dap: use container fs requests to get a more accurate state for the file system

The container filesystem request API that has been added to buildkit
allows a container created through the `NewContainer` API to also access
the filesystems. This is useful when an error occurs because it allows
us to grab the mutable state of the mounts used during the actual build
rather than the input version copies which don't contain any files that
were added as part of the failed command.

This gives us a more accurate view of the filesystem that was previously
only accessible through using `exec` and `ls`/`cat` commands that may
not always exist.

Author: jsternberg

dap/thread.go

@@ -16,6 +16,7 @@ import (
 	"github.com/moby/buildkit/solver/pb"
 	"github.com/opencontainers/go-digest"
 	"github.com/pkg/errors"
+	"github.com/tonistiigi/fsutil/types"
 	"golang.org/x/sync/errgroup"
 )
 
@@ -323,16 +324,12 @@ func (t *thread) pause(c Context, k string, refs map[string]gateway.Reference, e
 	}
 	t.paused = make(chan stepType, 1)
 
+	t.prepareResultHandle(c, k, refs, err)
+
 	ctx, cancel := context.WithCancelCause(c)
 	t.collectStackTrace(ctx, pos, refs)
 	t.cancel = cancel
 
-	// Used for exec. Only works if there was an error or if the step returns
-	// a root mount.
-	if ref, ok := refs[k]; ok || err != nil {
-		t.prepareResultHandle(c, ref, err)
-	}
-
 	event.ThreadId = t.id
 	c.C() <- &dap.StoppedEvent{
 		Event: dap.Event{Event: "stopped"},
@@ -341,7 +338,15 @@ func (t *thread) pause(c Context, k string, refs map[string]gateway.Reference, e
 	return t.paused
 }
 
-func (t *thread) prepareResultHandle(c Context, ref gateway.Reference, err error) {
+func (t *thread) prepareResultHandle(c Context, k string, refs map[string]gateway.Reference, err error) {
+	var ref gateway.Reference
+	if err == nil {
+		var ok bool
+		if ref, ok = refs[k]; !ok {
+			return
+		}
+	}
+
 	// Create a context for cancellations and make the cancel function
 	// block on the wait group.
 	var wg sync.WaitGroup
@@ -353,6 +358,26 @@ func (t *thread) prepareResultHandle(c Context, ref gateway.Reference, err error
 
 	t.rCtx = build.NewResultHandle(ctx, t.c, ref, t.meta, err)
 
+	if err != nil {
+		var solveErr *errdefs.SolveError
+		if errors.As(err, &solveErr) {
+			if exec, ok := solveErr.Op.Op.(*pb.Op_Exec); ok {
+				rCtx := t.rCtx
+
+				getContainer := sync.OnceValues(func() (gateway.Container, error) {
+					return rCtx.NewContainer(ctx, &build.InvokeConfig{})
+				})
+
+				for i, m := range exec.Exec.Mounts {
+					refs[m.Dest] = &mountReference{
+						getContainer: getContainer,
+						index:        i,
+					}
+				}
+			}
+		}
+	}
+
 	// Start the attach. Use the context we created and perform it in
 	// a goroutine. We aren't necessarily assuming this will actually work.
 	wg.Add(1)
@@ -680,3 +705,52 @@ func (t *thread) rewind(ctx Context, inErr error) (k string, result *step, mount
 	}
 	return k, result, mounts, inErr
 }
+
+type mountReference struct {
+	getContainer func() (gateway.Container, error)
+	index        int
+}
+
+func (r *mountReference) ToState() (llb.State, error) {
+	return llb.State{}, errors.New("unimplemented")
+}
+
+func (r *mountReference) Evaluate(ctx context.Context) error {
+	return nil
+}
+
+func (r *mountReference) ReadFile(ctx context.Context, req gateway.ReadRequest) ([]byte, error) {
+	ctr, err := r.getContainer()
+	if err != nil {
+		return nil, err
+	}
+
+	return ctr.ReadFile(ctx, gateway.ReadContainerRequest{
+		ReadRequest: req,
+		Index:       r.index,
+	})
+}
+
+func (r *mountReference) StatFile(ctx context.Context, req gateway.StatRequest) (*types.Stat, error) {
+	ctr, err := r.getContainer()
+	if err != nil {
+		return nil, err
+	}
+
+	return ctr.StatFile(ctx, gateway.StatContainerRequest{
+		StatRequest: req,
+		Index:       r.index,
+	})
+}
+
+func (r *mountReference) ReadDir(ctx context.Context, req gateway.ReadDirRequest) ([]*types.Stat, error) {
+	ctr, err := r.getContainer()
+	if err != nil {
+		return nil, err
+	}
+
+	return ctr.ReadDir(ctx, gateway.ReadDirContainerRequest{
+		ReadDirRequest: req,
+		Index:          r.index,
+	})
+}

