read the error after the call that sets it

[tamnd]

Every fallible call in the header reads

checked(zu_something(..., &err), err, "zu_something")

and the order a compiler evaluates those two arguments in is unspecified. On one that reads err first, it reads it before the call beside it has run, which is to say it reads the null it was initialised to. Everything went on throwing and everything thrown was empty: the right status, no code, no condition, no position, and the name of the C function where the engine's sentence should have been. GCC picks that order. Clang picks the other one, which is why a header that has never been anything but wrong here has looked fine.

Through the wrapper, before this:

threw [MATCH (p:Person RETURN p] what=zu_query status=3 code=(none) message=zu_query

and through the C ABI with the same library, at the same moment:

status=3 err=yes  42001: line 1, column 17: ...

The fix is not to hoist the call into a variable at forty three call sites, because that leaves the mistake available to the forty fourth. checked takes the slot by address now. What may be evaluated early is then where err lives rather than what it holds, and what it holds is read inside the call, after the call that fills it has certainly returned. It also clears the slot, so a caller who looks afterwards finds nothing to free twice.

test_errors already had a case for this and was failing on it. It has one more now that says the whole sentence: a statement that does not parse carries a five character code, a position, a severity and a message that is not the name of the C function. The case above it allows a missing position, because a variable that is not defined has no token to point at, and an allowance is how this got through.

Measured on Linux with gcc 13 against libzu at the engine's HEAD: test_errors and test_errors_cxx20 go from 7 of 11 to 11 of 12, and test_statement goes green.

The failures left over are the engine having moved to ABI 0.14 and having reserved two more words, and they are the next PR.