Make the gc-concurrent test more reliable

Use a fifo pipe to handle the synchronisation between the different
threads rather than relying on delays

tests/gc-concurrent.sh

@@ -2,7 +2,10 @@ source common.sh
 
 clearStore
 
-drvPath1=$(nix-instantiate gc-concurrent.nix -A test1)
+lockFifo1=$TEST_ROOT/test1.fifo
+mkfifo "$lockFifo1"
+
+drvPath1=$(nix-instantiate gc-concurrent.nix -A test1 --argstr lockFifo "$lockFifo1")
 outPath1=$(nix-store -q $drvPath1)
 
 drvPath2=$(nix-instantiate gc-concurrent.nix -A test2)
@@ -22,19 +25,16 @@ ln -s $outPath3 "$NIX_STATE_DIR"/gcroots/foo2
 nix-store -rvv "$drvPath1" &
 pid1=$!
 
-# Start build #2 in the background after 10 seconds.
-(sleep 10 && nix-store -rvv "$drvPath2") &
-pid2=$!
+# Wait for the build of $drvPath1 to start
+cat $lockFifo1
 
 # Run the garbage collector while the build is running.
-sleep 6
 nix-collect-garbage
 
-# Wait for build #1/#2 to finish.
+# Unlock the build of $drvPath1
+echo "" > $lockFifo1
 echo waiting for pid $pid1 to finish...
 wait $pid1
-echo waiting for pid $pid2 to finish...
-wait $pid2
 
 # Check that the root of build #1 and its dependencies haven't been
 # deleted.  The should not be deleted by the GC because they were
@@ -42,8 +42,9 @@ wait $pid2
 cat $outPath1/foobar
 cat $outPath1/input-2/bar
 
-# Check that build #2 has succeeded.  It should succeed because the
-# derivation is a GC root.
+# Check that the build build $drvPath2 succeeds.
+# It should succeed because the derivation is a GC root.
+nix-store -rvv "$drvPath2"
 cat $outPath2/foobar
 
 rm -f "$NIX_STATE_DIR"/gcroots/foo*