test: cover interrupted watch build trace events (#14802)

Add a standalone cram test for a watch build that is interrupted by a
file change. The test checks the trace event ordering and run-id
relationship: the build-restart event is emitted before the interrupted
build-finish event, and the restarted build completes successfully.

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

Author: rgrinberg

test/blackbox-tests/test-cases/trace/dune

@@ -1,5 +1,5 @@
 (cram
- (applies_to watch-build-events debug-event)
+ (applies_to watch-build-events debug-event interrupted-watch-build-events)
  (enabled_if
   (= %{system} linux)))
 

test/blackbox-tests/test-cases/trace/interrupted-watch-build-events.t

@@ -0,0 +1,82 @@
+Interrupted watch builds emit a restart event before the interrupted iteration's
+build-finish event.
+
+  $ make_dune_project 3.22
+
+  $ cat > x <<EOF
+  > original
+  > EOF
+
+  $ cat > build.sh <<'EOF'
+  > if [ "$(cat "$1")" = unstable ]; then
+  >   touch "$3"
+  >   sleep 1000
+  >   exit 1
+  > else
+  >   cat "$1" > "$2"
+  > fi
+  > EOF
+
+Keep the synchronization marker outside the workspace so it does not enqueue its
+own source invalidation.
+
+  $ marker="$TMPDIR/go-ahead"
+  $ cat > dune <<EOF
+  > (rule
+  >  (target y)
+  >  (deps x build.sh)
+  >  (action (run bash %{dep:build.sh} %{dep:x} y "$marker")))
+  > EOF
+
+  $ rm -f "$marker"
+  $ start_dune @idle
+
+  $ build y
+  Success
+  $ cat _build/default/y
+  original
+
+  $ echo unstable > x
+  $ (dune_cmd wait-for-file-to-appear "$marker"; echo updated > x) & build y
+  Success
+  $ cat _build/default/y
+  updated
+
+  $ stop_dune > /dev/null
+
+  $ dune trace cat | jq -s '
+  > include "dune";
+  > [ .[] | buildEvents | del(.args.rusage) ] | .[-4:]'
+  [
+    {
+      "args": {
+        "run_id": 2,
+        "restart": true
+      },
+      "name": "build-start"
+    },
+    {
+      "args": {
+        "run_id": 3,
+        "reasons": [
+          "x changed"
+        ]
+      },
+      "name": "build-restart"
+    },
+    {
+      "args": {
+        "run_id": 3,
+        "restart": true
+      },
+      "name": "build-start"
+    },
+    {
+      "args": {
+        "run_id": 3,
+        "outcome": "success",
+        "restart_duration": "number"
+      },
+      "name": "build-finish"
+    }
+  ]