[Libguestfs] [libnbd PATCH v2 3/5] docs: Clarify how callbacks should handle errors
Eric Blake
eblake at redhat.com
Thu Feb 3 20:25:56 UTC 2022
Recent patches have demonstrated confusion on the order in which
callbacks are reached, when it is safe or dangerous to ignore *error,
and what a completion callback should do when auto-retirement is in
use. Add wording to make it more obvious that:
- callbacks are reached in the following order: mid-command callback
(0, 1, or many times, if supplied), completion callback (exactly
once, if supplied), mid-command free (exactly once, if supplied),
completion free (exactly once, if supplied)
- returning -1 from a mid-command callback does does not prevent
future callbacks
- ignoring *error in a mid-command callback is safe
- completion callbacks are reached unconditionally, and must NOT ignore
*error
- if the user chooses to use auto-retirement instead of manual calls to
nbd_aio_command_completed, the completion callback should return 1 even
on error cases to avoid complicating command cleanup
- the contents of buf after nbd_pread and friends is undefined on
error (at present, if the user did not pre-initialize the buffer,
there are some code paths in libnbd that leave it uninitialized)
---
docs/libnbd.pod | 69 +++++++++++++++++++++++++++++++++++++-----------
generator/API.ml | 24 ++++++++++++-----
2 files changed, 71 insertions(+), 22 deletions(-)
diff --git a/docs/libnbd.pod b/docs/libnbd.pod
index eb8038b0..15bdf0a8 100644
--- a/docs/libnbd.pod
+++ b/docs/libnbd.pod
@@ -829,8 +829,12 @@ This can be used to free associated C<user_data>. For example:
NBD_NULL_COMPLETION,
0);
-will call L<free(3)> on C<my_data> after the last time that the
-S<C<chunk.callback = my_fn>> function is called.
+will call L<free(3)> once on C<my_data> after the point where it is
+known that the S<C<chunk.callback = my_fn>> function can no longer be
+called, regardless of how many times C<my_fn> was actually called. If
+both a mid-command and completion callback are supplied, the functions
+will be reached in this order: mid-function callbacks, completion
+callback, mid-function free, and finally completion free.
The free function is only accessible in the C API as it is not needed
in garbage collected programming languages.
@@ -858,27 +862,60 @@ same nbd object, as it would cause deadlock.
=head2 Completion callbacks
-All of the low-level commands have a completion callback variant that
-registers a callback function used right before the command is marked
-complete.
+All of the asychronous commands have an optional completion callback
+function that is used right before the command is marked complete,
+after any mid-command callbacks have finished, and before any free
+functions.
When the completion callback returns C<1>, the command is
automatically retired (there is no need to call
-L<nbd_aio_command_completed(3)>); for any other return value, the command
-still needs to be retired.
+L<nbd_aio_command_completed(3)>); for any other return value, the
+command still needs to be manually retired (otherwise, the command
+will tie up resources until L<nbd_close(3)> is eventually reached).
=head2 Callbacks with C<int *error> parameter
Some of the high-level commands (L<nbd_pread_structured(3)>,
-L<nbd_block_status(3)>) involve the use of a callback function invoked by
-the state machine at appropriate points in the server's reply before
-the overall command is complete. These callback functions, along with
-all of the completion callbacks, include a parameter C<error>
-containing the value of any error detected so far; if the callback
-function fails, it should assign back into C<error> and return C<-1>
-to change the resulting error of the overall command. Assignments
-into C<error> are ignored for any other return value; similarly,
-assigning C<0> into C<error> does not have an effect.
+L<nbd_block_status(3)>) involve the use of a callback function invoked
+by the state machine at appropriate points in the server's reply
+before the overall command is complete. These callback functions,
+along with all of the completion callbacks, include a parameter
+C<error> which is a pointer containing the value of any error detected
+so far. If a callback function fails and wants to change the
+resulting error of the overall command visible later in the API
+sequence, it should assign back into C<error> and return C<-1> in the
+C API. Assignments into C<error> are ignored for any other return
+value; similarly, assigning C<0> into C<error> does not have an
+effect. In other language bindings, reporting callback errors is
+generally done by raising an exception rather than by return value.
