3 files changed, 19 insertions, 5 deletions

diff --git a/src/libstore/gc.cc b/src/libstore/gc.cc
index 2d7734729..ff66c3938 100644
--- a/src/libstore/gc.cc
+++ b/src/libstore/gc.cc
@@ -112,6 +112,9 @@ void LocalStore::addTempRoot(const StorePath & path)
 
     }
 
+    if (!state->fdGCLock)
+        state->fdGCLock = openGCLock();
+
  restart:
     FdLock gcLock(state->fdGCLock.get(), ltRead, false, "");
 
@@ -653,8 +656,11 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
     if (state.shouldDelete)
         deletePath(reservedPath);
 
-    /* Acquire the global GC root. */
-    FdLock gcLock(_state.lock()->fdGCLock.get(), ltWrite, true, "waiting for the big garbage collector lock...");
+    /* Acquire the global GC root. Note: we don't use state->fdGCLock
+       here because then in auto-gc mode, another thread could
+       downgrade our exclusive lock. */
+    auto fdGCLock = openGCLock();
+    FdLock gcLock(fdGCLock.get(), ltWrite, true, "waiting for the big garbage collector lock...");
 
     /* Start the server for receiving new roots. */
     auto socketPath = stateDir.get() + gcSocketPath;
diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index faecdb00b..0e6556e11 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -382,11 +382,16 @@ LocalStore::LocalStore(const Params & params)
                     (select id from Realisations where drvPath = ? and outputName = ?));
             )");
     }
+}
+
 
+AutoCloseFD LocalStore::openGCLock()
+{
     Path fnGCLock = stateDir + "/gc.lock";
-    state->fdGCLock = open(fnGCLock.c_str(), O_RDWR | O_CREAT | O_CLOEXEC, 0600);
-    if (!state->fdGCLock)
+    auto fdGCLock = open(fnGCLock.c_str(), O_RDWR | O_CREAT | O_CLOEXEC, 0600);
+    if (!fdGCLock)
         throw SysError("opening global GC lock '%1%'", fnGCLock);
+    return fdGCLock;
 }
 
 
@@ -1509,7 +1514,8 @@ bool LocalStore::verifyStore(bool checkContents, RepairFlag repair)
 
     /* Acquire the global GC lock to get a consistent snapshot of
        existing and valid paths. */
-    FdLock gcLock(_state.lock()->fdGCLock.get(), ltWrite, true, "waiting for the big garbage collector lock...");
+    auto fdGCLock = openGCLock();
+    FdLock gcLock(fdGCLock.get(), ltRead, true, "waiting for the big garbage collector lock...");
 
     StringSet store;
     for (auto & i : readDirectory(realStoreDir)) store.insert(i.name);
diff --git a/src/libstore/local-store.hh b/src/libstore/local-store.hh
index 85c6c4495..12c057ec3 100644
--- a/src/libstore/local-store.hh
+++ b/src/libstore/local-store.hh
@@ -158,6 +158,8 @@ private:
 
     void findTempRoots(Roots & roots, bool censor);
 
+    AutoCloseFD openGCLock();
+
 public:
 
     Roots findRoots(bool censor) override;