[Lldb-commits] [PATCH] D37651: Fix for bug 34532 - A few rough corners related to post-mortem debugging (core/minidump)

Zachary Turner via lldb-commits lldb-commits at lists.llvm.org
Sat Sep 9 11:36:32 PDT 2017

On Sat, Sep 9, 2017 at 11:18 AM Jim Ingham <jingham at apple.com> wrote:

> On Sep 8, 2017, at 11:45 PM, Zachary Turner <zturner at google.com> wrote:
> On Fri, Sep 8, 2017 at 8:19 PM Jason Molenda <jmolenda at apple.com> wrote:
>> Also keep in mind that debug sessions have a tendency to be long lived.
>> I may be working through a problem for half an hour -- or this may be the
>> one rare instance where a bug reproduces -- and crashing because some bogus
>> piece of debug info that I don't care about is invalid is not acceptable.
>> I've gotten those bug reports, where someone has spent all day getting to
>> just the point where they have the problem in front of them, only to have
>> the debugger crash, and they are rightfully enraged at the debugger for
>> this.
>> An interactive tool like the debugger cannot use a model where asserting
>> is acceptable.  Even if lldb is out-of-process, you're losing all of the
>> work you'd done until then, and complex bugs can involve extraordinarily
>> long running debug sessions.  The fact that llvm/clang can assert out from
>> under us is to the detriment of lldb's quality.
>> J
> Fwiw i think we all agree that it shouldn't crash.  What we disagree on is
> the best way of making it crash less.  It's easy to see why someone would
> argue that adding an assert increases the incidence of crashes.  I mean on
> the surface it seems ridiculous to argue otherwise.
> But otherwise is precisely what we all continue to argue.
> Your position is that by going out of your way to avoid crashes, it will
> crash less and be more stable.  Pretty intuitive position if I'm being
> honest.  But I don't agree.
> I think you are trading short term metrics for long term technical debt
> You fix one crash by adding a null check, without figuring out why null
> ever got in there in the first place.
> You get fewer bug reports because instead of crashing, which would
> hopefully trigger some automatic crash uploader in Xcode that automatically
> files a rdar, you just never find out about the bug in the first place.
> You introduce even more bugs, because when you go to edit a function, its
> complexity and branching is through the roof, and you overlook something
> some corner case.
> You have less test coverage because you introduce another untested branch
> in the code, reducing the already abysmal code coverage even further.
> The way to reduce crashing is to *increase* the code coverage of your test
> suite.  That is the solution.  And you don't do that by adding null checks
> everywhere, you do it by removing them and asserting.
> Yes, it means Xcode might crash.  But you know the bug exists!  How many
> bugs are out there right now that nobody even knows about because they are
> hidden by a null pointer check and instead of the user trying to figure out
> what's going and filing a bug report, they just give up and use gdb
> instead.  That's one more person you're never going to get back.
> Crashing Xcode is annoying, but it's fixable.  But when your technical
> debt is going up and to the right, and your test coverage is going down and
> to the right, that's an existential threat to the project .
> Btw, i still don't understand why asserts cause anything to crash, given
> that they're supposed to be turned off in a release build
>  I was objecting to the use of llvm_report_fatal_error.  That crashes no
> matter what.  And I was suggesting replacing that with an lldb_assert.  So
> I’m not sure we are so much in disagreement about that.
You're right that crashes no matter what, but when an lldb_assert fires,
you only find out about it if someone is kind enough to submit a bug
report.  I'm assuming that if it were to crash you would find out about it
with an automatically filed rdar with an attached core file.  (I might be
wrong about this)

> I was also objecting to the approach where instead of figuring out why the
> errors from one call - DoResmume - were not getting handled properly - we
> add ANOTHER earlier check - CanResume - and then we have to deal with what
> happens when there’s a code path that doesn’t get caught by the added
> check.  That is just adding weakness, and a chance that some obscure
> programmer error gets us confused.
I agree with you here.

> I also am skeptical about the "just assert if things aren’t quite right"
> approach, because it removes the responsibility for thinking about how you
> would get to that state and how you would get out of it safely.  But more
> crucially, it exaggerates the importance of subsections of lldb’s
> functionality.  For instance, to the JIT, being asked to handle a
> relocation that it doesn’t support is a fatal error, so it seems okay by
> the fail soon lights to just fatal error.  But to lldb no expression is
> important enough to quit if it doesn’t work, so we would have really very
> much preferred if the JIT had been forced to back out from the error.
> Ditto with llvm & clang and handling types.  Even if the types from one CU
> are horked up beyond belief, there’s lots of useful debugging you can still
> do.
One way to handle this would be to have just the JIT run out of process.
Or to have it run in a separate thread with LLVM's crash recovery handling
where a single thread can terminate without taking down the rest of the
process.  See llvm::CrashRecoveryContext for example.  If the JIT thinks
it's in a state where it can't continue, no amount of backing out is going
to help.  Sure, there are often ways to redesign things so that it can fail
better, I'm not denying that.  I'm just saying that when you get a bug that
says "lldb crashed because x was null", the right fix is probably not a one
line change that says "add a null check for x".  Maybe you can return a
reference and propagate the requirements up through the API via the type
system.  Maybe after some investigation it turns out it was null because
you violated one of the API's constraints (which could happen for various
reasons, including the API not properly documenting its assumptions, which
definitely happens).

> It also seems like you are suggesting some conflict between adding more
> testing and trying to handle errors rather than exiting when they arise.
> There’s really no linkage between the two so far as I can see.

In the most trivial case, suppose you've got this program:

int main(int argc, char **argv) {
  return 0;

And you write a test that foo.exe returns 0.

You've got 100% code coverage.  Now suppose you add this:

int main(int argc, char **argv) {
  if (argc > 2)
    return 1;
  return 0;

Your same test now has 50% code coverage.  By adding a branch with no test,
you have reduced the code coverage of the system.  Now, you can obviously
add a test for this branch, but when the situation is "someone reported in
the field that x was null, so we're adding a null pointer check", then
you've just made the problem worse.  You can't reproduce it so there's no
test, and now maybe they're going to encounter another bug down the road
where y is null and caushes a crash, so we have to add a null pointer check
for y.  It's a domino effect.  Eventually you get to the point where the
code is unintelligible, because nobody understands when (or even if) any of
these conditions can happen.

But it gets worse.  If the problem is "foo() returned null for this guy on
line 42 of this file, so let's add a null check", now you have to go look
at all the other places foo() is called, of which there could be hundreds.

A good rule of thumb is that if the code owner of a particular piece of
code can look at a function and not explain in detail the circumstances
under which a branch can be entered, then you can delete the branch.
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