Port select portions of Makefile to script/build.go

This is to enable build tasks on Windows.

script/build.go

@@ -0,0 +1,196 @@
+// Build tasks for the GitHub CLI project.
+//
+// Usage:  go run script/build.go [<task>]
+//
+// Known tasks are:
+//
+//   bin/gh:
+//     Builds the main executable.
+//     Supported environment variables:
+//     - GH_VERSION: determined from source by default
+//     - GH_OAUTH_CLIENT_ID
+//     - GH_OAUTH_CLIENT_SECRET
+//     - SOURCE_DATE_EPOCH: enables reproducible builds
+//     - GO_LDFLAGS
+//
+//   manpages:
+//     Builds the man pages under `share/man/man1/`.
+//
+//   clean:
+//     Deletes all built files.
+//
+
+package main
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+	"strconv"
+	"strings"
+	"time"
+
+	"github.com/cli/safeexec"
+)
+
+var tasks = map[string]func(string) error{
+	"bin/gh": func(exe string) error {
+		info, err := os.Stat(exe)
+		if err == nil && !sourceFilesLaterThan(info.ModTime()) {
+			fmt.Printf("%s: `%s` is up to date.\n", self, exe)
+			return nil
+		}
+
+		ldflags := os.Getenv("GO_LDFLAGS")
+		ldflags = fmt.Sprintf("-X github.com/cli/cli/internal/build.Version=%s %s", version(), ldflags)
+		ldflags = fmt.Sprintf("-X github.com/cli/cli/internal/build.Date=%s %s", date(), ldflags)
+		if oauthSecret := os.Getenv("GH_OAUTH_CLIENT_SECRET"); oauthSecret != "" {
+			ldflags = fmt.Sprintf("-X github.com/cli/cli/internal/authflow.oauthClientSecret=%s %s", oauthSecret, ldflags)
+			ldflags = fmt.Sprintf("-X github.com/cli/cli/internal/authflow.oauthClientID=%s %s", os.Getenv("GH_OAUTH_CLIENT_ID"), ldflags)
+		}
+
+		return run("go", "build", "-trimpath", "-ldflags", ldflags, "-o", exe, "./cmd/gh")
+	},
+	"manpages": func(_ string) error {
+		return run("go", "run", "./cmd/gen-docs", "--man-page", "--doc-path", "./share/man/man1/")
+	},
+	"clean": func(_ string) error {
+		return rmrf("bin", "share")
+	},
+}
+
+var self string
+
+func main() {
+	task := "bin/gh"
+	if runtime.GOOS == "windows" {
+		task = "bin\\gh.exe"
+	}
+
+	if len(os.Args) > 1 {
+		task = os.Args[1]
+	}
+
+	self = filepath.Base(os.Args[0])
+	if self == "build" {
+		self = "build.go"
+	}
+
+	t := tasks[normalizeTask(task)]
+	if t == nil {
+		fmt.Fprintf(os.Stderr, "Don't know how to build task `%s`.\n", task)
+		os.Exit(1)
+	}
+
+	err := t(task)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		fmt.Fprintf(os.Stderr, "%s: building task `%s` failed.\n", self, task)
+		os.Exit(1)
+	}
+}
+
+func version() string {
+	if versionEnv := os.Getenv("GH_VERSION"); versionEnv != "" {
+		return versionEnv
+	}
+	if desc, err := cmdOutput("git", "describe", "--tags"); err == nil {
+		return desc
+	}
+	rev, _ := cmdOutput("git", "rev-parse", "--short", "HEAD")
+	return rev
+}
+
+func date() string {
+	t := time.Now()
+	if sourceDate := os.Getenv("SOURCE_DATE_EPOCH"); sourceDate != "" {
+		if sec, err := strconv.ParseInt(sourceDate, 10, 64); err == nil {
+			t = time.Unix(sec, 0)
+		}
+	}
+	return t.Format("2006-01-02")
+}
+
+func sourceFilesLaterThan(t time.Time) bool {
+	foundLater := false
+	_ = filepath.Walk(".", func(path string, info os.FileInfo, err error) error {
+		if err != nil {
+			return err
+		}
+		if foundLater {
+			return filepath.SkipDir
+		}
+		if len(path) > 1 && (path[0] == '.' || path[0] == '_') {
+			if info.IsDir() {
+				return filepath.SkipDir
+			} else {
+				return nil
+			}
+		}
+		if info.IsDir() {
+			return nil
+		}
+		if path == "go.mod" || path == "go.sum" || (strings.HasSuffix(path, ".go") && !strings.HasSuffix(path, "_test.go")) {
+			if info.ModTime().After(t) {
+				foundLater = true
+			}
+		}
+		return nil
+	})
+	return foundLater
+}
+
+func rmrf(targets ...string) error {
+	args := append([]string{"rm", "-rf"}, targets...)
+	announce(args...)
+	for _, target := range targets {
+		if err := os.RemoveAll(target); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func announce(args ...string) {
+	fmt.Println(shellInspect(args))
+}
+
+func run(args ...string) error {
+	exe, err := safeexec.LookPath(args[0])
+	if err != nil {
+		return err
+	}
+	announce(args...)
+	cmd := exec.Command(exe, args[1:]...)
+	return cmd.Run()
+}
+
+func cmdOutput(args ...string) (string, error) {
+	exe, err := safeexec.LookPath(args[0])
+	if err != nil {
+		return "", err
+	}
+	cmd := exec.Command(exe, args[1:]...)
+	cmd.Stderr = ioutil.Discard
+	out, err := cmd.Output()
+	return strings.TrimSuffix(string(out), "\n"), err
+}
+
+func shellInspect(args []string) string {
+	fmtArgs := make([]string, len(args))
+	for i, arg := range args {
+		if strings.ContainsAny(arg, " \t'\"") {
+			fmtArgs[i] = fmt.Sprintf("%q", arg)
+		} else {
+			fmtArgs[i] = arg
+		}
+	}
+	return strings.Join(fmtArgs, " ")
+}
+
+func normalizeTask(t string) string {
+	return filepath.ToSlash(strings.TrimSuffix(t, ".exe"))
+}