internal/pkg: exec prefix verbose output

This proxies program output through msg with a name and fd prefix. This also avoids introducing additional information to the container via process stdout/stderr.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/exec.go

@@ -1,11 +1,14 @@
 package pkg
 
 import (
+	"bufio"
 	"bytes"
 	"context"
 	"errors"
 	"fmt"
+	"io"
 	"os"
+	"os/exec"
 	"path"
 	"slices"
 	"strconv"
@@ -16,6 +19,7 @@ import (
 	"hakurei.app/container/check"
 	"hakurei.app/container/fhs"
 	"hakurei.app/container/std"
+	"hakurei.app/message"
 )
 
 // AbsWork is the container pathname [CureContext.GetWorkDir] is mounted on.
@@ -57,7 +61,8 @@ const (
 // Methods of execArtifact does not modify any struct field or underlying arrays
 // referred to by slices.
 type execArtifact struct {
-	// Caller-supplied user-facing reporting name.
+	// Caller-supplied user-facing reporting name, guaranteed to be nonzero
+	// during initialisation.
 	name string
 	// Caller-supplied inner mount points.
 	paths []ExecPath
@@ -219,6 +224,23 @@ const (
 	execWaitDelay = time.Nanosecond
 )
 
+// scanVerbose prefixes program output for a verbose [message.Msg].
+func scanVerbose(
+	msg message.Msg,
+	done chan<- struct{},
+	prefix string,
+	r io.Reader,
+) {
+	defer close(done)
+	s := bufio.NewScanner(r)
+	for s.Scan() {
+		msg.Verbose(prefix, s.Text())
+	}
+	if err := s.Err(); err != nil && !errors.Is(err, os.ErrClosed) {
+		msg.Verbose("*"+prefix, err)
+	}
+}
+
 // cure is like Cure but allows optional host net namespace. This is used for
 // the [KnownChecksum] variant where networking is allowed.
 func (a *execArtifact) cure(f *FContext, hostNet bool) (err error) {
@@ -250,8 +272,26 @@ func (a *execArtifact) cure(f *FContext, hostNet bool) (err error) {
 		z.Hostname = "cure-net"
 	}
 	z.Uid, z.Gid = (1<<10)-1, (1<<10)-1
-	if f.GetMessage().IsVerbose() {
+	if msg := f.GetMessage(); msg.IsVerbose() {
-		z.Stdout, z.Stderr = os.Stdout, os.Stderr
+		var stdout, stderr io.ReadCloser
+		if stdout, err = z.StdoutPipe(); err != nil {
+			return
+		}
+		if stderr, err = z.StderrPipe(); err != nil {
+			_ = stdout.Close()
+			return
+		}
+		defer func() {
+			if err != nil && !errors.As(err, new(*exec.ExitError)) {
+				_ = stdout.Close()
+				_ = stderr.Close()
+			}
+		}()
+
+		stdoutDone, stderrDone := make(chan struct{}), make(chan struct{})
+		go scanVerbose(msg, stdoutDone, "("+a.name+":1)", stdout)
+		go scanVerbose(msg, stderrDone, "("+a.name+":2)", stderr)
+		defer func() { <-stdoutDone; <-stderrDone }()
 	}
 
 	z.Dir, z.Env, z.Path, z.Args = a.dir, a.env, a.path, a.args