[lldb-dev] Renaming lldb_private::Error
Zachary Turner via lldb-dev
lldb-dev at lists.llvm.org
Mon May 1 16:52:19 PDT 2017
Yea, grouping the error and the result together is one of the most
compelling features of it. It's called Expected<T>, so where we would
currently write something like:
int getNumberOfSymbols(Error &Err) {}
or
Error getNumberOfSymbols(int &Count) {}
You would now write:
Expected<int> getNumberOfSymbols() {
if (foo) return 1;
else return make_error<DWARFError>("No symbols!");
}
and on the caller side you write:
Error processAllSymbols() {
if (auto Syms = getNumberOfSymbols()) {
outs() << "There are " << *Syms << " symbols!";
} else {
return Syms.takeError();
// alternatively, you could write:
// consumeError(Syms.takeError());
// return Error::success();
}
}
On Mon, May 1, 2017 at 4:47 PM Jim Ingham <jingham at apple.com> wrote:
>
> > On May 1, 2017, at 3:29 PM, Zachary Turner <zturner at google.com> wrote:
> >
> > I'm confused. By having the library assert, you are informed of places
> where you didn't do a good job of backing from errors, thereby allowing you
> to do a *better* job.
> >
> > You said this earlier:
> >
> > > 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!
> >
> > But when you're using llvm::Error, no developer is actively thinking
> about asserts. Nobody is thinking "well the library is going to assert, so
> my job is done here " because that doesn't make any sense. !!!!It's going
> to assert even if the operation was successful!!!!
> >
> > Your job can't possibly be done because if you don't check the error,
> you will assert 100% of the time you execute that codepath. You might as
> well have just written int x = *nullptr; Surely nobody could agree that
> their job is done after writing int x = *nullptr; in their code.
> >
> > If you write this:
> >
> > Error foo(int &x) {
> > x = 42;
> > return Error::success();
> > }
> >
> > void bar() {
> > int x;
> > foo(x);
> > cout << x;
> > }
> >
> > Then this code is going to assert. It doesn't matter that no error
> actually occurred. That is why I'm saying it is strictly a win, no matter
> what, in all situations. If you forget to check an error code, you
> *necessarily* aren't doing the best possible job backing out of the code in
> case an error does occur. Now you will find it and be able to fix it.
>
> Yeah, Lang was just explaining this. I think I was over-reacting to the
> asserts part because llvm's aggressive use of early failure was a real
> problem for lldb. So my hackles go up when something like it comes up
> again.
>
> In practical terms, lldb quite often uses another measure than the error
> to decide how it's going to proceed. I ask for some symbols, and I get
> some, but at the same time, one of 10 object files had some bad DWARF so an
> error was produced. I'll pass that error along for informational purposes,
> but I don't really care, I'm still going to set breakpoints on all the
> symbols I found. Lang said it is possible to gang something like the
> "number of symbols" and the error, so that checking the number of symbols
> automatically ticks the error box as well. If eventually ever comes we'll
> have to deal with this sort of complication.
>
> As for Error -> Status to avoid confusion, that seems fine, though if you
> are going to do it, I agree with Pavel it would be gross to have "Status
> error;" all over the place.
>
> Jim
>
>
> >
> > On Mon, May 1, 2017 at 3:19 PM Jim Ingham <jingham at apple.com> wrote:
> >
> > > On May 1, 2017, at 3:12 PM, Zachary Turner <zturner at google.com> wrote:
> > >
> > > 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?
> >
> > Not sure that's entirely relevant. The point is not to make failure
> points assert then turn them off in production because they shouldn't
> assert. The point is to make sure that instead of asserting you always do
> the best job you can backing out from any error.
> >
> > Jim
> >
> >
> > >
> > > 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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