misc/git_hook: extract shared githook package; auto-rebuild on version bump (#19440)

Pull the hook logic into a reusable githook library package so
tailscale/corp can share it via a thin wrapper main instead of
keeping a forked copy in sync.

The install flow also changes: a wrapper scripts now build the
binary and reinstall the git hooks. Pulling new shared code no
longer requires re-running the installer.

Updates tailscale/corp#39860

Change-Id: I4d606d11c8c883015c190c54e3387a7f9fe4dd32

Signed-off-by: Fernando Serboncini <fserb@tailscale.com>

misc/install-git-hooks.go

@@ -3,80 +3,19 @@
 
 //go:build ignore
 
-// The install-git-hooks program installs git hooks.
-//
-// It installs a Go binary at .git/hooks/ts-git-hook and a pre-hook
-// forwarding shell wrapper to .git/hooks/NAME.
+// The install-git-hooks program installs git hooks by delegating to
+// githook.Install. See that function's doc for what it does.
 package main
 
 import (
-	"fmt"
 	"log"
-	"os"
-	"os/exec"
-	"path/filepath"
-	"runtime"
-	"strings"
+
+	"tailscale.com/misc/git_hook/githook"
 )
 
-var hooks = []string{
-	"pre-push",
-	"pre-commit",
-	"commit-msg",
-	"post-checkout",
-}
-
-func fatalf(format string, a ...any) {
-	log.SetFlags(0)
-	log.Fatalf("install-git-hooks: "+format, a...)
-}
-
 func main() {
-	out, err := exec.Command("git", "rev-parse", "--git-common-dir").CombinedOutput()
-	if err != nil {
-		fatalf("finding git dir: %v, %s", err, out)
-	}
-	gitDir := strings.TrimSpace(string(out))
-
-	hookDir := filepath.Join(gitDir, "hooks")
-	if fi, err := os.Stat(hookDir); err != nil {
-		fatalf("checking hooks dir: %v", err)
-	} else if !fi.IsDir() {
-		fatalf("%s is not a directory", hookDir)
-	}
-
-	buildOut, err := exec.Command(goBin(), "build",
-		"-o", filepath.Join(hookDir, "ts-git-hook"+exe()),
-		"./misc/git_hook").CombinedOutput()
-	if err != nil {
-		log.Fatalf("go build git-hook: %v, %s", err, buildOut)
-	}
-
-	for _, hook := range hooks {
-		content := fmt.Sprintf(hookScript, hook)
-		file := filepath.Join(hookDir, hook)
-		// Install the hook. If it already exists, overwrite it, in case there's
-		// been changes.
-		if err := os.WriteFile(file, []byte(content), 0755); err != nil {
-			fatalf("%v", err)
-		}
+	log.SetFlags(0)
+	if err := githook.Install(); err != nil {
+		log.Fatalf("install-git-hooks: %v", err)
 	}
 }
-
-const hookScript = `#!/usr/bin/env bash
-exec "$(dirname ${BASH_SOURCE[0]})/ts-git-hook" %s "$@"
-`
-
-func goBin() string {
-	if p, err := exec.LookPath("go"); err == nil {
-		return p
-	}
-	return "go"
-}
-
-func exe() string {
-	if runtime.GOOS == "windows" {
-		return ".exe"
-	}
-	return ""
-}