Re: [Xen-devel] [PATCH v7 03/11] scripts: add coccinelle script to use auto propagated errp

[Markus Armbruster <armbru@xxxxxxxxxx>]

Vladimir Sementsov-Ogievskiy <vsementsov@xxxxxxxxxxxxx> writes:

> 23.02.2020 11:55, Markus Armbruster wrote:
>> Vladimir Sementsov-Ogievskiy <vsementsov@xxxxxxxxxxxxx> writes:
>>
>>> Script adds ERRP_AUTO_PROPAGATE macro invocation where appropriate and
>>> does corresponding changes in code (look for details in
>>> include/qapi/error.h)
>>>
>>> Usage example:
>>> spatch --sp-file scripts/coccinelle/auto-propagated-errp.cocci \
>>>   --macro-file scripts/cocci-macro-file.h --in-place --no-show-diff \
>>>   blockdev-nbd.c qemu-nbd.c {block/nbd*,nbd/*,include/block/nbd*}.[hc]
>>>
>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@xxxxxxxxxxxxx>
>>> ---
>>>
>>> CC: Eric Blake <eblake@xxxxxxxxxx>
>>> CC: Kevin Wolf <kwolf@xxxxxxxxxx>
>>> CC: Max Reitz <mreitz@xxxxxxxxxx>
>>> CC: Greg Kurz <groug@xxxxxxxx>
>>> CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
>>> CC: Anthony Perard <anthony.perard@xxxxxxxxxx>
>>> CC: Paul Durrant <paul@xxxxxxx>
>>> CC: Stefan Hajnoczi <stefanha@xxxxxxxxxx>
>>> CC: "Philippe Mathieu-Daudé" <philmd@xxxxxxxxxx>
>>> CC: Laszlo Ersek <lersek@xxxxxxxxxx>
>>> CC: Gerd Hoffmann <kraxel@xxxxxxxxxx>
>>> CC: Stefan Berger <stefanb@xxxxxxxxxxxxx>
>>> CC: Markus Armbruster <armbru@xxxxxxxxxx>
>>> CC: Michael Roth <mdroth@xxxxxxxxxxxxxxxxxx>
>>> CC: qemu-block@xxxxxxxxxx
>>> CC: xen-devel@xxxxxxxxxxxxxxxxxxxx
>>>
>>>   include/qapi/error.h                          |   3 +
>>>   scripts/coccinelle/auto-propagated-errp.cocci | 158 ++++++++++++++++++
>>>   2 files changed, 161 insertions(+)
>>>   create mode 100644 scripts/coccinelle/auto-propagated-errp.cocci
>>>
>>> diff --git a/include/qapi/error.h b/include/qapi/error.h
>>> index b9452d4806..79f8e95214 100644
>>> --- a/include/qapi/error.h
>>> +++ b/include/qapi/error.h
>>> @@ -141,6 +141,9 @@
>>>    *         ...
>>>    *     }
>>>    *
>>> + * For mass conversion use script
>>> + *   scripts/coccinelle/auto-propagated-errp.cocci
>>> + *
>>>    *
>>>    * Receive and accumulate multiple errors (first one wins):
>>>    *     Error *err = NULL, *local_err = NULL;
>>
>> Extra blank line.
>>
>>> diff --git a/scripts/coccinelle/auto-propagated-errp.cocci 
>>> b/scripts/coccinelle/auto-propagated-errp.cocci
>>> new file mode 100644
>>> index 0000000000..fb03c871cb
>>> --- /dev/null
>>> +++ b/scripts/coccinelle/auto-propagated-errp.cocci
>>> @@ -0,0 +1,158 @@
>>> +// Use ERRP_AUTO_PROPAGATE (see include/qapi/error.h)
>>> +//
>>> +// Copyright (c) 2020 Virtuozzo International GmbH.
>>> +//
>>> +// This program is free software; you can redistribute it and/or modify
>>> +// it under the terms of the GNU General Public License as published by
>>> +// the Free Software Foundation; either version 2 of the License, or
>>> +// (at your option) any later version.
>>> +//
>>> +// This program 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 General Public License for more details.
>>> +//
>>> +// You should have received a copy of the GNU General Public License
>>> +// along with this program.  If not, see <http://www.gnu.org/licenses/>.
>>> +//
>>> +// Usage example:
>>> +// spatch --sp-file scripts/coccinelle/auto-propagated-errp.cocci \
>>> +//  --macro-file scripts/cocci-macro-file.h --in-place --no-show-diff \
>>> +//  blockdev-nbd.c qemu-nbd.c {block/nbd*,nbd/*,include/block/nbd*}.[hc]
>>> +
>>> +@rule0@
>>> +// Add invocation to errp-functions where necessary
>>> +// We should skip functions with "Error *const *errp"
>>> +// parameter, but how to do it with coccinelle?
>>> +// I don't know, so, I skip them by function name regex.
