[Bitcoin-development] # error "Bitcoin cannot be compiled without assertions." <<<<NOT

Jannis Froese s9jafroe at stud.uni-saarland.de
Wed Jun 4 10:42:37 UTC 2014

There are reasons to have assertions enabled by default in software like
Bitcoin Core, where incorrect behaviour can be costly. But this comes at
a prize: our assertions have to satisfy certain performance
requirements. It's no longer possible to do expensive, redundant checks
in performance critical code, which is one of the main advantages of
asserts. Imho, asserts are not intended for small range checks etc, but
are meant for checks that a hash hasn't changed, that a tree structure
is still a tree, that data is still sorted, or that data structures are
in sync.

I think most concerns about the current use of asserts would be resolved
if the currently used asserts would be changed to a nicer definition
which is independent of NDEBUG, and a second class of debugging asserts
would be introduced, which is exclusively for expensive, redundant
checks and is disabled by NDEBUG.

Am 2014-06-04 12:15, schrieb Gregory Maxwell:
> On Wed, Jun 4, 2014 at 2:51 AM, Mike Hearn <mike at plan99.net
> <mailto:mike at plan99.net>> wrote:
>     Hi Ron,
>     FYI your mail is being spamfoldered due to Yahoo's DMARC policy
>     and the brokenness of the SF.net mailing list software. I would
>     not expect to get replies reliably whilst this is the case. I
>     think we should move away from SF.net for hosting mailing lists
>     personally, because it's this list that's at fault not Yahoo, but
>     until then you may wish to send to the list with a different email
>     address.
>     As to your question,
>         assert() should have *no* side effects, that is the problem.
>         See
>         http://books.google.com/books?id=L5ZbzVnpkXAC&pg=PA72&lpg=PA72&dq=Gotcha+%2328+Side+Effects&source=bl&ots=Rn15TlPmje&sig=tymHqta0aSANwaM2GaXC-1Di_tk&hl=en&sa=X&ei=uVKNU47fCcvTsAT6goHIBA&ved=0CCAQ6AEwAA#v=onepage&q=Gotcha%20%2328%20Side%20Effects&f=false
>         a great book, BTW.  Everyone who thinks they know what they
>         are doing when they write C++ should read this book!  They
>         will realize that they don't know Jack Roll Eyes
>         Why weren't these and all the other examples of amateur, i.e.,
>         non-professional, software fixed way back in version 0.3.0 in
>         2010, before any more releases were done?  And why were these
>         and other sub-standard coding practices continued and expanded
>         in later releases, right up until the present?
>     Back in 2010 most code was still being written by Satoshi so
>     perhaps you should ask him. Regardless, it's very common for
>     professional codebases to require assertions be enabled. For
>     example the entire Google C++ codebase uses always-on assertions
>     that have side effects liberally: it's convenient and safe, when
>     you have the guarantee the code will always run, and the
>     performance benefits of compiling out assertions are usually
>     non-existent.
>     So for this reason I think Bitcoin Core currently will fail to
>     build if assertions are disabled, and that seems OK to me.
> As a matter of procedure we do not use assertions with side effects---
> the codebase did at one point, but have cleaned them up.  In an
> abundance of caution we also made it refuse to compile without
> assertions enabled: A decision who's wisdom was clearly demonstrated
> when not long after, some additional side-effect having assert was
> contributed. In the real world errors happen here and there, and
> making robust software involves defense in depth.
> Considering the normal criticality of the software it should always be
> with the assertions. Without them is an untested configuration.  It
> would probably be superior to use our own assertion macros (for one,
> they can give some better reporting...) that don't have the baggage
> ordinary assertions have, but as a the codebase is a production thing,
> making larger changes all at once to satisfy aesthetics would be
> unwise... simply refusing to compile in that untested, unsupported
> configuration is prudent, for the time being.
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