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[CAS] Protect the CASOutputBackend from multi-threaded updates #82859

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Jul 8, 2025
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[CAS] Protect the CASOutputBackend from multi-threaded updates #82859

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39 changes: 27 additions & 12 deletions lib/Frontend/CASOutputBackends.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -22,6 +22,7 @@
#include "llvm/CAS/HierarchicalTreeBuilder.h"
#include "llvm/CAS/ObjectStore.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/Mutex.h"
#include <optional>

#define DEBUG_TYPE "swift-cas-backend"
Expand Down Expand Up @@ -112,6 +113,9 @@ class SwiftCASOutputBackend::Implementation {
Error storeImpl(StringRef Path, StringRef Bytes, unsigned InputIndex,
file_types::ID OutputKind);

// Return true if all the outputs are produced for the given index.
bool addProducedOutput(unsigned InputIndex, file_types::ID OutputKind,
ObjectRef BytesRef);
Error finalizeCacheKeysFor(unsigned InputIndex);

private:
Expand All @@ -123,6 +127,9 @@ class SwiftCASOutputBackend::Implementation {
const FrontendOptions &Opts;
FrontendOptions::ActionType Action;

// Lock for updating output file status.
llvm::sys::SmartMutex<true> OutputLock;

// Map from output path to the input index and output kind.
StringMap<std::pair<unsigned, file_types::ID>> OutputToInputMap;

Expand Down Expand Up @@ -197,8 +204,10 @@ Error SwiftCASOutputBackend::storeMCCASObjectID(StringRef OutputFilename,
return createStringError("Invalid CASID: " + ID.toString() +
". No associated ObjectRef found!");

Impl.OutputRefs[InputIndex].insert({file_types::TY_Object, *MCRef});
return Impl.finalizeCacheKeysFor(InputIndex);
if (Impl.addProducedOutput(InputIndex, file_types::TY_Object, *MCRef))
return Impl.finalizeCacheKeysFor(InputIndex);

return Error::success();
}

void SwiftCASOutputBackend::Implementation::initBackend(
Expand Down Expand Up @@ -249,23 +258,29 @@ Error SwiftCASOutputBackend::Implementation::storeImpl(
<< "\' for input \'" << InputIndex << "\': \'"
<< CAS.getID(*BytesRef).toString() << "\'\n";);

OutputRefs[InputIndex].insert({OutputKind, *BytesRef});
if (addProducedOutput(InputIndex, OutputKind, *BytesRef))
return finalizeCacheKeysFor(InputIndex);
return Error::success();
}

bool SwiftCASOutputBackend::Implementation::addProducedOutput(
unsigned InputIndex, file_types::ID OutputKind,
ObjectRef BytesRef) {
llvm::sys::SmartScopedLock<true> LockOutput(OutputLock);
auto &ProducedOutputs = OutputRefs[InputIndex];
ProducedOutputs.insert({OutputKind, BytesRef});

return finalizeCacheKeysFor(InputIndex);
return llvm::all_of(OutputToInputMap, [&](auto &E) {
return (E.second.first != InputIndex ||
ProducedOutputs.count(E.second.second));
});
}

Error SwiftCASOutputBackend::Implementation::finalizeCacheKeysFor(
unsigned InputIndex) {
auto ProducedOutputs = OutputRefs[InputIndex];
const auto &ProducedOutputs = OutputRefs[InputIndex];
assert(!ProducedOutputs.empty() && "Expect outputs for this input");

// If not all outputs for the input are emitted, return.
if (!llvm::all_of(OutputToInputMap, [&](auto &E) {
return (E.second.first != InputIndex ||
ProducedOutputs.count(E.second.second));
}))
return Error::success();

std::vector<std::pair<file_types::ID, ObjectRef>> OutputsForInput;
llvm::for_each(ProducedOutputs, [&OutputsForInput](auto E) {
OutputsForInput.emplace_back(E.first, E.second);
Expand Down