4:08 a.m.
On Mon, Sep 05, 2022 at 02:00:22PM +0200, Laszlo Ersek wrote:
On 09/03/22 00:14, Eric Blake wrote:
> The next patch will add some APIs that expose long-term statistics
> counters. A counter is always accessible (no need to return -1; if
> the handle is new the count is still 0, and even after the handle is
> in DEAD the counter is still useful). But it is also long-running;
> the existing RUInt might only be 32 bits, while it is easy to prove
> some counters (bytes transmitted, for example) will need 64-bits. So
> time to plumb in a new return type.
>
> Using signed int64 in OCaml bindings is okay (actually sending 2^63
> bytes of data to overflow the counter takes a LOOONG time, so we don't
> worry if we can't get to 2^64).
Should be OK for counters (the first use of the new data type in the
generator), could be a problem for other purposes. See e.g. commit
0e714a6e06e6 ("ocaml: map C's uint32_t to OCaml's int64", 2022-01-17);
mishandling the sign bit there caused infinite loops there.
This is not an argument against the patch / new return data type in
general, just a remark that in the future, once we try to use Uint64 for
other things (not just byte / chunk counters), we could be surprised.
Perhaps add a comment somewhere near the RUint64 data constructor in
"generator/OCaml.ml"?
... Well, at the same time, we already silently map UInt64 (not RUInt64)
to "int64", so there's already prior art for this. OK, feel free to
ignore this remark.
And I did have a comment about RUInt64 being a counter in API.mli.
...
> @@ -3368,10 +3369,12 @@ let () =
> failwithf "%s: if may_set_error is false, permitted_states must be
empty (any permitted state)"
> name
>
> - (* may_set_error = true is incompatible with RUInt, REnum, and RFlags
> + (* may_set_error = true is incompatible with RUInt*, REnum, and RFlags
> * because these calls cannot return an error indication.
> *)
> | name, { ret = RUInt; may_set_error = true }
> + | name, { ret = RUIntPtr; may_set_error = true }
Silent fix for a different omission?
Yes. I failed to call that out, but the commit is already in now.
Missed in commit 85798518.
> +++ b/generator/C.ml
> @@ -730,24 +732,25 @@ let
> pr " nbd_internal_printable_string (ret);\n"
> | RBool | RErr | RFd | RInt
> | RInt64 | RCookie | RSizeT
> - | RUInt | RUIntPtr | REnum _ | RFlags _ -> ()
> + | RUInt | RUIntPtr | RUInt64 | REnum _ | RFlags _ -> ()
> );
> pr " debug (h, \"leave: ret=\" ";
> (match ret with
> | RBool | RErr | RFd | RInt | REnum _ -> pr "\"%%d\",
ret"
> - | RInt64 | RCookie -> pr "\"%%\" PRIi64 \"\",
ret"
> + | RInt64 | RCookie -> pr "\"%%\" PRIi64, ret"
another silent fix for an earlier problem?
> | RSizeT -> pr "\"%%zd\", ret"
> | RStaticString | RString ->
> pr "\"%%s\", ret_printable ? ret_printable :
\"\""
> | RUInt | RFlags _ -> pr "\"%%u\", ret"
> | RUIntPtr -> pr "\"%%\" PRIuPTR, ret"
> + | RUInt64 -> pr "\"%%\" PRIu64, ret"
Here, it was more of a consistency issue. Inserting a bare "" during
string constant concatenation looks funny; we already omitted it in
RUIntPtr (again, 85798518), and I liked that style better for RUInt64,
so I copied it to RInt64 and RCookie.
We could further compress things to have:
pr " debug (h, \"leave: ret=\"";
(match ret with
| RBool -> pr "%%d\", ret"
and so on, to get rid of yet another spurious pair of "" in the
generated C code (since the return format is always exacty one
argument, there's no need to have the generated api.c have
"... ret=" "%<>" when "... ret=%<>" would do).
I can touch that
up in a followup, if it makes sense.
> );
> pr ");\n";
> (match ret with
> | RStaticString | RString -> pr " free (ret_printable);\n"
> | RBool | RErr | RFd | RInt
> | RInt64 | RCookie | RSizeT
> - | RUInt | REnum _ | RFlags _ | RUIntPtr -> ()
> + | RUInt | RUIntPtr | RUInt64 | REnum _ | RFlags _ -> ()
looks like there were multiple RUintPtr omissions, those could go into a
unified (but from this, separate) patch.
Actually, this line rearranged RUIntPtr to be earlier in the line (we
don't have a strong precedent on whether branches to the match should
be in alphabetical, declaration, or abitrary order).
> );
> pr " }\n";
> pr " }\n"
> @@ -904,6 +907,8 @@ let
> pr "This call returns a bitmask.\n";
> | RUIntPtr ->
> pr "This call returns a C<uintptr_t>.\n";
> + | RUInt64 ->
> + pr "This call returns a counter.\n";
Hmmm. On one hand, I like that here we do tie the new type to counters.
On the other hand, the explanation doesn't match the generic name
"RUint64". Not sure what to suggest :/
If we later find ourselves needing an actual 64-bit unsigned value,
unrelated to counters, we can add RCounter at that point and
repurpose RUInt64 to the new purpose. We've done that sort of enum
splitting before; the enum names are less important that the
underlying abstract type that then has to be ported to all the
language bindings for the semantics it will be representing.
> +++ b/generator/Python.ml
> @@ -525,7 +525,7 @@ let
> | RErr ->
> pr " py_ret = Py_None;\n";
> pr " Py_INCREF (py_ret);\n"
> - | RFd | RInt | REnum _ | RFlags _ | RSizeT | RUInt | RUIntPtr ->
> + | RFd | RInt | REnum _ | RFlags _ | RSizeT | RUInt | RUIntPtr | RUInt64 ->
> pr " py_ret = PyLong_FromLong (ret);\n"
> | RInt64 | RCookie ->
> pr " py_ret = PyLong_FromLongLong (ret);\n"
>
This does not look right; I think RUInt64 belongs with RInt64 and
RCookie (so that we invoke PyLong_FromLongLong()).
D'oh. Actual bug; now fixed as followup commit f79a1ac1
Thanks for reviewing.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org