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

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

23.02.2020 11:55, Markus Armbruster wrote:
> Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com> 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@virtuozzo.com>
>> ---
>>
>> CC: Eric Blake <eblake@redhat.com>
>> CC: Kevin Wolf <kwolf@redhat.com>
>> CC: Max Reitz <mreitz@redhat.com>
>> CC: Greg Kurz <groug@kaod.org>
>> CC: Stefano Stabellini <sstabellini@kernel.org>
>> CC: Anthony Perard <anthony.perard@citrix.com>
>> CC: Paul Durrant <paul@xen.org>
>> CC: Stefan Hajnoczi <stefanha@redhat.com>
>> CC: "Philippe Mathieu-Daudé" <philmd@redhat.com>
>> CC: Laszlo Ersek <lersek@redhat.com>
>> CC: Gerd Hoffmann <kraxel@redhat.com>
>> CC: Stefan Berger <stefanb@linux.ibm.com>
>> CC: Markus Armbruster <armbru@redhat.com>
>> CC: Michael Roth <mdroth@linux.vnet.ibm.com>
>> CC: qemu-block@nongnu.org
>> CC: xen-devel@lists.xenproject.org
>>
>>   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?
> 
>> +
>> +@@
>> +// 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);
> 
> 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.
> 
> 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.
> 


-- 
Best regards,
Vladimir