tests/e2e/188: the "did not rerun" assertion could not fail

It read `grep -q "GENERATE" b3.log && FAIL`, on the assumption that ninja's
description line for the edge reaches that log. It does not. `mcpp build`
passes `--quiet` to ninja whenever it is not `--verbose` and surfaces ninja's
captured stdout only on failure, so a from-scratch build -- which
unquestionably runs the action -- prints no such line either. The grep never
matched, and the check could not fail whether the action reran or not.

Found while designing the depfile test, whose first draft copied this
convention and would have inherited the same emptiness.

The generated file's modification time measures the thing being asserted
directly: the action rewrites it whenever it runs. Verified in both directions
against the same binary -- unchanged after touching an unrelated source, and
the assertion fails naming both timestamps when the touched file is a declared
input of the action instead.

Author: speak-agent

tests/e2e/188_build_actions.sh

@@ -122,10 +122,29 @@ out="$("$MCPP" run 2>&1 | grep '^VALUE=' | tail -1)"
     cat b2.log; echo "FAIL: action did not re-run when its input changed: $out"; exit 1; }
 
 # ...and an unrelated rebuild must NOT re-run it (that is the whole point).
+#
+# THIS ASSERTION USED TO BE VACUOUS, AND THAT IS WORTH THE COMMENT. It read
+# `grep -q "GENERATE" b3.log && FAIL`, on the assumption that ninja's
+# description line for the edge reaches this log. It does not: `mcpp build`
+# passes `--quiet` to ninja whenever it is not `--verbose`, and surfaces
+# ninja's captured stdout only on failure. A from-scratch build, which
+# unquestionably runs the action, prints no such line either -- so the grep
+# never matched and the check could not fail whether the action reran or not.
+#
+# The generated file's modification time is a direct measurement of the thing
+# being asserted: the action rewrites it whenever it runs.
+before=$(stat -c %Y "$(find target -name gen.cpp -print -quit)" 2>/dev/null \
+       || stat -f %m "$(find target -name gen.cpp -print -quit)")
 touch src/main.cpp
+sleep 1        # coarser than any filesystem's mtime granularity here
 "$MCPP" build > b3.log 2>&1 || { cat b3.log; echo "FAIL: rebuild failed"; exit 1; }
-grep -q "GENERATE" b3.log && {
-    cat b3.log; echo "FAIL: the action re-ran although its inputs were unchanged"; exit 1; }
+after=$(stat -c %Y "$(find target -name gen.cpp -print -quit)" 2>/dev/null \
+      || stat -f %m "$(find target -name gen.cpp -print -quit)")
+[[ -n "$before" && "$before" == "$after" ]] || {
+    cat b3.log
+    echo "FAIL: the action re-ran although its inputs were unchanged"
+    echo "      (gen.cpp mtime $before -> $after)"
+    exit 1; }
 
 # ── 2b. changing the action's COMMAND also takes effect ────────────────────
 # Distinct from 2: there the action's declared INPUT changed and ninja noticed.