Add an undocumented general "run with RPC service"

Makefile

@@ -60,7 +60,7 @@ export GOLANGCI_LINT_VERSION := 2.8.0
 #     Note: Uses the -N -l go compiler options to disable compiler optimizations
 #           and inlining. Using these build options allows you to subsequently
 #           use source debugging tools like delve.
-all: bin/buildah bin/imgtype bin/copy bin/inet bin/tutorial bin/dumpspec bin/passwd bin/crash bin/wait docs
+all: bin/buildah bin/imgtype bin/copy bin/inet bin/tutorial bin/dumpspec bin/passwd bin/crash bin/wait bin/grpcnoop docs
 
 bin/buildah: $(SOURCES) internal/mkcw/embed/entrypoint_amd64.gz
 	$(GO_BUILD) $(BUILDAH_LDFLAGS) $(GO_GCFLAGS) "$(GOGCFLAGS)" -o $@ $(BUILDFLAGS) ./cmd/buildah
@@ -131,6 +131,9 @@ bin/inet: tests/inet/inet.go
 bin/passwd: tests/passwd/passwd.go
 	$(GO_BUILD) $(BUILDAH_LDFLAGS) -o $@ $(BUILDFLAGS) ./tests/passwd/passwd.go
 
+bin/grpcnoop: tests/rpc/noop/noop.go
+	$(GO_BUILD) $(BUILDAH_LDFLAGS) -o $@ $(BUILDFLAGS) ./tests/rpc/noop/noop.go
+
 .PHONY: clean
 clean:
 	$(RM) -r bin tests/testreport/testreport tests/conformance/testdata/mount-targets/true internal/mkcw/embed/entrypoint_amd64 internal/mkcw/embed/entrypoint_arm64 internal/mkcw/embed/entrypoint_ppc64le internal/mkcw/embed/entrypoint_s390x internal/mkcw/embed/*.gz internal/mkcw/embed/asm/*.o

cmd/buildah/rpc.go

@@ -0,0 +1,98 @@
+package main
+
+import (
+	"os"
+	"os/exec"
+	"path/filepath"
+	"slices"
+
+	"github.com/containers/buildah/internal/rpc/listen"
+	"github.com/containers/buildah/internal/rpc/noop"
+	"github.com/sirupsen/logrus"
+	"github.com/spf13/cobra"
+	"golang.org/x/sync/errgroup"
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/reflection"
+)
+
+var (
+	rpcDescription = "\n  Runs a command with a rudimentary RPC server available."
+	rpcCommand     = &cobra.Command{
+		Use:     "rpc",
+		Short:   "Run a command with a rudimentary RPC server available",
+		Long:    rpcDescription,
+		RunE:    rpcCmd,
+		Hidden:  true,
+		Example: `buildah rpc [-e|--env NAME] [-l|--listen PATH] command ...`,
+		Args:    cobra.MinimumNArgs(1),
+	}
+)
+
+func rpcCmd(c *cobra.Command, args []string) error {
+	store, err := getStore(c)
+	if err != nil {
+		return err
+	}
+
+	socketPath := c.Flag("listen").Value.String()
+	if socketPath == "" {
+		socketDir, err := os.MkdirTemp(store.RunRoot(), "buildah-socket")
+		if err != nil {
+			return err
+		}
+		defer func() {
+			if err := os.RemoveAll(socketDir); err != nil {
+				logrus.Errorf("removing %s: %v", socketDir, err)
+			}
+		}()
+		socketPath = filepath.Join(socketDir, "socket")
+	}
+	listener, cleanup, err := listen.Listen(socketPath)
+	if err != nil {
+		return err
+	}
+	logrus.Debugf("listening for rpc requests at %q", socketPath)
+	defer func() {
+		if err := cleanup(); err != nil {
+			logrus.Errorf("cleaning up: %v", err)
+		}
+	}()
+
+	s := grpc.NewServer()
+	noop.Register(s)
+	reflection.Register(s)
+
+	var errgroup errgroup.Group
+	errgroup.Go(func() error {
+		if err := s.Serve(listener); err != nil {
+			return err
+		}
+		return nil
+	})
+	defer func() {
+		s.GracefulStop() // closes the listening socket
+		if err := errgroup.Wait(); err != nil {
+			logrus.Errorf("while waiting for rpc service to shut down: %v", err)
+		}
+	}()
+	cmd := exec.Command(args[0], args[1:]...)
+	cmd.Stdin, cmd.Stdout, cmd.Stderr = os.Stdin, os.Stdout, os.Stderr
+	envVar := c.Flag("env").Value.String()
+	if envVar != "" {
+		cmd.Env = append(slices.Clone(cmd.Environ()), envVar+"="+"unix://"+socketPath)
+	}
+	return cmd.Run()
+}
+
+func init() {
+	var rpcOptions struct {
+		envVar     string
+		listenPath string
+	}
+	rpcCommand.SetUsageTemplate(UsageTemplate())
+	flags := rpcCommand.Flags()
+	flags.SetInterspersed(false)
+	flags.StringVarP(&rpcOptions.envVar, "env", "e", "", "set environment `variable` to point to listening socket path")
+	flags.StringVarP(&rpcOptions.listenPath, "listen", "l", "", "listening socket `path`")
+	rootCmd.AddCommand(rpcCommand)
+}

