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'Gracefully' fallback from failing substituters #13301

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Merged
merged 1 commit into from
Sep 12, 2025
+31 −19
Merged

'Gracefully' fallback from failing substituters #13301

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25 changes: 15 additions & 10 deletions src/libstore/build/substitution-goal.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -55,9 +55,14 @@ Goal::Co PathSubstitutionGoal::init()
auto subs = settings.useSubstitutes ? getDefaultSubstituters() : std::list<ref<Store>>();

bool substituterFailed = false;
std::optional<Error> lastStoresException = std::nullopt;

for (const auto & sub : subs) {
trace("trying next substituter");
if (lastStoresException.has_value()) {
logError(lastStoresException->info());
lastStoresException.reset();
}

cleanup();

Expand All @@ -80,19 +85,13 @@ Goal::Co PathSubstitutionGoal::init()
try {
// FIXME: make async
info = sub->queryPathInfo(subPath ? *subPath : storePath);
} catch (InvalidPath &) {
} catch (InvalidPath & e) {
continue;
} catch (SubstituterDisabled & e) {
if (settings.tryFallback)
continue;
else
throw e;
continue;
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@Ericson2314 Ericson2314 Sep 12, 2025 •
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Suggested change
continue;
if (!settings.tryFallback)
throw e;
continue;

I think we want this, the saving exception thing should just be for the no-fallback case, right?

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oh my bad, I think I am wrong and this is fine. You tell can confirm, though.

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i think there has been some confusion when implementing this loop that means tryFallback has been coerced into meaning both "try the next substituter" and "try building the path ourselves" - I am quite sure it should only be used for the latter. Otherwise... that would mean we are ok with trying the next substituter if the first one doesn't have a path, but not if it errors out and gets disabled... really weird.

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There was some other PR where I left a big comment trying to explain the theory.

Here's what I think: "!tryFallback" is a coherence trick: you only build/use a substituter if you are sure that the previous substituters don't have the store object in question. If you can access them and they don't have it, you are sure. If you can't access it, you don't know, and thus you could be getting/build the "wrong version".

That actually would be a reason to have what I wrote above.

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#7188 (comment) ah this is the comment I had in mind

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perhaps im ignorant, but in what case could we get a path that has different versions depending on the substituter used, would this not result in a different path entirely?

I don't think the state of the substituter should affect the contents of the path we get back?

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@Ericson2314 Ericson2314 Sep 26, 2025 •
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With input-addressing, the a store path may be allowed to point to different contents if the derivation is non-deterministic.

} catch (Error & e) {
if (settings.tryFallback) {
logError(e.info());
continue;
} else
throw e;
lastStoresException = std::make_optional(std::move(e));
continue;
}

if (info->path != storePath) {
Expand Down Expand Up @@ -156,6 +155,12 @@ Goal::Co PathSubstitutionGoal::init()
worker.failedSubstitutions++;
worker.updateProgress();
}
if (lastStoresException.has_value()) {
if (!settings.tryFallback) {
throw *lastStoresException;
} else
logError(lastStoresException->info());
}

/* Hack: don't indicate failure if there were no substituters.
In that case the calling derivation should just do a
Expand Down
25 changes: 16 additions & 9 deletions src/libstore/store-api.cc
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Original file line number Diff line number Diff line change
@@ -1,3 +1,4 @@
#include "nix/util/logging.hh"
#include "nix/util/signature/local-keys.hh"
#include "nix/util/source-accessor.hh"
#include "nix/store/globals.hh"
Expand Down Expand Up @@ -392,11 +393,14 @@ void Store::querySubstitutablePathInfos(const StorePathCAMap & paths, Substituta
{
if (!settings.useSubstitutes)
return;
for (auto & sub : getDefaultSubstituters()) {
for (auto & path : paths) {
if (infos.count(path.first))
// Choose first succeeding substituter.
continue;

for (auto & path : paths) {
std::optional<Error> lastStoresException = std::nullopt;
for (auto & sub : getDefaultSubstituters()) {
if (lastStoresException.has_value()) {
logError(lastStoresException->info());
lastStoresException.reset();
}

auto subPath(path.first);

Expand Down Expand Up @@ -437,12 +441,15 @@ void Store::querySubstitutablePathInfos(const StorePathCAMap & paths, Substituta
} catch (InvalidPath &) {
} catch (SubstituterDisabled &) {
} catch (Error & e) {
if (settings.tryFallback)
logError(e.info());
else
throw;
lastStoresException = std::make_optional(std::move(e));
}
}
if (lastStoresException.has_value()) {
if (!settings.tryFallback) {
throw *lastStoresException;
} else
logError(lastStoresException->info());
}
}
}

Expand Down
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