>>> +// It's safe: if we did not skip some functions with
>>> +// "Error *const *errp", ERRP_AUTO_PROPAGATE invocation
>>> +// will fail to compile, because of const violation.
>>
>> Not skipping a function we should skip fails to compile.
>>
>> What about skipping a function we should not skip?
>>
>>> +identifier fn !~ "error_append_.*_hint";
>>> +identifier local_err, ERRP;
>>
>> A few of our coccinelle scripts use ALL_CAPS for meta-variables.  Most
>> don't.  Either is fine with me.  Mixing the two styles feels a bit
>> confusing, though.
>>
>>> +@@
>>> +
>>> + fn(..., Error **ERRP, ...)
>>> + {
>>> ++   ERRP_AUTO_PROPAGATE();
>>> +    <+...
>>> +        when != ERRP_AUTO_PROPAGATE();
>>> +(
>>> +    error_append_hint(ERRP, ...);
>>> +|
>>> +    error_prepend(ERRP, ...);
>>> +|
>>> +    Error *local_err = NULL;
>>> +)
>>> +    ...+>
>>> + }
>>
>> Misses error_vprepend().  Currently harmless, but as long as we commit
>> the script, we better make it as robust as we reasonably can.
>>
>> The previous patch explains this Coccinelle script's intent:
>>
>>    To achieve these goals, later patches will add invocations
>>    of this macro at the start of functions with either use
>>    error_prepend/error_append_hint (solving 1) or which use
>>    local_err+error_propagate to check errors, switching those
>>    functions to use *errp instead (solving 2 and 3).
>>
>> This rule matches "use error_prepend/error_append_hint" directly.  It
>> appears to use presence of a local Error * variable as proxy for "use
>> local_err+error_propagate to check errors".  Hmm.
>>
>> We obviously have such a variable when we use "local_err+error_propagate
>> to check errors".  But we could also have such variables without use of
>> error_propagate().  In fact, error.h documents such use:
>>
>>   * Call a function and receive an error from it:
>>   *     Error *err = NULL;
>>   *     foo(arg, &err);
>>   *     if (err) {
>>   *         handle the error...
>>   *     }
>>
>> where "handle the error" frees it.
>>
>> I figure such uses typically occur in functions without an Error **errp
>> parameter.  This rule doesn't apply then.  But they could occur even in
>> functions with such a parameter.  Consider:
>>
>>      void foo(Error **errp)
>>      {
>>          Error *err = NULL;
>>
>>          bar(&err);
>>          if (err) {
>>              error_free(err);
>>              error_setg(errp, "completely different error");
>>          }
>>      }
>>
>> Reasonable enough when bar() gives us an error that's misleading in this
>> context, isn't it?
>>
>> The script transforms it like this:
>>
>>      void foo(Error **errp)
>>      {
>>     -    Error *err = NULL;
>>     +    ERRP_AUTO_PROPAGATE();
>>
>>     -    bar(&err);
>>     -    if (err) {
>>     -        error_free(err);
>>     +    bar(errp);
>>     +    if (*errp) {
>>     +        error_free_errp(errp);
>>              error_setg(errp, "completely different error");
>>          }
>>      }
>>
>> Unwanted.
>>
>> Now, if this script applied in just a few dozen places, we could rely on
>> eyeballing its output to catch unwanted transformations.  Since it
>> applies in so many more, I don't feel comfortable relying on reviewer
>> eyeballs.
>>
>> Can we make rule0 directly match error_propagate(errp, local_err)
>> somehow?
>>
>> Another observation: the rule does not match error_reportf_err() and
>> warn_reportf_err().
>
> They are unrelated, as they take Error* argument, not Error**
>
>> These combine error_prepend(),
>> error_report()/warn_report() and error_free(), for convenience.  Don't
>> their users need ERRP_AUTO_PROPAGATE() just like error_prepend()'s
>> users?

