Fix deadlock in nix-store when max-connections=1

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`.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/libstore/remote-store.cc b/src/libstore/remote-store.cc
index b6f70057d..a392f3b8c 100644
--- a/src/libstore/remote-store.cc
+++ b/src/libstore/remote-store.cc
@@ -409,10 +409,10 @@ StorePathSet RemoteStore::queryValidDerivers(const StorePath & path)
 
 StorePathSet RemoteStore::queryDerivationOutputs(const StorePath & path)
 {
-    auto conn(getConnection());
-    if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 0x16) {
+    if (GET_PROTOCOL_MINOR(getProtocol()) >= 0x16) {
         return Store::queryDerivationOutputs(path);
     }
+    auto conn(getConnection());
     conn->to << wopQueryDerivationOutputs << printStorePath(path);
     conn.processStderr();
     return worker_proto::read(*this, conn->from, Phantom<StorePathSet> {});