[Libguestfs] [PATCH libnbd 1/7] api: Add semi-private function for freeing persistent data.
Richard W.M. Jones
rjones at redhat.com
Mon Aug 12 16:08:45 UTC 2019
This adds a C-only semi-private function for freeing various types of
persistent data passed to libnbd.
There are some similarities with nbd_add_close_callback which we
removed in commit 7f191b150b52ed50098976309a6af883d245fc56.
---
generator/generator | 57 ++++++++++++++++++++
lib/Makefile.am | 1 +
lib/free.c | 129 ++++++++++++++++++++++++++++++++++++++++++++
lib/handle.c | 6 +++
lib/internal.h | 14 +++++
5 files changed, 207 insertions(+)
diff --git a/generator/generator b/generator/generator
index c52200b..d6b9352 100755
--- a/generator/generator
+++ b/generator/generator
@@ -3284,6 +3284,7 @@ let generate_lib_libnbd_syms () =
pr "LIBNBD_%d.%d {\n" major minor;
pr " global:\n";
if (major, minor) = (1, 0) then (
+ pr " nbd_add_free_callback;\n";
pr " nbd_create;\n";
pr " nbd_close;\n";
pr " nbd_get_errno;\n";
@@ -3581,6 +3582,13 @@ let generate_include_libnbd_h () =
pr "extern int nbd_get_errno (void);\n";
pr "#define LIBNBD_HAVE_NBD_GET_ERRNO 1\n";
pr "\n";
+ pr "typedef void (*nbd_free_callback) (void *ptr, void *user_data);\n";
+ pr "extern int nbd_add_free_callback (struct nbd_handle *h,\n";
+ pr " void *ptr,\n";
+ pr " nbd_free_callback cb,\n";
+ pr " void *user_data);\n";
+ pr "#define LIBNBD_HAVE_NBD_ADD_FREE_CALLBACK 1\n";
+ pr "\n";
print_closure_typedefs ();
List.iter (
fun (name, { args; optargs; ret }) ->
@@ -4005,6 +4013,55 @@ C<E<lt>errno.hE<gt>>.
List.iter print_api handle_calls;
pr "\
+=head1 FREE CALLBACKS
+
+B<Note:> The API described in this section is only
+available from C. It is designed to help when writing
+bindings to libnbd in other programming languages.
+As such it is B<not> covered by the usual API stability
+guarantee offered by other parts of the C API. Use it with care.
+
+Some pointers passed to libnbd calls are saved in the
+S<C<struct nbd_handle>>. These include pointers to buffers
+passed to C<nbd_aio_pread>, C<nbd_aio_pwrite>, etc.,
+and pointers to the C<user_data> for callbacks. If you
+want to know when it is safe to free these objects then
+you can register a free callback using:
+
+ typedef void (*nbd_free_callback) (void *ptr, void *user_data);
+ int nbd_add_free_callback (struct nbd_handle *h,
+ void *ptr,
+ nbd_free_callback cb,
+ void *user_data);
+
+C<ptr> is a pointer to the object (ie. the buffer or
+callback C<user_data>). C<cb (ptr)> is called when libnbd
+no longer holds a reference to that object.
+
+To illustrate how this works with an example:
+
+ struct write_completed_user_data *my_user_data;
+ void *buf;
+
+ my_user_data = malloc (sizeof *my_user_data);
+ buf = malloc (512);
+ nbd_add_free_callback (nbd, my_user_data, my_free, NULL);
+ nbd_add_free_callback (nbd, buf, my_free, NULL);
+ nbd_aio_pwrite_callback (nbd, buf, 512, 0,
+ write_completed, my_user_data, 0);
+
+where C<my_free> is a simple wrapper around the L<free(3)>
+function (we are not using the extra C<user_data> pointer
+in this case):
+
+ void my_free (void *ptr, void *)
+ {
+ free (ptr);
+ }
+
+This would free both C<my_user_data> and C<buf> once libnbd
+has finished with them.
+
=head1 SEE ALSO
L<libnbd(3)>.
diff --git a/lib/Makefile.am b/lib/Makefile.am
index 3657b9f..08e9d25 100644
--- a/lib/Makefile.am
+++ b/lib/Makefile.am
@@ -42,6 +42,7 @@ libnbd_la_SOURCES = \
disconnect.c \
errors.c \
flags.c \
+ free.c \
handle.c \
internal.h \
is-state.c \
diff --git a/lib/free.c b/lib/free.c
new file mode 100644
index 0000000..67ba752
--- /dev/null
+++ b/lib/free.c
@@ -0,0 +1,129 @@
+/* NBD client library in userspace
+ * Copyright (C) 2013-2019 Red Hat Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include <config.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <errno.h>
+#include <assert.h>
+
+#include "internal.h"
+
+/* This is not generated because we don't want to offer it to other
+ * programming languages. Also it's a semi-private API liable to
+ * evolve in its exact semantics.
