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().
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.
