Move version.go out of main and use BuildInfo.

Move version.go from main to the router module (lib/) so that it can
(later) be accessed from Router itself.

Make it use standard debug.BuildInfo that the Go toolchain produced
automatically, so we no longer have to mess about with shell scripts
and linker flags.

This eliminates some unnecessary differences between the Dockerfile and
Makefile builds.

.dockerignore

@@ -1,6 +1,8 @@
-.git
-.gitignore
-Dockerfile
-README.md
-docs
-integration_tests
+#
+# Only files that are untracked by Git should be added here.
+#
+# The builder container needs to see a pristine checkout, otherwise
+# vcs.modified in the BuildInfo will always be true, i.e. the build will always
+# be marked as "dirty".
+#
+/router

.gitignore

@@ -1,2 +1,2 @@
-router
+/router
 __build

Dockerfile

@@ -1,8 +1,22 @@
-FROM golang:1.20-alpine AS builder
+ARG go_registry=""
+ARG go_version=1.20
+ARG go_tag_suffix=-alpine
+
+FROM ${go_registry}golang:${go_version}${go_tag_suffix} AS builder
 ARG TARGETARCH TARGETOS
+ARG GOARCH=$TARGETARCH GOOS=$TARGETOS
+ARG CGO_ENABLED=0
+ARG GOFLAGS="-trimpath"
+ARG go_ldflags="-s -w"
+# Go needs git for `-buildvcs`, but the alpine version lacks git :( It's still
+# way cheaper to `apk add git` than to pull the Debian-based golang image.
+# hadolint ignore=DL3018
+RUN apk add --no-cache git
 WORKDIR /src
 COPY . ./
-RUN CGO_ENABLED=0 GOARCH=$TARGETARCH GOOS=$TARGETOS go build -trimpath -ldflags="-s -w"
+RUN go build -ldflags="$go_ldflags" && \
+    ./router -version && \
+    go version -m ./router
 
 FROM scratch
 COPY --from=builder /src/router /bin/router

Makefile

@@ -1,35 +1,29 @@
-.PHONY: all clean build lint test unit_tests integration_tests start_mongo stop_mongo
+.PHONY: all clean build test lint unit_tests integration_tests start_mongo stop_mongo
 .NOTPARALLEL:
 
-BINARY ?= router
+TARGET_MODULE := router
+GO_BUILD_ENV := CGO_ENABLED=0
 SHELL := /bin/dash
 
-ifdef RELEASE_VERSION
-VERSION := $(RELEASE_VERSION)
-else
-VERSION := $(shell git describe --always | tr -d '\n'; test -z "`git status --porcelain`" || echo '-dirty')
-endif
-
-all: build test
+all: build
 
 clean:
-	rm -f $(BINARY)
+	rm -f $(TARGET_MODULE)
 
 build:
-	go build -ldflags "-X main.version=$(VERSION)" -o $(BINARY)
+	env $(GO_BUILD_ENV) go build
+	./$(TARGET_MODULE) -version
+
+test: lint unit_tests integration_tests
 
 lint:
 	golangci-lint run
 
-test: lint unit_tests integration_tests
-
-unit_tests: build
+unit_tests:
 	go test -race $$(go list ./... | grep -v integration_tests)
 
 integration_tests: build start_mongo
-	ROUTER_PUBADDR=localhost:8080 \
-	ROUTER_APIADDR=localhost:8081 \
-		go test -race -v ./integration_tests
+	go test -race -v ./integration_tests
 
 start_mongo:
 	./mongo.sh start

lib/version.go

@@ -0,0 +1,42 @@
+package router
+
+import (
+	"fmt"
+	"runtime/debug"
+)
+
+// VersionInfo returns human-readable version information in a format suitable
+// for concatenation with other messages.
+func VersionInfo() (v string) {
+	v = "(version info unavailable)"
+
+	bi, ok := debug.ReadBuildInfo()
+	if !ok {
+		return
+	}
+
+	rev, commitTime, dirty := buildSettings(bi.Settings)
+	if rev == "" {
+		return
+	}
+
+	commitTimeOrDirty := "dirty"
+	if dirty == "false" {
+		commitTimeOrDirty = commitTime
+	}
+	return fmt.Sprintf("built from commit %.8s (%s) using %s", rev, commitTimeOrDirty, bi.GoVersion)
+}
+
+func buildSettings(bs []debug.BuildSetting) (rev, commitTime, dirty string) {
+	for _, b := range bs {
+		switch b.Key {
+		case "vcs.modified":
+			dirty = b.Value
+		case "vcs.revision":
+			rev = b.Value
+		case "vcs.time":
+			commitTime = b.Value
+		}
+	}
+	return
+}

main.go

@@ -35,7 +35,7 @@ ROUTER_BACKEND_HEADER_TIMEOUT=15s  Timeout for backend response headers to be re
 ROUTER_FRONTEND_READ_TIMEOUT=60s   See https://cs.opensource.google/go/go/+/master:src/net/http/server.go?q=symbol:ReadTimeout
 ROUTER_FRONTEND_WRITE_TIMEOUT=60s  See https://cs.opensource.google/go/go/+/master:src/net/http/server.go?q=symbol:WriteTimeout
 `
-	fmt.Fprintf(os.Stderr, helpstring, versionInfo(), os.Args[0])
+	fmt.Fprintf(os.Stderr, helpstring, router.VersionInfo(), os.Args[0])
 	const ErrUsage = 64
 	os.Exit(ErrUsage)
 }
@@ -89,7 +89,7 @@ func main() {
 	flag.Usage = usage
 	flag.Parse()
 	if *returnVersion {
-		fmt.Printf("GOV.UK Router %s\n", versionInfo())
+		fmt.Printf("GOV.UK Router %s\n", router.VersionInfo())
 		os.Exit(0)
 	}