Fix deadlock in nix-store when max-connections=1 #4262
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This fixes a bug I encountered where `nix-store -qR` will deadlock when the `--include-outputs` flag is passed and `max-connections=1`. The deadlock occurs because `RemoteStore::queryDerivationOutputs` takes the only connection from the connection pool and uses it to check the daemon version. If the version is new enough, it calls `Store::queryDerivationOutputs`, which eventually calls `RemoteStore::queryPartialDerivationOutputMap`, where we take another connection from the connection pool to check the version again. Because we still haven't released the connection from the caller, this waits for a connection to be available, causing a deadlock. This diff solves the issue by using `getProtocol` to check the protocol version in the caller `RemoteStore::queryDerivationOutputs`, which immediately frees the connection back to the pool before returning the protocol version. That way we've already freed the connection by the time we call `RemoteStore::queryPartialDerivationOutputMap`.
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This fixes a bug I encountered where
nix-store -qRwill deadlock whenthe
--include-outputsflag is passed andmax-connections=1.The deadlock occurs because
RemoteStore::queryDerivationOutputstakesthe only connection from the connection pool and uses it to check the
daemon version. If the version is new enough, it calls
Store::queryDerivationOutputs, which eventually callsRemoteStore::queryPartialDerivationOutputMap, where we take anotherconnection from the connection pool to check the version again. Because
we still haven't released the connection from the caller, this waits for
a connection to be available, causing a deadlock.
This diff solves the issue by using
getProtocolto check the protocolversion in the caller
RemoteStore::queryDerivationOutputs, whichimmediately frees the connection back to the pool before returning the
protocol version. That way we've already freed the connection by the
time we call
RemoteStore::queryPartialDerivationOutputMap.