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Fix non-deterministic parser printing

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In _very_ rare cases (I had about 7 cases out of 32200 files!),
the order of how inherit-from bindings are printed when using
`nix-instantiate --parse` gets messed up.

The cause of this seems to be because the std::map the bindings are
placed in is keyed on a _pointer_, which then uses an
[implementation-defined strict total order](https://en.cppreference.com/w/cpp/language/operator_comparison#Pointer_total_order).

The fix here is to key the bindings on their displacement instead,
which maintains the same order as they appear in the file.

Unfortunately I wasn't able to make a reproducible test for this in the
source, there's something about the local environment that makes it
unreproducible for me.

However I was able to make a reproducible test in a Nix build on a Nix
version from a very recent master:

    nix build github:infinisil/non-det-nix-parsing-repro

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>
This commit is contained in:
Silvan Mosberger 2024-07-17 02:05:38 +02:00 committed by Silvan Mosberger
parent 463256b9e8
commit a6dccae223
1 changed files with 5 additions and 3 deletions
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8
src/libexpr/nixexpr.cc
View file

@ -82,7 +82,9 @@ void ExprAttrs::showBindings(const SymbolTable & symbols, std::ostream & str) co
return sa < sb;
});
std::vector<Symbol> inherits;
std::map<ExprInheritFrom *, std::vector<Symbol>> inheritsFrom;
// We can use the displacement as a proxy for the order in which the symbols were parsed.
// The assignment of displacements should be deterministic, so that showBindings is deterministic.
std::map<Displacement, std::vector<Symbol>> inheritsFrom;
for (auto & i : sorted) {
switch (i->second.kind) {
case AttrDef::Kind::Plain:
@ -93,7 +95,7 @@ void ExprAttrs::showBindings(const SymbolTable & symbols, std::ostream & str) co
case AttrDef::Kind::InheritedFrom: {
auto & select = dynamic_cast<ExprSelect &>(*i->second.e);
auto & from = dynamic_cast<ExprInheritFrom &>(*select.e);
inheritsFrom[&from].push_back(i->first);
inheritsFrom[from.displ].push_back(i->first);
break;
}
}
@ -105,7 +107,7 @@ void ExprAttrs::showBindings(const SymbolTable & symbols, std::ostream & str) co
}
for (const auto & [from, syms] : inheritsFrom) {
str << "inherit (";
(*inheritFromExprs)[from->displ]->show(symbols, str);
(*inheritFromExprs)[from]->show(symbols, str);
str << ")";
for (auto sym : syms) str << " " << symbols[sym];
str << "; ";