[lldb-dev] Renaming lldb_private::Error

Jim Ingham via lldb-dev lldb-dev at lists.llvm.org
Mon May 1 16:47:48 PDT 2017 

> 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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