Extract the reusable parts of a command pipeline

PipeCommands runs a chain of commands to completion and gathers what
they wrote to stderr. Rendering a diff through a renderer needs the same
chain, but has to read the last command's output as it arrives, so it
can't use PipeCommands as it stands.

Pull out what both need: naming the chain for a log, wiring each
command's output to the next one's input, and starting them all with the
cleanup a failure to start requires.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

pkg/commands/oscommands/os.go

@@ -4,12 +4,10 @@ import (
 	"bytes"
 	"io"
 	"os"
-	"os/exec"
 	"path/filepath"
 	"strings"
 
 	"github.com/go-errors/errors"
-	"github.com/samber/lo"
 
 	"github.com/atotto/clipboard"
 	"github.com/jesseduffield/lazygit/pkg/common"
@@ -203,61 +201,25 @@ func (c *OSCommand) FileExists(path string) (bool, error) {
 
 // PipeCommands runs a heap of commands and pipes their inputs/outputs together like A | B | C
 func (c *OSCommand) PipeCommands(cmdObjs ...*CmdObj) error {
-	cmds := lo.Map(cmdObjs, func(cmdObj *CmdObj, _ int) *exec.Cmd {
-		return cmdObj.GetCmd()
-	})
+	c.LogCommand(pipelineString(cmdObjs), true)
 
-	logCmdStr := strings.Join(
-		lo.Map(cmdObjs, func(cmdObj *CmdObj, _ int) string {
-			return cmdObj.ToString()
-		}),
-		" | ",
-	)
-
-	c.LogCommand(logCmdStr, true)
-
-	for i := range len(cmds) - 1 {
-		stdout, err := cmds[i].StdoutPipe()
-		if err != nil {
-			return err
-		}
-
-		cmds[i+1].Stdin = stdout
+	cmds, err := wirePipeline(cmdObjs)
+	if err != nil {
+		return err
 	}
 
-	// keeping this here in case I adapt this code for some other purpose in the future
-	// cmds[len(cmds)-1].Stdout = os.Stdout
-
 	stderrs := make([]bytes.Buffer, len(cmds))
 	for i := range cmds {
 		cmds[i].Stderr = &stderrs[i]
 	}
 
-	// Start every command before waiting for any of them: waiting for a command
-	// closes our end of the pipe that feeds the next one, and a command that
-	// hasn't been started by then would inherit a closed stdin.
-	started := 0
-	var startErr error
-	for _, cmd := range cmds {
-		if err := cmd.Start(); err != nil {
-			startErr = err
-			break
-		}
-
-		started++
-	}
+	started, startErr := startPipeline(cmds)
 
 	finalErrors := []string{}
 
 	if startErr != nil {
 		c.Log.Error(startErr)
 		finalErrors = append(finalErrors, startErr.Error())
-
-		// Without the rest of the pipeline to drain them, the commands we did
-		// start could block forever writing to a full pipe.
-		for _, cmd := range cmds[:started] {
-			_ = cmd.Process.Kill()
-		}
 	}
 
 	for i, cmd := range cmds[:started] {

pkg/commands/oscommands/pipeline.go

@@ -0,0 +1,60 @@
+package oscommands
+
+import (
+	"os/exec"
+	"strings"
+
+	"github.com/samber/lo"
+)
+
+// pipelineString names a chain of commands the way a shell would write it.
+func pipelineString(cmdObjs []*CmdObj) string {
+	return strings.Join(
+		lo.Map(cmdObjs, func(cmdObj *CmdObj, _ int) string {
+			return cmdObj.ToString()
+		}),
+		" | ",
+	)
+}
+
+// wirePipeline connects each command's output to the next one's input, like
+// A | B | C. The last command's output is left for the caller to direct.
+func wirePipeline(cmdObjs []*CmdObj) ([]*exec.Cmd, error) {
+	cmds := lo.Map(cmdObjs, func(cmdObj *CmdObj, _ int) *exec.Cmd {
+		return cmdObj.GetCmd()
+	})
+
+	for i := range len(cmds) - 1 {
+		stdout, err := cmds[i].StdoutPipe()
+		if err != nil {
+			return nil, err
+		}
+
+		cmds[i+1].Stdin = stdout
+	}
+
+	return cmds, nil
+}
+
+// startPipeline starts every command and reports how many it got going. Every
+// one is started before any of them is waited for: waiting closes our end of
+// the pipe that feeds the next command, and one that hasn't been started by
+// then would inherit a closed stdin.
+//
+// When a command fails to start, the ones already running are killed, since
+// without the rest of the pipeline to drain them they could block forever
+// writing to a full pipe. They still have to be reaped, so the count covers
+// them too.
+func startPipeline(cmds []*exec.Cmd) (int, error) {
+	for i, cmd := range cmds {
+		if err := cmd.Start(); err != nil {
+			for _, started := range cmds[:i] {
+				_ = started.Process.Kill()
+			}
+
+			return i, err
+		}
+	}
+
+	return len(cmds), nil
+}