cmd: Pipe output through pager in terminals

Replicate git's behavior of piping output through a pager when it
doesn't fit a single screen. This is not only more convenient than
piping into less on the command line, but preserves terminal features
like colored output as well.

cmd/ci_status.go

@@ -43,6 +43,9 @@ lab ci status --wait`,
 
 		pid := rn
 
+		pager := NewPager(cmd.Flags())
+		defer pager.Close()
+
 		w := tabwriter.NewWriter(os.Stdout, 2, 4, 1, byte(' '), 0)
 
 		wait, err := cmd.Flags().GetBool("wait")

cmd/ci_trace.go

@@ -53,6 +53,9 @@ var ciTraceCmd = &cobra.Command{
 		}
 		projectID = project.ID
 
+		pager := NewPager(cmd.Flags())
+		defer pager.Close()
+
 		err = doTrace(context.Background(), os.Stdout, projectID, pipelineID, jobName)
 		if err != nil {
 			log.Fatal(err)

cmd/issue_list.go

@@ -37,6 +37,10 @@ lab issue list remote "search terms"  # search "remote" for issues with "search
 		if err != nil {
 			log.Fatal(err)
 		}
+
+		pager := NewPager(cmd.Flags())
+		defer pager.Close()
+
 		for _, issue := range issues {
 			fmt.Printf("#%d %s\n", issue.IID, issue.Title)
 		}

cmd/issue_show.go

@@ -42,6 +42,9 @@ var issueShowCmd = &cobra.Command{
 			renderMarkdown = !noMarkdown
 		}
 
+		pager := NewPager(cmd.Flags())
+		defer pager.Close()
+
 		printIssue(issue, rn, renderMarkdown)
 
 		showComments, _ := cmd.Flags().GetBool("comments")

cmd/label_list.go

@@ -35,6 +35,9 @@ lab label list remote "search term"  # search "remote" for labels with "search t
 			log.Fatal(err)
 		}
 
+		pager := NewPager(cmd.Flags())
+		defer pager.Close()
+
 		for _, label := range labels {
 			// GitLab API has no search for labels, so we do it ourselves
 			if labelSearch != "" &&

cmd/mr_list.go

@@ -50,6 +50,10 @@ lab mr list remote "search terms"  # search "remote" for merge requests with "se
 			log.Print(err)
 			config.UserConfigError()
 		}
+
+		pager := NewPager(cmd.Flags())
+		defer pager.Close()
+
 		for _, mr := range mrs {
 			fmt.Printf("!%d %s\n", mr.IID, mr.Title)
 		}

cmd/mr_show.go

@@ -65,6 +65,9 @@ var mrShowCmd = &cobra.Command{
 			}
 			git.Show(remote+"/"+mr.TargetBranch, mr.SHA, mrShowPatchReverse)
 		} else {
+			pager := NewPager(cmd.Flags())
+			defer pager.Close()
+
 			printMR(mr, rn, renderMarkdown)
 		}
 

cmd/project_list.go

@@ -48,6 +48,10 @@ var projectListCmd = &cobra.Command{
 		if err != nil {
 			log.Fatal(err)
 		}
+
+		pager := NewPager(cmd.Flags())
+		defer pager.Close()
+
 		for _, p := range projects {
 			fmt.Println(p.PathWithNamespace)
 		}

cmd/root.go

@@ -102,6 +102,7 @@ func init() {
 	RootCmd.SetHelpCommand(helpCmd)
 	RootCmd.SetHelpFunc(helpFunc)
 	RootCmd.Flags().Bool("version", false, "Show the lab version")
+	RootCmd.PersistentFlags().Bool("no-pager", false, "Do not pipe output into a pager")
 }
 
 var (

cmd/snippet_list.go

@@ -28,6 +28,8 @@ var snippetListCmd = &cobra.Command{
 		if err != nil {
 			log.Fatal(err)
 		}
+		pager := NewPager(cmd.Flags())
+		defer pager.Close()
 		for _, snip := range snips {
 			fmt.Printf("#%d %s\n", snip.ID, snip.Title)
 		}

cmd/util.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"log"
 	"os"
+	"os/exec"
 	"strconv"
 	"strings"
 	"syscall"
@@ -419,6 +420,50 @@ func isOutputTerminal() bool {
 	return true
 }
 
+type Pager struct {
+	proc   *os.Process
+	stdout int
+}
+
+// If standard output is a terminal, redirect output to an external
+// pager until the returned object's Close() method is called
+func NewPager(fs *flag.FlagSet) *Pager {
+	cmdLine, env := git.PagerCommand()
+	args := strings.Split(cmdLine, " ")
+
+	noPager, _ := fs.GetBool("no-pager")
+	if !isOutputTerminal() || noPager || args[0] == "cat" {
+		return &Pager{}
+	}
+
+	pr, pw, _ := os.Pipe()
+	defer pw.Close()
+
+	name, _ := exec.LookPath(args[0])
+	proc, _ := os.StartProcess(name, args, &os.ProcAttr{
+		Env:   env,
+		Files: []*os.File{pr, os.Stdout, os.Stderr},
+	})
+
+	savedStdout, _ := syscall.Dup(syscall.Stdout)
+	_ = syscall.Dup2(int(pw.Fd()), syscall.Stdout)
+
+	return &Pager{
+		proc:   proc,
+		stdout: savedStdout,
+	}
+}
+
+func (pager *Pager) Close() {
+	if pager.stdout > 0 {
+		_ = syscall.Dup2(pager.stdout, syscall.Stdout)
+		_ = syscall.Close(pager.stdout)
+	}
+	if pager.proc != nil {
+		pager.proc.Wait()
+	}
+}
+
 func LabPersistentPreRun(cmd *cobra.Command, args []string) {
 	flagConfig(cmd.Flags())
 }

internal/git/git.go

@@ -86,6 +86,37 @@ func CommentChar() string {
 	return "#"
 }
 
+// PagerCommand returns the commandline and environment for the pager
+func PagerCommand() (string, []string) {
+	// Set up environment for common pagers, see the documentation
+	// for "core.pager" in git-config(1)
+	env := os.Environ()
+	if _, ok := os.LookupEnv("LESS"); !ok {
+		env = append(env, "LESS=FRX")
+	}
+	if _, ok := os.LookupEnv("LESSSECURE"); !ok {
+		env = append(env, "LESSSECURE=1")
+	}
+	if _, ok := os.LookupEnv("LV"); !ok {
+		env = append(env, "LV=-c")
+	}
+
+	// Find an appropriate pager command, following git's preference
+	cmd, ok := os.LookupEnv("GIT_PAGER")
+	if ok {
+		return cmd, env
+	}
+	cmd, err := gitconfig.Entire("core.pager")
+	if err == nil {
+		return cmd, env
+	}
+	cmd, ok = os.LookupEnv("PAGER")
+	if ok {
+		return cmd, env
+	}
+	return "less", env
+}
+
 // LastCommitMessage returns the last commits message as one line
 func LastCommitMessage() (string, error) {
 	cmd := New("show", "-s", "--format=%s%n%+b", "HEAD")