+ */
+int
+nbd_add_free_callback (struct nbd_handle *h, void *ptr,
+ nbd_free_callback cb, void *user_data)
+{
+ int ret = -1;
+ size_t i;
+
+ if (ptr == NULL)
+ return 0;
+
+ nbd_internal_set_error_context ("nbd_add_free_callback");
+ pthread_mutex_lock (&h->lock);
+
+ /* Extend the list of callbacks in the handle. */
+ if (h->nr_free_callbacks >= h->alloc_free_callbacks) {
+ size_t new_alloc;
+ struct free_callback *new_callbacks;
+
+ if (h->alloc_free_callbacks == 0)
+ new_alloc = 8;
+ else
+ new_alloc = 2 * h->alloc_free_callbacks;
+
+ new_callbacks = realloc (h->free_callbacks,
+ sizeof (struct free_callback) * new_alloc);
+ if (new_callbacks == NULL) {
+ set_error (errno, "realloc");
+ goto out;
+ }
+ h->alloc_free_callbacks = new_alloc;
+ h->free_callbacks = new_callbacks;
+ }
+
+ /* Need to keep the list sorted by pointer so we can use bsearch.
+ * Insert the new entry before the i'th entry.
+ *
+ * Note the same pointer can be added multiple times (eg. if a
+ * buffer is shared between commands), and that is not a bug. Which
+ * free function is called is undefined, but they should all be
+ * called eventually.
+ */
+ for (i = 0; i < h->nr_free_callbacks; ++i) {
+ if (ptr <= h->free_callbacks[i].ptr)
+ break;
+ }
+ memmove (&h->free_callbacks[i+1], &h->free_callbacks[i],
+ (h->nr_free_callbacks - i) * sizeof (struct free_callback));
+
+ h->free_callbacks[i].ptr = ptr;
+ h->free_callbacks[i].cb = cb;
+ h->free_callbacks[i].user_data = user_data;
+ h->nr_free_callbacks++;
+
+ ret = 0;
+ out:
+ pthread_mutex_unlock (&h->lock);
+ return ret;
+}
+
+static int
+compare_free_callbacks (const void *v1, const void *v2)
+{
+ const void *ptr = v1;
+ const struct free_callback *cb = v2;
+
+ if (ptr < cb->ptr) return -1;
+ else if (ptr > cb->ptr) return 1;
+ else return 0;
+}
+
+/* Called before the library frees 'ptr', where 'ptr' is something
+ * that might potentially be associated with a free callback. This is
+ * called often so must be fast.
+ */
+void
+nbd_internal_free_callback (struct nbd_handle *h, void *ptr)
+{
+ struct free_callback *free_cb;
+
+ if (ptr == NULL)
+ return;
+
+ free_cb = bsearch (ptr, h->free_callbacks, h->nr_free_callbacks,
+ sizeof (struct free_callback),
+ compare_free_callbacks);
+ if (free_cb) {
+ assert (ptr == free_cb->ptr);
+
+ free_cb->cb (ptr, free_cb->user_data);
+
+ /* Remove it from the free list. */
+ memmove (free_cb, free_cb+1,
+ sizeof (struct free_callback) *
+ (&h->free_callbacks[h->nr_free_callbacks] - (free_cb+1)));
+ h->nr_free_callbacks--;
+ }
+}
diff --git a/lib/handle.c b/lib/handle.c
index 054c8a7..ae0d196 100644
--- a/lib/handle.c
+++ b/lib/handle.c
@@ -126,6 +126,12 @@ nbd_close (struct nbd_handle *h)
free_cmd_list (h->cmds_to_issue);
free_cmd_list (h->cmds_in_flight);
free_cmd_list (h->cmds_done);
+
+ /* Any remaining free callbacks indicate an error. */
+ if (h->nr_free_callbacks != 0)
+ abort ();
+ free (h->free_callbacks);
+
nbd_internal_free_string_list (h->argv);
free (h->unixsocket);
free (h->hostname);
diff --git a/lib/internal.h b/lib/internal.h
index 301b798..d8b0eed 100644
--- a/lib/internal.h
+++ b/lib/internal.h
@@ -47,6 +47,7 @@
struct meta_context;
struct socket;
struct command;
+struct free_callback;
struct nbd_handle {
/* Unique name assigned to this handle for debug messages
@@ -87,6 +88,10 @@ struct nbd_handle {
nbd_debug_callback debug_callback;
void *debug_data;
+ /* Free callbacks kept in pointer order. */
+ struct free_callback *free_callbacks;
+ size_t nr_free_callbacks, alloc_free_callbacks;
+
/* State machine.
*
* The actual current state is ‘state’. ‘public_state’ is updated
@@ -220,6 +225,12 @@ struct nbd_handle {
bool disconnect_request; /* True if we've queued NBD_CMD_DISC */
};
+struct free_callback {
+ void *ptr;
+ nbd_free_callback cb;
+ void *user_data;
+};
+
struct meta_context {
struct meta_context *next; /* Linked list. */
char *name; /* Name of meta context. */
@@ -318,6 +329,9 @@ extern int nbd_internal_set_size_and_flags (struct nbd_handle *h,
uint64_t exportsize,
uint16_t eflags);
+/* free.c */
+extern void nbd_internal_free_callback (struct nbd_handle *h, void *ptr);
+
/* is-state.c */
extern bool nbd_internal_is_state_created (enum state state);
extern bool nbd_internal_is_state_connecting (enum state state);
--
2.22.0
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