* Add a test to demonstrate the quadratic complexity of referrer

(de)registration, in particular garbage collection (NIX-23).

tests/Makefile.am

@@ -39,7 +39,7 @@ gc-concurrent.sh: gc-concurrent.nix gc-concurrent2.nix
 
 TESTS = init.sh hash.sh lang.sh simple.sh dependencies.sh locking.sh parallel.sh \
   build-hook.sh substitutes.sh substitutes2.sh fallback.sh nix-push.sh gc.sh \
-  gc-concurrent.sh verify.sh nix-pull.sh
+  gc-concurrent.sh verify.sh nix-pull.sh referrers.sh
 
 XFAIL_TESTS =
 

tests/referrers.sh

@@ -0,0 +1,15 @@
+max=5000
+
+reference=$NIX_STORE_DIR/abcdef
+touch $reference
+(echo $reference && echo && echo 0) | $TOP/src/nix-store/nix-store --register-validity 
+
+echo "registering..."
+time for ((n = 0; n < $max; n ++)); do
+    storePath=$NIX_STORE_DIR/$n
+    touch $storePath
+    (echo $storePath && echo && echo 1 && echo $reference)
+done | $TOP/src/nix-store/nix-store --register-validity 
+
+echo "collecting garbage..."
+time $TOP/src/nix-store/nix-store --gc 2> /dev/null