hi Edmon, Here's an example of a function that does some schema validation:
https://github.com/apache/arrow/blob/master/cpp/src/arrow/table.cc#L450 The issue is less about the magnitude of the cost and more of a software engineering question about layering of concerns. Consider two code paths: * Path A: Eventually constructs tables from input known to be valid (validated by some other procedure) * Path B: Constructs tables for user input I don't believe that Path A should have to pay for the additional validation requirements that Path B has. Overly defensive coding practices can also create a false sense of security re: unit testing edge cases. By the way, we need more help with systematic and automated benchmarking so we can use commit-by-commit numbers to assist in our decision making. - Wes On Sun, Mar 10, 2019 at 6:29 PM Edmon Begoli <ebeg...@berkeley.edu> wrote: > > Do you guys have an example somewhere of this validated vs. unvalidated > code, and suspected performance impacts, and has anyone benchmarked any of > this? > > > > On Sun, Mar 10, 2019 at 5:45 PM Wes McKinney <wesmck...@gmail.com> wrote: > > > I think having consistent methods for both validated and unvalidated > > construction is a good idea. Being fairly passionate about > > microperformance, I don't think we should penalize responsible users > > of unsafe/unvalidated APIs (e.g. by taking them away and replacing > > them with variants featuring unavoidable computation), this can > > partially be handled through developer documentation (which, ah, we > > will need to write more of) > > > > On Sun, Mar 10, 2019 at 4:01 PM Micah Kornfield <emkornfi...@gmail.com> > > wrote: > > > > > > I agree there should always be a path to avoid the validation but I > > think there should also be an easy way to have validation included and a > > clear way to tell the difference. IMO, having strong naming convention so > > callers can tell the difference, and code reviewers can focus more on less > > safe method usage, is important. I will help new-comers to the project > > write safer code. Which can either be refactored or called out in code > > review for performance issues. It also provides a cue for all developers > > to consider if they are meeting the necessary requirements when using less > > safe methods. > > > > > > A straw-man proposal for naming conventions: > > > - Constructors are always unvalidated (should still have appropriate > > DCHECKs) > > > - "Make" calls are always unvalidated (should still have appropriate > > DHCECKs) > > > - "MakeValidated" ensures proper structural validation occur (but not > > data is validation). > > > - "MakeSanitized" ensures proper structural and data is validations occur > > > > > > As noted above it might only pay to refactor a small amount of current > > usage to the safer APIs. > > > > > > We could potentially go even further down the rabbit hole and try to > > define standard for a Hungarian notation [1] to make it more obvious what > > invariants are expected for a particular data-structure variable (I'm > > actually -.5 on this). > > > > > > As a personal bias, I would rather have slower code that has lower risk > > of crashing in production than faster code that does. Obviously, there is > > a tradeoff here, and the ideal is faster code that won't segfault. > > > > > > Thoughts? > > > > > > -Micah > > > > > > [1] > > https://www.joelonsoftware.com/2005/05/11/making-wrong-code-look-wrong/ > > > > > > On Sun, Mar 10, 2019 at 9:38 AM Wes McKinney <wesmck...@gmail.com> > > wrote: > > >> > > >> hi folks, > > >> > > >> I think some issues are being conflated here, so let me try to dig > > >> through them. Let's first look at the two cited bugs that were fixed, > > >> if I have this right: > > >> > > >> * ARROW-4766: root cause dereferencing a null pointer > > >> * ARROW-4774: root cause unsanitized Python user input > > >> > > >> None of the 4 remedies listed could have prevented ARROW-4766 AFAICT > > >> since we currently allow for null buffers (the object, not the pointer > > >> inside) in ArrayData. This has been discussed on the mailing list in > > >> the past; "sanitizing" ArrayData to be free of null objects would be > > >> expensive and my general attitude in the C++ library is that we should > > >> not be in the business of paying extra CPU cycles for the 1-5% case > > >> when it is unneeded in the 95-99% of cases. We have DCHECK assertions > > >> to check these issues in debug builds while avoiding the costs in > > >> release builds. In the case of checking edge cases in computational > > >> kernels, suffice to say that we should check every kernel on length-0 > > >> input with null buffers to make sure