Add little tool to archive go.mod code to git.

LICENSE

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+BSD 3-Clause License
+
+Copyright (c) 2021 Tailscale & AUTHORS.
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+1. Redistributions of source code must retain the above copyright notice, this
+   list of conditions and the following disclaimer.
+
+2. Redistributions in binary form must reproduce the above copyright notice,
+   this list of conditions and the following disclaimer in the documentation
+   and/or other materials provided with the distribution.
+
+3. Neither the name of the copyright holder nor the names of its
+   contributors may be used to endorse or promote products derived from
+   this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

archive.go

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+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// The go-mod-archiver command pushes a git tag named after the SHA-256 of the
+// go.mod, containing all the vendored code from go.mod.
+package main
+
+import (
+	"bytes"
+	"crypto/sha256"
+	"fmt"
+	"io/ioutil"
+	"log"
+	"os"
+	"os/exec"
+)
+
+func main() {
+	goMod, err := ioutil.ReadFile("go.mod")
+	if err != nil {
+		log.Fatal(err)
+	}
+	modSum := fmt.Sprintf("%x", sha256.Sum256(goMod))
+	ref := fmt.Sprintf("archive/gomod/%v", modSum)
+
+	lsRemote, err := exec.Command("git", "ls-remote").CombinedOutput()
+	if err != nil {
+		log.Fatalf("git ls-remote: %v, %s", err, lsRemote)
+	}
+	if bytes.Contains(lsRemote, []byte(ref)) {
+		// A ref already exists, so go.mod hasn't changed and
+		// it's already archived. This is the common case.
+		log.Printf("go.mod already archived as %v", ref)
+		return
+	}
+	run(exec.Command("go", "mod", "vendor"))
+	if _, err := os.Stat("vendor"); os.IsNotExist(err) {
+		out, err := exec.Command("go", "mod", "graph").Output()
+		if err != nil {
+			log.Fatalf("go mod graph: %v", err)
+		}
+		if len(out) == 0 {
+			log.Printf("no deps; nothing to do")
+			return
+		}
+		log.Fatalf("'go mod graph' shows dependencies but vendor folder absent after a 'go mod vendor'")
+	}
+	run(exec.Command("git", "add", "vendor"))
+	msg := fmt.Sprintf("go mod vendor from a go.mod with SHA-256 of %v", modSum)
+	run(exec.Command("git", "commit", "-m", msg))
+	run(exec.Command("git", "tag", "-a", ref, "-m", msg))
+	run(exec.Command("git", "push", "origin", ref))
+	log.Printf("Pushed %v", ref)
+}
+
+func run(cmd *exec.Cmd) {
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		log.Fatalf("Running %v: %v, %s", cmd.Args, err, out)
+	}
+}

go.mod

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+module github.com/tailscale/go-mod-archiver
+
+go 1.16