solver: fix nil result handling

Before this, you could return worker ref results from ops that have nil
refs but once they were attempted to be used, various nil exceptions
would get hit. Now, those cases should be handled.

Signed-off-by: Erik Sipsma <erik@sipsma.dev>

solver/exporter.go

@@ -136,7 +136,7 @@ func (e *exporter) ExportTo(ctx context.Context, t CacheExporterTarget, opt Cach
 				return nil, err
 			}
 			res.Release(context.TODO())
-			if remote == nil {
+			if remote == nil && len(remotes) > 0 {
 				remote, remotes = remotes[0], remotes[1:] // pop the first element
 			}
 			if opt.CompressionOpt != nil {

solver/llbsolver/ops/diff.go

@@ -77,7 +77,9 @@ func (d *diffOp) Exec(ctx context.Context, g session.Group, inputs []solver.Resu
 				return nil, errors.Errorf("invalid lower reference for diff op %T", lowerInp.Sys())
 			}
 			lowerRef = wref.ImmutableRef
-			lowerRefID = wref.ImmutableRef.ID()
+			if lowerRef != nil {
+				lowerRefID = wref.ImmutableRef.ID()
+			}
 		} else {
 			return nil, errors.New("invalid nil lower input for diff op")
 		}
@@ -93,7 +95,9 @@ func (d *diffOp) Exec(ctx context.Context, g session.Group, inputs []solver.Resu
 				return nil, errors.Errorf("invalid upper reference for diff op %T", upperInp.Sys())
 			}
 			upperRef = wref.ImmutableRef
-			upperRefID = wref.ImmutableRef.ID()
+			if upperRef != nil {
+				upperRefID = wref.ImmutableRef.ID()
+			}
 		} else {
 			return nil, errors.New("invalid nil upper input for diff op")
 		}

solver/llbsolver/ops/merge.go

@@ -70,6 +70,9 @@ func (m *mergeOp) Exec(ctx context.Context, g session.Group, inputs []solver.Res
 		if !ok {
 			return nil, errors.Errorf("invalid reference for merge %T", inp.Sys())
 		}
+		if wref.ImmutableRef == nil {
+			continue
+		}
 		refs[index] = wref.ImmutableRef
 		ids[index] = wref.ImmutableRef.ID()
 		index++

worker/base/worker.go

@@ -221,6 +221,11 @@ func (w *Worker) LoadRef(ctx context.Context, id string, hidden bool) (cache.Imm
 	if hidden {
 		opts = append(opts, cache.NoUpdateLastUsed)
 	}
+	if id == "" {
+		// results can have nil refs if they are optimized out to be equal to scratch,
+		// i.e. Diff(A,A) == scratch
+		return nil, nil
+	}
 
 	ref, err := w.CacheMgr.Get(ctx, id, opts...)
 	var needsRemoteProviders cache.NeedsRemoteProvidersError

worker/cacheresult.go

@@ -75,7 +75,9 @@ func (s *cacheResultStorage) LoadRemotes(ctx context.Context, res solver.CacheRe
 	if err != nil {
 		return nil, err
 	}
-	defer ref.Release(context.TODO())
+	if ref != nil {
+		defer ref.Release(context.TODO())
+	}
 	wref := WorkerRef{ref, w}
 	all := true // load as many compression blobs as possible
 	if compressionopt == nil {

worker/result.go

@@ -34,6 +34,9 @@ func (wr *WorkerRef) GetRemotes(ctx context.Context, createIfNeeded bool, compre
 	}); ok {
 		return w.GetRemotes(ctx, wr.ImmutableRef, createIfNeeded, compressionopt, all, g)
 	}
+	if wr.ImmutableRef == nil {
+		return nil, nil
+	}
 	return wr.ImmutableRef.GetRemotes(ctx, createIfNeeded, compressionopt, all, g)
 }