Add script to reproduce issue by inducing heavy load.

repro-7998.sh

@@ -0,0 +1,33 @@
+#!/usr/bin/env bash
+set -eux
+LOG_FILE=/tmp/repro-7998.log
+rm -f "$LOG_FILE"
+for i in {1..8}; do
+  (
+    while true; do
+      nix-build \
+        --argstr uuid $(uuidgen) \
+        --arg drvCount $((RANDOM % 256)) \
+        -E '
+          { uuid ? "00000000-0000-0000-0000-000000000000", drvCount ? 0 }:
+          with import <nixpkgs> { };
+          let
+            mkDrv = name: buildInputs:
+              stdenv.mkDerivation {
+                inherit name;
+                inherit buildInputs;
+                unpackPhase = "date +\"${uuid} %F %T\" >date.txt";
+                installPhase = "mkdir -p $out; cp date.txt $out/";
+              };
+            mkDrvs = n:
+              let
+                name = "repro-7998-${toString n}";
+                buildInputs = if n == 0 then [ ] else [ (mkDrvs (n - 1)) ];
+              in mkDrv name buildInputs;
+          in mkDrvs drvCount
+        '
+    done 2>&1 | tee -a "$LOG_FILE"
+  ) &
+done
+read # Press enter to stop
+pkill -KILL -f repro-7998.sh