this case is properly handled > > >> > > >> In the case of ARROW-4774, we should work at the language binding > > >> interface to make sure we have convenient "validating" constructors > > >> that check user input for common problems. This can prevent the > > >> duplication of this code across the binding layers (GLib, Python, R, > > >> MATLAB, etc.) > > >> > > >> On the specific 4 steps mentioned by Francois, here are my thoughts: > > >> > > >> 1. Having StatusOr would be useful, but this is a programming > > convenience > > >> > > >> 2. There are a couple purposes of the static factory methods that > > >> exist now, like Table::Make and RecordBatch::Make. One of the reasons > > >> that I added them initially was because of the implicit constructor > > >> behavior of std::vector inside a call to std::make_shared. If you have > > >> a std::vector<T> argument in a class's constructor, then > > >> std::make_shared<Klass>(..., {...}) will not result in the initializer > > >> list constructing the std::vector<T>. This means some awkwardness like > > >> having to assign a std::vector<T> lvalue and _then_ pass that to > > >> std::make_shared<Klass>(..., vector_arg, ...). > > >> > > >> I do not agree with refactoring these methods to use "validating" > > >> constructors. Users of these C++ APIs should know what their > > >> requirements are, and we provide in some cases a Validate() to spend > > >> the extra cycles to assert preconditions. Therefore: > > >> > > >> 3. -1 on this > > >> 4. -1 also > > >> > > >> Thanks > > >> Wes > > >> > > >> On Sat, Mar 9, 2019 at 9:58 PM Micah Kornfield <emkornfi...@gmail.com> > > wrote: > > >> > > > >> > HI François, > > >> > This sounds great. I would hope that as part of this we document the > > >> > invariants (and possible sharp edges like zero length/all null and no > > null > > >> > Arrays). > > >> > > > >> > Is your intent to allow languages binding to the C++ library go > > through the > > >> > new API or will they still be able to use the "dangerous" ones? > > >> > > > >> > -Micah > > >> > > > >> > On Fri, Mar 8, 2019 at 6:16 PM Francois Saint-Jacques < > > >> > fsaintjacq...@gmail.com> wrote: > > >> > > > >> > > Greetings, > > >> > > > > >> > > I noted that the current C++ API permits constructing core objects > > breaking > > >> > > said classes invariants. The following recent issues were affected > > by this: > > >> > > > > >> > > - ARROW-4766: segfault due to invalid ArrayData with nullptr buffer > > >> > > - ARROW-4774: segfault due to invalid Table with columns of > > different > > >> > > length > > >> > > > > >> > > Consumers often assumes that the objects respect the invariants, > > e.g. by > > >> > > dereferencing `array_data->buffers[i]->data()` or > > >> > > `Array::GetValue(table.n_rows - 1)` . > > >> > > > > >> > > Sample of classes which requires invariant in the constructor: > > >> > > - ArrayData/Array: number and size of buffers depending on type > > >> > > - ChunkedArray: Arrays of same type > > >> > > - Column: same as ChunkedArray and Field must match array's type > > >> > > - RecordBatch: number of columns and schema must match, columns of > > same > > >> > > size > > >> > > - Table: columns must be of same size > > >> > > > > >> > > Some classes provide static factory methods, notably: > > >> > > - Most `shared_ptr<T> *::Make` methods, but they lack the Status > > return > > >> > > type to indicate > > >> > > failure, the consumer can at least check for nullptr > > >> > > - `Status Table::FromRecordBatches(..., shared_ptr<T> *out)` is a > > good > > >> > > candidate to follow > > >> > > > > >> > > I suspect that mis-usage is only going to grow with the number of > > users and > > >> > > language that binds to C++. I would like to propose a plan to > > tackle for > > >> > > the > > >> > > 0.14.0 release > > >> > > > > >> > > 1. Implement `StatusOr` (ARROW-4800), providing a cleaner API by > > minimizing > > >> > > output parameters. > > >> > > 2. Refactor normalized factory methods for each core object > > (ArrayData, > > >> > > ChunkedArray, Column, RecordBatch, Table) > > >> > > - Common naming, I suggest we stay with `Make`. > > >> > > - Common return type, `StatusOr<shared_ptr<T>>` > > >> > > 3. Refactor existing Make methods to use new methods but preserve > > original > > >> > > signature by losing error message, on top of marking them > > deprecated. > > >> > > 4. Mark non-validating constructors as deprecated and ideailly make > > every > > >> > > "dangerous" constructor non-public. > > >> > > > > >> > > We'd give 1-2 release for consumers to stop using the deprecated > > >> > > methods/constructors. > > >> > > > > >> > > François > > >> > > > >
