[lldb-dev] Renaming lldb_private::Error

[Zachary Turner via lldb-dev]

Does Xcode ship with a build of LLDB that has asserts enabled?  Because if
not it shouldn't affect anything.  If so, can you shed some light on why?

On Mon, May 1, 2017 at 3:08 PM Jim Ingham <jingham at apple.com> wrote:

>
> > On May 1, 2017, at 2:51 PM, Zachary Turner <zturner at google.com> wrote:
> >
> > I think we agree about the SB layer.  You can't have mandatory checking
> on things that go through the SB API.  I think we could code around that
> though.
> >
> > Regarding the other point, I actually think force checked errors *help*
> you think about how to back out and leave the debug session alive.
> Specifically because they force you think think about it at all.  With
> unchecked errors, a caller might forget that a function even returns an
> error (or Status) at all, and so they may just call a function and proceed
> on assuming it worked as expected.  With a checked error, this would never
> happen because the first time you called that function in a test,
> regardless of whether it passed or failed, you would get an assertion
> saying you forgot to check the error.  Then you can go back and think about
> what the most appropriate thing to do is in that situation, and if the
> appropriate thing to do is ignore it and continue, then you can do that.
> >
> > Most of these error conditions are things that rarely happen
> (obviously), and it's hard to get test coverage to make sure the debugger
> does the right thing when it does happen.  Checked errors is at least a way
> to help you identify all the places in your code that you may have
> overlooked a possible failure condition.  And you can always just
> explicitly ignore it.
> >
>
> Sure, it is the policy not the tool to enforce it that really matters.
> But for instance lldb supports many debug sessions in one process (a mode
> it gets used in all the time under Xcode) and no matter how bad things go
> in one debug session, none of the other debug sessions care about that.  So
> unless you know you're about to corrupt memory in some horrible and
> unavoidable way, no action in lldb should take down the whole lldb
> session.  Playing with tools that do just that - and automatically too -
> means you've equipped yourself with something you are going to have to be
> very careful handling.
>
> Jim
>
>
> > On Mon, May 1, 2017 at 2:42 PM Jim Ingham <jingham at apple.com> wrote:
> >
> > > On May 1, 2017, at 12:54 PM, Zachary Turner <zturner at google.com>
> wrote:
> > >
> > > The rename is just to avoid the spelling collision.  Nothing in
> particular leads me to believe that unchecked errors are a source of major
> bugs.
> > >
> > > That said, I have some short term plans to begin making use of some
> llvm library classes which deal in llvm::Error, and doing this first should
> make those changes less confusing.  Additionally I'd like to be able to
> start writing new LLDB code against llvm::Error where appropriate, so it
> would be nice if this collision weren't present.
> > >
> > > BTW, I'm curious why you think asserting is still bad even in the test
> suite when errors don't need to be checked.
> >
> > I think I was making a more limited statement that you took it to be.
> >
> > Errors that should be checked locally because you know locally that it
> is fatal not to check them should always be checked - testsuite or no.  But
> a lot of lldb's surface area goes out to the SB API's, and we don't control
> the callers of those.  All the errors of that sort can't be checked before
> they pass the boundary (and are more appropriate as Status's instead.)  The
> failure to check those errors shouldn't propagate to the SB API's or we are
> just making an annoying API set...  So test suite asserting for this class
> of errors would not be appropriate.
> >
> > But a larger point about asserting as a result of errors is that it
> makes it seem to the lldb developer like once you've raised an assert on
> error your job is done.  You've stopped the error from propagating, two
> points!  For the debugger, you should really be thinking "oh, that didn't
> go right, how can I back out of that so I can leave the debug session
> alive."   There's nothing about force checked errors for code you can
> reason on locally that enforces one way of resolving errors or the other.
> But IME it does favor the "bag out early" model.
> >
> > Jim
> >
> >
> > > I think of it as something like this:
> > >
> > > void foo(int X) {
> > >   return;
> > > }
> > >
> > > And your compiler giving you a warning that you've got an unused
> parameter.  So to silence it, you write:
> > >
> > > void foo(int X) {
> > >   (void)X;
> > > }
> > >
> > > The point here being, it's only the function foo() that knows whether
> the parameter is needed.  Just like if you write:
> > >
> > > Error E = foo();
> > >
> > > the function foo() cannot possibly know whether the error needs to be
> checked, because it depends on the context in which foo() is called.  One
> caller might care about the error, while the other doesn't.  So foo()
> should assume that the caller will check the error (otherwise why even
> bother returning one if it's just going to be ignored), and the caller can
> explicitly opt out of this behavior by writing:
> > > consumeError(foo());
> > >
> > > which suppresses the assertion.
> > >
> > > So yes, the error has to be "checked", but you can "check" it by
> explicitly ignoring it at a particular callsite.
> > >
> > > On Mon, May 1, 2017 at 12:38 PM Jim Ingham <jingham at apple.com> wrote:
> > > BTW, I'm interested to know if you have some analysis which leads you
> to think that propagating unchecked errors actually is a big problem in
> lldb, or are you just doing this to remove a spelling collision?  I see a
> lot of bugs for lldb come by, but I really haven't seen many that this sort