Right, we never pass *errp to error_reportf_err() and
warn_reportf_err(), so there's no need to wrap it.

But see below.

>>
>>> +
>>> +@@
>>> +// Switch unusual (Error **) parameter names to errp
>>> +// (this is necessary to use ERRP_AUTO_PROPAGATE).
>>
>> Please put your rule comments right before the rule, i.e. before the
>> @-line introducing metavariable declarations, not after.  Same
>> elsewhere.
>>
>>> +identifier rule0.fn;
>>> +identifier rule0.ERRP != errp;
>>> +@@
>>> +
>>> + fn(...,
>>> +-   Error **ERRP
>>> ++   Error **errp
>>> +    ,...)
>>> + {
>>> +     <...
>>> +-    ERRP
>>> ++    errp
>>> +     ...>
>>> + }
>>
>> This normalizes errp parameter naming.  It matches exactly when rule0
>> matches (and inserts ERRP_AUTO_PROPAGATE()) and the Error ** parameter
>> is unusual.  Good.
>>
>>> +
>>> +@rule1@
>>> +// We want to patch error propagation in functions regardless of
>>> +// whether the function already uses ERRP_AUTO_PROPAGATE prior to
>>> +// applying rule0, hence this one does not inherit from it.
>>
>> I'm not sure I get this comment.  Let's see what the rule does.
>>
>>> +identifier fn !~ "error_append_.*_hint";
>>> +identifier local_err;
>>> +symbol errp;
>>> +@@
>>> +
>>> + fn(..., Error **errp, ...)
>>> + {
>>> +     <...
>>> +-    Error *local_err = NULL;
>>> +     ...>
>>> + }
>>
>> rule1 matches like rule0, except the Error ** parameter match is
>> tightened from any C identifier to the C identifier errp, and the
>> function body match tightened from "either use
>> error_prepend/error_append_hint or which use local_err+error_propagate
>> to check errors" to just the latter.
>>
>> I figure tightening the Error ** parameter match has no effect, because
>> we already normalized the parameter name.
>>
>> So rule1 deletes variable local_err where rule0 applied.  Correct?
>>
>>> +
>>> +@@
>>> +// Handle pattern with goto, otherwise we'll finish up
>>> +// with labels at function end which will not compile.
>>> +identifier rule1.fn, rule1.local_err;
>>> +identifier OUT;
>>> +@@
>>> +
>>> + fn(...)
>>> + {
>>> +     <...
>>> +-    goto OUT;
>>> ++    return;
>>> +     ...>
>>> +- OUT:
>>> +-    error_propagate(errp, local_err);
>>> + }
>>
>> This is one special case of error_propagate() deletion.  It additionally
>> gets rid of a goto we no longer want.  For the general case, see below.
>>
>> The rule applies only where rule1 just deleted the variable.  Thus, the
>> two rules work in tandem.  Makes sense.
>>
>>> +
>>> +@@
>>> +identifier rule1.fn, rule1.local_err;
>>
>> This rule also works in tandem with rule1.
>>
>>> +expression list args; // to reindent error_propagate_prepend
>>
>> What is the comment trying to tell me?
>>
>>> +@@
>>> +
>>> + fn(...)
>>> + {
>>> +     <...
>>> +(
>>> +-    error_free(local_err);
>>> +-    local_err = NULL;
>>> ++    error_free_errp(errp);
>>
>> Reminder:
>>
>>      static inline void error_free_errp(Error **errp)
>>      {
>>          assert(errp && *errp);
>>          error_free(*errp);
>>          *errp = NULL;
>>      }
>>
>> Now let's examine the actual change.
>>
>> The assertion's first half trivially holds, ERRP_AUTO_PROPAGATE()
>> ensures it.
>>
>> The second half is new.  We now crash when we haven't set an error.  Why
>> is this safe?  Note that error_free(local_err) does nothing when
>> !local_err.
>>
>> The zapping of the variable pointing to the Error just freed is
>> unchanged.
>>
>>> +|
>>> +-    error_free(local_err);
>>> ++    error_free_errp(errp);
>>
>> Here, the zapping is new.  Zapping dangling pointers is obviously safe.
>> Needed, or else the automatic error_propagate() due to
>> ERRP_AUTO_PROPAGATE() would propagate the dangling pointer.
>>
>>> +|
>>> +-    error_report_err(local_err);
>>> ++    error_report_errp(errp);

error_reportf_err() is just like error_report_err(), except it
additionally modifies the error message.  Does it need a similar
transformation?

>> The only difference to the previous case is that we also report the
>> error.
>>
>> The previous case has a buddy that additionally matches *errp = NULL.
>> Why not this one?
>>
>>> +|
>>> +-    warn_report_err(local_err);
>>> ++    warn_report_errp(errp);

Likewise.

>> Likewise.
>>
>> What about error_reportf_err(), warn_reportf_err()?
>>
>> Up to here, this rule transforms the various forms of error_free().
>> Next: error_propagate().
>>
>>> +|
>>> +-    error_propagate_prepend(errp, local_err, args);
>>> ++    error_prepend(errp, args);
>>> +|
>>> +-    error_propagate(errp, local_err);
>>
>> rule0's adding of ERRP_AUTO_PROPAGATE() made error_propagate()
>> redundant.
>>
>> This is the general case of error_propagate() deletion.
>>
>> I'd put the plain error_propagate() first, variations second, like you
>> do with error_free().
>>
>> If neither of these two patterns match on a path from
>> ERRP_AUTO_PROPAGATE() to return, we effectively insert error_propagate()
>> where it wasn't before.  Does nothing when the local error is null
>> there.  Bug fix when it isn't: it's at least a memory leak, and quite
>> possibly worse.
>>
>> Identifying these bug fixes would be nice, but I don't have practical
>> ideas on how to do that.
>>
>> Can we explain this in the commit message?
>>
>>> +)
>>> +     ...>
>>> + }
>>> +
>>> +@@
>>> +identifier rule1.fn, rule1.local_err;
>>> +@@
>>> +
>>> + fn(...)
>>> + {
>>> +     <...
>>> +(
>>> +-    &local_err
>>> ++    errp
>>> +|
>>> +-    local_err
>>> ++    *errp
>>> +)
>>> +     ...>
>>> + }
>>
>> Also in tandem with rule1, fixes up uses of local_err.  Good.
>>
>>> +
>>> +@@
>>> +identifier rule1.fn;
>>> +@@
>>> +
>>> + fn(...)
>>> + {
>>> +     <...
>>> +- *errp != NULL
>>> ++ *errp
>>> +     ...>
>>> + }
>>
>> Still in tandem with rule1, normalizes style.  Good.
>>


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