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[c++] avrogencpp: emit deterministic include guard #3778

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[c++] avrogencpp: emit deterministic include guard #3778

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9 changes: 8 additions & 1 deletion lang/c++/impl/avrogencpp.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -805,7 +805,14 @@ void CodeGen::emitGeneratedWarning() {
string CodeGen::guard() {
string h = headerFile_;
makeCanonical(h, true);
return h + "_" + std::to_string(random_()) + "_H";
// headerFile_ is already a unique-per-output path, so the canonicalised
// form is already a valid, unique include guard. Avoid mixing in a
// time-seeded RNG here so the generated output is byte-deterministic
// across invocations -- otherwise build systems that key their cache on
// input-content digests (e.g. Bazel remote cache, Nix store paths) end
// up rebuilding every downstream consumer on every invocation, even on
// byte-identical schemas.
return h + "_H";
}

void CodeGen::generate(const ValidSchema &schema) {
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