dap: fix the check to determine whether exec will succeed

This refines the check for determining whether exec will succeed to work
when an error occurs. This check previously relied on the `Ref` being
populated in the result context but this would only happen if we were
paused from a breakpoint or by stepping. An error would not fill in this
field.

The check is now refined to use the new gateway filesystem exec API so
we can create the container and then check even if we don't have a
returned gateway reference. The logic to determine which mount to check
has also been moved.

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

build/invoke.go

@@ -4,6 +4,9 @@ import (
 	"context"
 	_ "crypto/sha256" // ensure digests can be computed
 	"io"
+	"io/fs"
+	"os"
+	"path"
 	"sync"
 	"sync/atomic"
 	"syscall"
@@ -146,6 +149,57 @@ func (c *Container) Exec(ctx context.Context, cfg *InvokeConfig, stdin io.ReadCl
 	return err
 }
 
+func (c *Container) CanInvoke(ctx context.Context, cfg *InvokeConfig) error {
+	var cmd string
+	if len(cfg.Entrypoint) > 0 {
+		cmd = cfg.Entrypoint[0]
+	} else if len(cfg.Cmd) > 0 {
+		cmd = cfg.Cmd[0]
+	}
+
+	if cmd == "" {
+		return errors.New("no command specified")
+	}
+
+	const symlinkResolutionLimit = 40
+	for range symlinkResolutionLimit {
+		fpath, index, err := c.resultCtx.inferMountIndex(cmd, cfg)
+		if err != nil {
+			return err
+		}
+
+		st, err := c.container.StatFile(ctx, gateway.StatContainerRequest{
+			StatRequest: gateway.StatRequest{
+				Path: fpath,
+			},
+			MountIndex: index,
+		})
+		if err != nil {
+			return errors.Wrapf(err, "stat error: %s", cmd)
+		}
+
+		mode := fs.FileMode(st.Mode)
+		if mode&os.ModeSymlink != 0 {
+			// Follow the link.
+			if path.IsAbs(st.Linkname) {
+				cmd = st.Linkname
+			} else {
+				cmd = path.Join(path.Dir(fpath), st.Linkname)
+			}
+			continue
+		}
+
+		if !mode.IsRegular() {
+			return errors.Errorf("%s: not a file", cmd)
+		}
+		if mode&0o111 == 0 {
+			return errors.Errorf("%s: not an executable", cmd)
+		}
+		return nil
+	}
+	return errors.Errorf("%s: reached symlink resolution limit", cmd)
+}
+
 func (c *Container) ReadFile(ctx context.Context, req gateway.ReadContainerRequest) ([]byte, error) {
 	return c.container.ReadFile(ctx, req)
 }

build/result.go

@@ -7,7 +7,7 @@ import (
 	"encoding/json"
 	"io"
 	iofs "io/fs"
-	"path/filepath"
+	"path"
 	"slices"
 	"strings"
 	"sync"
@@ -19,7 +19,6 @@ 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
@@ -81,38 +80,37 @@ 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) {
+func (r *ResultHandle) inferMountIndex(fpath string, cfg *InvokeConfig) (string, int, error) {
 	containerCfg, err := r.getContainerConfig(cfg)
 	if err != nil {
-		return nil, err
+		return "", 0, err
 	}
 
-	candidateMounts := make([]gateway.Mount, 0, len(containerCfg.Mounts))
-	for _, m := range containerCfg.Mounts {
+	type mountCandidate struct {
+		gateway.Mount
+		Index int
+	}
+
+	candidateMounts := make([]mountCandidate, 0, len(containerCfg.Mounts))
+	for i, m := range containerCfg.Mounts {
 		if strings.HasPrefix(fpath, m.Dest) {
-			candidateMounts = append(candidateMounts, m)
+			candidateMounts = append(candidateMounts, mountCandidate{
+				Mount: m,
+				Index: i,
+			})
 		}
 	}
 	if len(candidateMounts) == 0 {
-		return nil, iofs.ErrNotExist
+		return "", 0, iofs.ErrNotExist
 	}
 
-	slices.SortFunc(candidateMounts, func(a, b gateway.Mount) int {
+	slices.SortFunc(candidateMounts, func(a, b mountCandidate) 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)
+	relpath := strings.TrimPrefix(fpath, m.Dest)
+	return path.Join("/", relpath), m.Index, nil
 }
 
 func (r *ResultHandle) getContainerConfig(cfg *InvokeConfig) (containerCfg gateway.NewContainerRequest, _ error) {