> of forced checking would have fixed.
> > >
> > > Jim
> > >
> > >
> > > > On May 1, 2017, at 12:36 PM, Jim Ingham <jingham at apple.com> wrote:
> > > >
> > > >>
> > > >> On May 1, 2017, at 11:48 AM, Zachary Turner <zturner at google.com>
> wrote:
> > > >>
> > > >>
> > > >>
> > > >> On Mon, May 1, 2017 at 11:28 AM Jim Ingham <jingham at apple.com>
> wrote:
> > > >> I'm mostly but not entirely tongue in cheek wondering why we aren't
> calling llvm::Error llvm::LLVMError, as the lldb error class much preceded
> it, but that's a minor point.
> > > >> FWIW I think the naming chosen by LLVM is correct.  It's intended
> to be a generic utility class, extensible enough to be used by anything
> that links against LLVM.  As such, calling it LLVMError kind of gives off
> the false impression that it should only be used by errors that originate
> from LLVM, when in fact it's much more general purpose.
> > > >>
> > > >>
> > > >> If it is actually causing confusion (I haven't experienced such
> yet) I don't mind typing some extra letters.
> > > >> I think that's in part because llvm::Error isn't very prevalent
> inside of LLDB (yet).
> > > >>
> > > >>
> > > >> As we've discussed several times in the past, we often use errors
> for informational purposes (for instance in the ValueObject system) with no
> programmatic requirement they be checked.  So the llvm::Error class is not
> a drop-in replacement for our uses of lldb_private::Error in subset of its
> uses.  More generally, the environment the debugger lives in is often
> pretty dirty, bad connections to devices, uncertain debug information,
> arguments with clang about what types mean, weird user input, etc.  But the
> job of the debugger is to keep going as well/long as it can in the face of
> this. For something like a compiler, if some operation goes bad that whole
> execution is likely rendered moot, and so bagging out early is the right
> thing to do.  For lldb, if the debug info for a frame is all horked up,
> users can still resort to memory reading and casts, or some other
> workaround, provided the debugger stays alive.  This makes me a little
> leery of adopting an infrastructure whose default action is to abort on
> mishandling.
> > > >> Just re-iterating from previous discussions, but it only does that
> in debug mode.  When you have a release build, it will happily continue on
> without aborting.  The point of all this is that you catch unhandled errors
> immediately the first time you run the test suite.
> > > >
> > > > Yup, we do that for assertions.  But asserting isn't appropriate
> even in the testsuite for cases where we don't require the errors be
> checked.
> > > >
> > > >>
> > > >> Even if you have a bad connection, uncertain debug information, etc
> you still have to propagate that up the callstack some number of levels
> until someone knows what to do.  All this class does is make sure (when in
> debug mode) that you're doing that instead of silently ignoring some
> condition.
> > > >>
> > > >> That said, it certainly seems plausible that we could come up with
> some kind of abstraction for informational status messages.  With that in
> mind, I'll change my original renaming proposal from LLDBError to Status.
> This way we will have llvm::Error and lldb_private::Status.
> > > >
> > > > That seems better.
> > > >
> > > >>
> > > >> In the future, perhaps we can discuss with Lang and the larger
> community about whether such a class makes in LLVM as well.  Maybe there's
> a way to get both checked and unchecked errors into LLVM using a single
> consistent interface.  But at least then the person who generates the error
> is responsible for deciding how important it is.
> > > >>
> > > >
> > > > It's not "how important it is" but "does this error need to be dealt
> with programmatically proximate to the code that produces it."  For
> instance, if an error makes it to the SB API level - something that is
> quite appropriate for the SBValues for instance, we wouldn't want to use an
> llvm::Error.  After all forcing everybody to check this at the Python layer
> would be really annoying.  I guess you could work around this by
> hand-checking off any error when you go from lldb_private -> SBError.  But
> that seems like now you're just pretending to be doing something you
> aren't, which I don't think is helpful.
> > > >
> > > > Probably better as you say to make everything into
> lldb_private::Status behaving as it does now, to side-step the name
> collision, and then start with all the uses where the error doesn't
> propagate very far, and try converting those to use llvm::Error and working
> judiciously out from there.  'Course you can't change the SB API names, so
> there will always be a little twist there.
> > > >
> > > > Jim
> > > >
> > > >>
> > > >>
> > > >> BTW, I don't think the comment Lang cited had to do with replacing
> the errors with some other error backend.  It was more intended to handle a
> problem that came up with gdb where we tried to multiplex all various
> system error numbers into one single error.  lldb errors have a flavor
> (eErrorTypePosix, eErrorTypeWin32, etc) which allows you to use each native
> error number by annotating it with the flavor.
> > > >>
> > > >> FWIW, using the llvm::Error model, the way this is handled is by
> doing something like this:
> > > >>
> > > >> return make_error<WindowsError>(::GetLastError());
> > > >>
> > > >> return make_error<ErrnoError>(errno);
> > > >>
> > > >> but it is general enough to handle completely different categories
> of errors as well, so you can "namespace" out your command interpreter
> errors, debug info errors, etc.
> > > >>
> > > >> return make_error<CommandInterpreterError>("Incorrect command
> usage");
> > > >>
> > > >> return make_error<DWARFFormatError>("Invalid DIE specification");
> > > >>
> > > >> etc
> > >
> >
>
>
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