go.mod

@@ -180,3 +180,5 @@ require (
 
 // restore junctions to have os.ModeSymlink flag set on Windows: https://github.com/docker/buildx/issues/3221
 godebug winsymlink=0
+
+replace github.com/moby/buildkit => github.com/jsternberg/buildkit v0.12.1-0.20260108212518-d47b53f94598

go.sum

@@ -189,6 +189,8 @@ github.com/josharian/intern v1.0.0 h1:vlS4z54oSdjm0bgjRigI+G1HpF+tI+9rE5LLzOg8Hm
 github.com/josharian/intern v1.0.0/go.mod h1:5DoeVV0s6jJacbCEi61lwdGj/aVlrQvzHFFd8Hwg//Y=
 github.com/json-iterator/go v1.1.12 h1:PV8peI4a0ysnczrg+LtxykD8LfKY9ML6u2jnxaEnrnM=
 github.com/json-iterator/go v1.1.12/go.mod h1:e30LSqwooZae/UwlEbR2852Gd8hjQvJoHmT4TnhNGBo=
+github.com/jsternberg/buildkit v0.12.1-0.20260108212518-d47b53f94598 h1:cWId6kOBQvY6PHRhlRTnFx/TTrZaKjol6rlh1ogfPsY=
+github.com/jsternberg/buildkit v0.12.1-0.20260108212518-d47b53f94598/go.mod h1:LtUhyzVLAD0ilniDZRbghEyRq9CgZAY7ywMXlVo9MBI=
 github.com/kisielk/errcheck v1.5.0/go.mod h1:pFxgyoBC7bSaBwPgfKdkLd5X25qrDl4LWUI2bnpBCr8=
 github.com/kisielk/gotool v1.0.0/go.mod h1:XhKaO+MFFWcvkIS/tQcRk01m1F5IRFswLeQ+oQHNcck=
 github.com/klauspost/compress v1.18.2 h1:iiPHWW0YrcFgpBYhsA6D1+fqHssJscY/Tm/y2Uqnapk=
@@ -210,8 +212,6 @@ github.com/mitchellh/go-wordwrap v1.0.1 h1:TLuKupo69TCn6TQSyGxwI1EblZZEsQ0vMlAFQ
 github.com/mitchellh/go-wordwrap v1.0.1/go.mod h1:R62XHJLzvMFRBbcrT7m7WgmE1eOyTSsCt+hzestvNj0=
 github.com/mitchellh/hashstructure/v2 v2.0.2 h1:vGKWl0YJqUNxE8d+h8f6NJLcCJrgbhC4NcD46KavDd4=
 github.com/mitchellh/hashstructure/v2 v2.0.2/go.mod h1:MG3aRVU/N29oo/V/IhBX8GR/zz4kQkprJgF2EVszyDE=
-github.com/moby/buildkit v0.26.1-0.20260106154623-ed6dc749ce40 h1:h45o8dTo7Cq1Su49bs9VHP83Fdp3olgMfgo3tZULXUw=
-github.com/moby/buildkit v0.26.1-0.20260106154623-ed6dc749ce40/go.mod h1:LtUhyzVLAD0ilniDZRbghEyRq9CgZAY7ywMXlVo9MBI=
 github.com/moby/docker-image-spec v1.3.1 h1:jMKff3w6PgbfSa69GfNg+zN/XLhfXJGnEx3Nl2EsFP0=
 github.com/moby/docker-image-spec v1.3.1/go.mod h1:eKmb5VW8vQEh/BAr2yvVNvuiJuY6UIocYsFu/DxxRpo=
 github.com/moby/go-archive v0.2.0 h1:zg5QDUM2mi0JIM9fdQZWC7U8+2ZfixfTYoHL7rWUcP8=