libstore/gc: fix auto-GC blocking indefinitely during evaluation

The first auto-GC request would not be registered as a waiter due to a logic
error. As a result, if that request was synchronous (as happens during
evaluation) it would be stuck forever waiting on a promise that will never be
fulfilled.
Register also the first request properly so that it is notified and unblocked
again when the GC has finished. Also add a test verifying that auto-GC
triggering during evaluation will not get stuck.

Fixes: https://git.lix.systems/lix-project/lix/issues/844
Change-Id: I157afdc737415261e48d6d01d46c586a2927a1ad
(cherry picked from commit 4505bfac8e)

lix/libstore/gc.cc

@@ -966,9 +966,9 @@ try {
 
     {
         auto state(_gcState.lock());
+        state->gcWaiters.push_back(std::move(pfp.fulfiller));
 
         if (state->gcRunning) {
-            state->gcWaiters.push_back(std::move(pfp.fulfiller));
             debug("waiting for auto-GC to finish");
             goto sync;
         }

tests/functional/gc-auto.sh

@@ -4,6 +4,13 @@ needLocalStore "“min-free” and “max-free” are daemon options"
 
 clearStore
 
+fake_free=$TEST_ROOT/fake-free
+export _NIX_TEST_FREE_SPACE_FILE=$fake_free
+echo 1100 > $fake_free
+
+# Check that auto-GC during evaluation progresses.
+timeout --signal=KILL 10s nix eval --expr 'builtins.toFile "meow" "meow"' --min-free 2000
+
 garbage1=$(nix store add-path --name garbage1 ./nar-access.sh)
 garbage2=$(nix store add-path --name garbage2 ./nar-access.sh)
 garbage3=$(nix store add-path --name garbage3 ./nar-access.sh)
@@ -11,10 +18,6 @@ garbage3=$(nix store add-path --name garbage3 ./nar-access.sh)
 ls -l $garbage3
 POSIXLY_CORRECT=1 du $garbage3
 
-fake_free=$TEST_ROOT/fake-free
-export _NIX_TEST_FREE_SPACE_FILE=$fake_free
-echo 1100 > $fake_free
-
 fifoLock=$TEST_ROOT/fifoLock
 mkfifo "$fifoLock"