tests: regen tests and add CI to catch stale generated code

.github/workflows/build.yml

@@ -12,3 +12,12 @@ jobs:
         with:
           go-version-file: go.mod
       - run: make test
+      - name: Check tests/ generated code is up to date
+        run: |
+          make test-gen
+          if [ -n "$(git status --porcelain)" ]; then
+            echo "::error::Generated code is out of date; run 'make test-gen' and commit the result."
+            git status --porcelain
+            git diff
+            exit 1
+          fi

Makefile

@@ -11,7 +11,7 @@ SHELL := /bin/bash
 
 BIN = $(GOBIN)/msgp
 
-.PHONY: clean wipe install get-deps bench all
+.PHONY: clean wipe install get-deps bench all test-gen
 
 $(BIN): */*.go
 	@go install ./...
@@ -25,6 +25,14 @@ test: all
 	go test -covermode=atomic -coverprofile=cover.out ./...
 	cd tests && go test ./...
 
+# Regenerate tests/*_gen*.go with the msgp built from this tree, rather than
+# whatever msgp happens to be on PATH. CI runs this and fails on any diff.
+test-gen:
+	@bindir=$$(mktemp -d) && \
+	go build -o "$$bindir/msgp" . && \
+	(cd tests && PATH="$$bindir:$$PATH" go generate ./...); \
+	status=$$?; rm -rf "$$bindir"; exit $$status
+
 bench: all
 	go test -bench ./...