply to the source memory buffer
 131  * @len         Length of the memory contents to reference.
 132  *
 133  * Note that a payload view never assumes the ownership of the memory it
 134  * references.
 135  */
 136 struct lttng_payload_view
 137 lttng_payload_view_init_from_buffer(const char *src, size_t offset, ptrdiff_t len);
 138
 139 /**
 140  * Get the number of file descriptor handles left in a payload view.
 141  *
 142  * @payload     Payload instance
 143  *
 144  * Returns the number of file descriptor handles left on success, -1 on error.
 145  */
 146 int lttng_payload_view_get_fd_handle_count(const struct lttng_payload_view *payload_view);
 147
 148 /**
 149  * Pop an fd handle from a payload view.
 150  *
 151  * A reference to the returned fd_handle is acquired on behalf of the caller.
 152  *
 153  * @payload     Payload instance
 154  *
 155  * Returns an fd_handle on success, -1 on error.
 156  */
 157 struct fd_handle *lttng_payload_view_pop_fd_handle(struct lttng_payload_view *payload_view);
 158
 159 #endif /* LTTNG_PAYLOAD_VIEW_H */