qemu/docs
Eric Blake d9f62dde13 qapi: Simplify semantics of visit_next_list()
The semantics of the list visit are somewhat baroque, with the
following pseudocode when FooList is used:

start()
for (prev = head; cur = next(prev); prev = &cur) {
    visit(&cur->value)
}

Note that these semantics (advance before visit) requires that
the first call to next() return the list head, while all other
calls return the next element of the list; that is, every visitor
implementation is required to track extra state to decide whether
to return the input as-is, or to advance.  It also requires an
argument of 'GenericList **' to next(), solely because the first
iteration might need to modify the caller's GenericList head, so
that all other calls have to do a layer of dereferencing.

Thankfully, we only have two uses of list visits in the entire
code base: one in spapr_drc (which completely avoids
visit_next_list(), feeding in integers from a different source
than uint8List), and one in qapi-visit.py.  That is, all other
list visitors are generated in qapi-visit.c, and share the same
paradigm based on a qapi FooList type, so we can refactor how
lists are laid out with minimal churn among clients.

We can greatly simplify things by hoisting the special case
into the start() routine, and flipping the order in the loop
to visit before advance:

start(head)
for (tail = *head; tail; tail = next(tail)) {
    visit(&tail->value)
}

With the simpler semantics, visitors have less state to track,
the argument to next() is reduced to 'GenericList *', and it
also becomes obvious whether an input visitor is allocating a
FooList during visit_start_list() (rather than the old way of
not knowing if an allocation happened until the first
visit_next_list()).  As a minor drawback, we now allocate in
two functions instead of one, and have to pass the size to
both functions (unless we were to tweak the input visitors to
cache the size to start_list for reuse during next_list, but
that defeats the goal of less visitor state).

The signature of visit_start_list() is chosen to match
visit_start_struct(), with the new parameters after 'name'.

The spapr_drc case is a virtual visit, done by passing NULL for
list, similarly to how NULL is passed to visit_start_struct()
when a qapi type is not used in those visits.  It was easy to
provide these semantics for qmp-output and dealloc visitors,
and a bit harder for qmp-input (several prerequisite patches
refactored things to make this patch straightforward).  But it
turned out that the string and opts visitors munge enough other
state during visit_next_list() to make it easier to just
document and require a GenericList visit for now; an assertion
will remind us to adjust things if we need the semantics in the
future.

Several pre-requisite cleanup patches made the reshuffling of
the various visitors easier; particularly the qmp input visitor.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1461879932-9020-24-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
2016-05-12 09:47:55 +02:00
..
specs fw_cfg: Adopt /opt/RFQDN convention 2016-04-19 16:09:50 +02:00
aio_notify_accept.promela AioContext: optimize clearing the EventNotifier 2015-07-22 12:41:40 +01:00
aio_notify_bug.promela AioContext: fix broken placement of event_notifier_test_and_clear 2015-07-22 12:41:40 +01:00
aio_notify.promela AioContext: fix broken ctx->dispatching optimization 2015-07-22 12:41:40 +01:00
atomics.txt docs/atomics.txt: fix two typos 2015-04-30 16:05:48 +03:00
bitmaps.md docs: update bitmaps.md 2015-11-17 18:35:56 +08:00
blkdebug.txt blkdebug: Merge hand-rolled and qapi BlkdebugEvent enum 2015-12-17 08:21:27 +01:00
blkverify.txt
bootindex.txt Typo, spelling and grammatical fixes 2013-04-12 14:33:20 +02:00
build-system.txt docs: describe the QEMU build system structure / design 2015-09-25 12:20:18 +02:00
ccid.txt libcacard: improve documentation 2014-06-10 07:44:01 +02:00
ich9-ehci-uhci.cfg usb: update documentation 2011-07-08 11:25:56 +02:00
image-fuzzer.txt docs: List all image elements currently supported by the fuzzer 2014-09-22 11:39:35 +01:00
live-block-ops.txt docs: describe live block operations 2012-01-26 14:49:18 +01:00
memory-hotplug.txt docs: update documentation for memory hot unplug 2015-04-27 21:07:38 +02:00
memory.txt docs: fix typo in memory.txt 2016-04-08 00:07:43 +02:00
migration.txt postcopy: Remove the x- 2016-03-11 17:53:59 +05:30
multi-thread-compression.txt docs: Add a doc about multiple thread compression 2015-05-06 17:36:57 +02:00
multiple-iothreads.txt docs/multiple-iothreads.txt: add documentation on IOThread programming 2014-08-15 15:07:13 +02:00
multiseat.txt maint: remove / fix many doubled words 2015-09-11 10:21:38 +03:00
pci_expander_bridge.txt pxb: cleanup 2016-03-11 16:59:12 +02:00
q35-chipset.cfg q35: Add PCIe switch to example q35 configuration 2013-08-21 12:39:15 +03:00
qapi-code-gen.txt qapi: Simplify semantics of visit_next_list() 2016-05-12 09:47:55 +02:00
qcow2-cache.txt docs: document how to configure the qcow2 L2/refcount caches 2015-09-04 21:00:32 +02:00
qdev-device-use.txt docs: Fix qdev-device-use.txt typo in -chardev serial, path=COM<NUM> 2011-09-09 09:43:17 +01:00
qemupciserial.inf Docs: Introduce multiport serial support in qemupciserial.inf. 2014-03-12 10:33:23 +01:00
qmp-events.txt qmp event: Refactor QUORUM_REPORT_BAD 2016-03-14 16:46:43 +01:00
qmp-intro.txt docs: Move files from docs/qmp/ to docs/ 2015-10-12 18:44:53 +02:00
qmp-spec.txt qapi: Update docs to match recent generator changes 2016-03-05 10:41:16 +01:00
rcu.txt doc/rcu: fix g_free_rcu() usage example 2015-10-19 10:13:07 +02:00
rdma.txt rdma: Fix incorrect description in comments 2014-09-20 17:55:53 +04:00
replay.txt replay: introduce block devices record/replay 2016-03-30 12:15:57 +02:00
spice-port-fqdn.txt docs: add spice-port-fqdn.txt 2012-12-17 14:01:41 +01:00
throttle.txt docs: Document the throttling infrastructure 2016-02-22 14:08:07 +01:00
tracing.txt docs: Update documentation for stderr (now log) tracing backend. 2016-03-31 09:48:59 +01:00
usb2.txt usb: update docs for bus name change 2013-04-03 11:39:43 +02:00
usb-storage.txt Fix typos and misspellings 2013-03-22 13:25:07 +01:00
virtio-balloon-stats.txt virtio-balloon: return empty data when no stats are available 2014-05-25 12:46:58 +03:00
virtio-migration.txt virtio: add some migration doc 2015-10-22 14:34:48 +03:00
vnc-ledstate-Pseudo-encoding.txt doc: document the Pseudo-encoding of LED state 2013-04-29 08:28:51 -05:00
win32-qemu-event.promela qemu-thread: add a fast path to the Win32 QemuEvent 2015-09-24 20:52:28 +02:00
writing-qmp-commands.txt error: More error_setg() usage 2015-11-11 18:56:26 +01:00
xbzrle.txt xbzrle: optimize XBZRLE to decrease the cache misses 2015-01-15 17:49:43 +05:30
xen-save-devices-state.txt Introduce "xen-save-devices-state" 2012-03-19 18:21:00 +00:00