fix(boatstack): guard retries helper on Linux ETXTBSY under concurrent first use

labs/12-product-engineering-loop/boatstack-distribution/release-notes/2026-07-26-guard-etxtbsy-retry.md

@@ -0,0 +1,15 @@
+### Concurrent first use no longer fails with "Text file busy" on Linux
+
+When several tool calls hit an empty shared-runtime slot at the same time, one guard
+hydrates the slot and the others wait. On Linux the kernel refuses to run a file while
+another process still holds it open for writing. It reports this as "Text file busy". A
+waiting guard could reach the run step in that brief window, fail to start the helper, and
+deny the tool call by mistake. This showed up as a flaky Linux CI failure under contention.
+macOS and Windows do not enforce this rule, so only Linux saw the denial.
+
+The guard now retries the helper a bounded number of times when the start fails with this
+exact condition, then hands off as before. The retry is short and self-clearing: the peer
+closes the file the moment its write finishes, so the next attempt starts the helper. A
+helper that genuinely cannot run still returns the same status after the retries, so no real
+failure is hidden. The runtime binary is still written atomically, so the fix only closes
+the read-side race.

labs/12-product-engineering-loop/product-engineering-loop/hooks.go

@@ -223,7 +223,24 @@ if [[ -z "$EXPECTED" || "$ACTUAL" != "$EXPECTED" ]]; then
   exit 2
 fi
 
-exec "$HELPER" bootstrap-safety-hook --host "$HOST" --repo "$ROOT"
+# Linux refuses to exec a file another process still holds open for writing
+# (ETXTBSY, surfaced as exit 126). Under concurrent first use a peer guard can be
+# finishing hydration at this instant, even though the writer replaces the binary
+# atomically. Retry briefly, then hand off. A genuinely non-executable helper keeps
+# returning 126 and the final status still propagates unchanged. Running the helper
+# as a child (not exec) is required so a failed start is observable; stdio and the
+# exit code pass through, and an ETXTBSY start never consumes stdin.
+ATTEMPT=0
+while :; do
+  "$HELPER" bootstrap-safety-hook --host "$HOST" --repo "$ROOT"
+  HELPER_STATUS=$?
+  if [[ $HELPER_STATUS -eq 126 && $ATTEMPT -lt 30 ]]; then
+    ATTEMPT=$((ATTEMPT + 1))
+    sleep 0.1
+    continue
+  fi
+  exit $HELPER_STATUS
+done
 `, Version, SourceCommit, Version, SourceCommit, Version, Version, runtimeHydrateCommandBash(Version), runtimeHydrateCommandBash(Version)))
 }