dap: make exec shell persistent across the build

Invoking the shell will cause it to persist across the entire build and
to re-execute whenever the builder pauses at another location again.

This still requires using `exec` to launch the shell. Launching by frame
id is also removed since it no longer applies to this version.

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>

build/result.go

@@ -1,10 +1,15 @@
 package build
 
 import (
+	"cmp"
 	"context"
 	_ "crypto/sha256" // ensure digests can be computed
 	"encoding/json"
 	"io"
+	iofs "io/fs"
+	"path/filepath"
+	"slices"
+	"strings"
 	"sync"
 
 	"github.com/moby/buildkit/exporter/containerimage/exptypes"
@@ -14,6 +19,7 @@ import (
 	ocispecs "github.com/opencontainers/image-spec/specs-go/v1"
 	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
+	"github.com/tonistiigi/fsutil/types"
 )
 
 // NewResultHandle stores a gateway client, gateway reference, and the error from
@@ -75,6 +81,40 @@ func (r *ResultHandle) NewContainer(ctx context.Context, cfg *InvokeConfig) (gat
 	return r.gwClient.NewContainer(ctx, req)
 }
 
+func (r *ResultHandle) StatFile(ctx context.Context, fpath string, cfg *InvokeConfig) (*types.Stat, error) {
+	containerCfg, err := r.getContainerConfig(cfg)
+	if err != nil {
+		return nil, err
+	}
+
+	candidateMounts := make([]gateway.Mount, 0, len(containerCfg.Mounts))
+	for _, m := range containerCfg.Mounts {
+		if strings.HasPrefix(fpath, m.Dest) {
+			candidateMounts = append(candidateMounts, m)
+		}
+	}
+	if len(candidateMounts) == 0 {
+		return nil, iofs.ErrNotExist
+	}
+
+	slices.SortFunc(candidateMounts, func(a, b gateway.Mount) int {
+		return cmp.Compare(len(a.Dest), len(b.Dest))
+	})
+
+	m := candidateMounts[len(candidateMounts)-1]
+	relpath, err := filepath.Rel(m.Dest, fpath)
+	if err != nil {
+		return nil, err
+	}
+
+	if m.Ref == nil {
+		return nil, iofs.ErrNotExist
+	}
+
+	req := gateway.StatRequest{Path: filepath.ToSlash(relpath)}
+	return m.Ref.StatFile(ctx, req)
+}
+
 func (r *ResultHandle) getContainerConfig(cfg *InvokeConfig) (containerCfg gateway.NewContainerRequest, _ error) {
 	if r.ref != nil && r.solveErr == nil {
 		logrus.Debugf("creating container from successful build")