go.mod

@@ -41,6 +41,8 @@ require (
 	golang.org/x/sync v0.19.0
 	golang.org/x/sys v0.41.0
 	golang.org/x/term v0.40.0
+	google.golang.org/grpc v1.76.0
+	google.golang.org/protobuf v1.36.11
 	tags.cncf.io/container-device-interface v1.1.0
 )
 
@@ -132,8 +134,6 @@ require (
 	golang.org/x/text v0.34.0 // indirect
 	google.golang.org/genproto/googleapis/api v0.0.0-20250929231259-57b25ae835d4 // indirect
 	google.golang.org/genproto/googleapis/rpc v0.0.0-20251022142026-3a174f9686a8 // indirect
-	google.golang.org/grpc v1.76.0 // indirect
-	google.golang.org/protobuf v1.36.11 // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
 	k8s.io/klog v1.0.0 // indirect
 	sigs.k8s.io/yaml v1.6.0 // indirect

internal/rpc/listen/listen.go

@@ -0,0 +1,39 @@
+package listen
+
+import (
+	"errors"
+	"fmt"
+	"net"
+	"os"
+	"path/filepath"
+
+	"github.com/containers/buildah/internal/tmpdir"
+)
+
+func Listen(location string) (net.Listener, func() error, error) {
+	cleanup := func() error { return nil }
+	if location == "" {
+		newParentDir, err := os.MkdirTemp(tmpdir.GetTempDir(), "buildah-socket")
+		if err != nil {
+			return nil, nil, fmt.Errorf("creating a temporary directory to hold a listening socket: %w", err)
+		}
+		location = filepath.Join(newParentDir, "build.sock")
+		cleanup = func() error { return os.RemoveAll(newParentDir) }
+	}
+	l, err := net.ListenUnix("unix", &net.UnixAddr{Net: "unix", Name: location})
+	if err != nil {
+		cerr := cleanup()
+		return nil, nil, errors.Join(err, cerr)
+	}
+	closeAndCleanup := func() error {
+		lerr := l.Close()
+		if lerr != nil && errors.Is(lerr, net.ErrClosed) {
+			// if the listening socket was used by an rpc server that was explicitly
+			// stopped, the rpc server will have closed the socket
+			lerr = nil
+		}
+		cerr := cleanup()
+		return errors.Join(lerr, cerr)
+	}
+	return l, closeAndCleanup, nil
+}

internal/rpc/noop/noop.go

@@ -0,0 +1,26 @@
+package noop
+
+import (
+	"context"
+	"syscall"
+
+	"github.com/containers/buildah/internal/rpc/noop/pb"
+	"google.golang.org/grpc"
+)
+
+type noopServer struct {
+	pb.UnimplementedNoopServer
+}
+
+func (n *noopServer) Noop(_ context.Context, req *pb.NoopRequest) (*pb.NoopResponse, error) {
+	if req == nil {
+		return nil, syscall.EINVAL
+	}
+	resp := &pb.NoopResponse{}
+	resp.Ignored = req.Ignored
+	return resp, nil
+}
+
+func Register(s grpc.ServiceRegistrar) {
+	pb.RegisterNoopServer(s, &noopServer{})
+}

internal/rpc/noop/pb/build.sh

@@ -0,0 +1,12 @@
+#!/bin/bash
+set -e
+cd $(dirname ${BASH_SOURCE[0]})
+TOP=../../../..
+PATH=${TOP}/tests/tools/build:${PATH}
+set -x
+for proto in *.proto ; do
+	protoc \
+		--go_opt=paths=source_relative --go_out . \
+		--go-grpc_opt=paths=source_relative --go-grpc_out . \
+	${proto}
+done