+
+Note that a mid-command callback might never be reached, such as if
+libnbd detects that the command was invalid to send (see
+L<nbd_set_strict_mode(3)>) or if the server reports a failure that
+concludes the command. It is safe for a mid-command callback to
+ignore non-zero C<error>: all the other parameters to the mid-command
+callback will still be valid (corresponding to the current portion of
+the server's reply), and the overall command will still fail (at the
+completion callback or L<nbd_aio_command_completed(3)> for an
+asynchronous command, or as the result of the overall synchronous
+command). Returing C<-1> from a mid-command callback does not prevent
+that callback from being reached again, if the server sends more
+mid-command replies that warrant another use of that callback. A
+mid-command callback may be reached more times than expected if the
+server is non-compliant.
+
+On the other hand, if a completion callback is supplied (only possible
+with asynchronous commands), it will always be reached exactly once,
+and the completion callback must not ignore the value pointed to by
+C<error>. In particular, the content of a buffer passed to
+L<nbd_aio_pread(3)> or L<nbd_aio_pread_structured(3)> is undefined
+if C<*error> is non-zero on entry to the completion callback. It is
+recommended that if you choose to use automatic command retirement
+(where the completion callback returns C<1> to avoid needing to call
+L<nbd_aio_command_completed(3)> later), your completion function
+should return C<1> on all control paths, even when handling errors
+(note that with automatic retirement, assigning into C<error> is
+pointless as there is no later API to see that value).
=head1 COMPILING YOUR PROGRAM
diff --git a/generator/API.ml b/generator/API.ml
index cf2e7543..012016bc 100644
--- a/generator/API.ml
+++ b/generator/API.ml
@@ -1,6 +1,6 @@
(* hey emacs, this is OCaml code: -*- tuareg -*- *)
(* nbd client library in userspace: the API
- * Copyright (C) 2013-2021 Red Hat Inc.
+ * Copyright (C) 2013-2022 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
@@ -1829,7 +1829,9 @@ "pread", {
L<nbd_get_block_size(3)>.
The C<flags> parameter must be C<0> for now (it exists for future NBD
-protocol extensions)."
+protocol extensions).
+
+Note that if this command fails, the contents of C<buf> are undefined."
^ strict_call_description;
see_also = [Link "aio_pread"; Link "pread_structured";
Link "get_block_size"; Link "set_strict_mode"];
@@ -1914,7 +1916,9 @@ "pread_structured", {
C<LIBNBD_CMD_FLAG_DF> meaning that the server should not reply with
more than one fragment (if that is supported - some servers cannot do
this, see L<nbd_can_df(3)>). Libnbd does not validate that the server
-actually obeys the flag."
+actually obeys the flag.
+
+Note that if this command fails, the contents of C<buf> are undefined."
^ strict_call_description;
see_also = [Link "can_df"; Link "pread";
Link "aio_pread_structured"; Link "get_block_size";
@@ -2155,7 +2159,8 @@ "block_status", {
for contexts known to libnbd, B<E<lt>libnbd.hE<gt>> contains constants
beginning with C<LIBNBD_STATE_> that may help decipher the values.
On entry to the callback, the C<error> parameter contains the errno
-value of any previously detected error.
+value of any previously detected error, but even if an earlier error
+was detected, the current C<metacontext> and C<entries> are valid.
It is possible for the extent function to be called
more times than you expect (if the server is buggy),
@@ -2454,7 +2459,10 @@ "aio_pread", {
as described in L<libnbd(3)/Completion callbacks>.
Note that you must ensure C<buf> is valid until the command has
-completed. Other parameters behave as documented in L<nbd_pread(3)>."
+completed. Furthermore, the contents of C<buf> are undefined if the
+C<error> parameter to C<completion_callback> is set, or if
+L<nbd_aio_command_completed(3)> reports failure. Other parameters behave
+as documented in L<nbd_pread(3)>."
^ strict_call_description;
example = Some "examples/aio-connect-read.c";
see_also = [SectionLink "Issuing asynchronous commands";
@@ -2478,7 +2486,11 @@ "aio_pread_structured", {
Or supply the optional C<completion_callback> which will be invoked
as described in L<libnbd(3)/Completion callbacks>.
-Other parameters behave as documented in L<nbd_pread_structured(3)>."
+Note that you must ensure C<buf> is valid until the command has
+completed. Furthermore, the contents of C<buf> are undefined if the
+C<error> parameter to C<completion_callback> is set, or if
+L<nbd_aio_command_completed(3)> reports failure. Other parameters behave
+as documented in L<nbd_pread_structured(3)>."
^ strict_call_description;
see_also = [SectionLink "Issuing asynchronous commands";
Link "aio_pread"; Link "pread_structured";
--
2.34.1
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