fix more entrypoint situations

this was hard to figure out, but it works, and should be close to what
Dagger does. there is probably be a better way to model this, but I
think it achieves the desired semantics.

pkg/bass/thunk.go

@@ -443,22 +443,45 @@ func (thunk Thunk) WithImage(image ThunkImage) Thunk {
 
 // WithEntrypoint sets the thunk's entrypoint.
 func (thunk Thunk) WithEntrypoint(entrypoint []string) Thunk {
-	if len(entrypoint) == 0 {
-		thunk.ClearEntrypoint = true
-	}
+	thunk = thunk.encapsulate()
 	thunk.Entrypoint = entrypoint
+	thunk.ClearEntrypoint = len(entrypoint) == 0
 	return thunk
 }
 
 // WithDefaultArgs sets the thunk's default arguments.
 func (thunk Thunk) WithDefaultArgs(args []string) Thunk {
-	if len(args) == 0 {
-		thunk.ClearDefaultArgs = true
-	}
+	thunk = thunk.encapsulate()
 	thunk.DefaultArgs = args
+	thunk.ClearDefaultArgs = len(args) == 0
 	return thunk
 }
 
+// encapsulate is used to transition from a "build-time" thunk to a "run-time"
+// thunk by configuring an entrypoint and/or default args. It returns a thunk
+// with no args and an image that points to the original thunk. The returned
+// thunk also inherits the original thunk's ports, labels, and workdir.
+func (thunk Thunk) encapsulate() Thunk {
+	if len(thunk.Args) == 0 {
+		// already encapsulated
+		return thunk
+	} else {
+		return Thunk{
+			Image: &ThunkImage{
+				Thunk: &thunk,
+			},
+			// no args
+			Dir:              thunk.Dir,
+			Ports:            thunk.Ports,
+			Labels:           thunk.Labels,
+			Entrypoint:       thunk.Entrypoint,
+			ClearEntrypoint:  thunk.ClearEntrypoint,
+			DefaultArgs:      thunk.DefaultArgs,
+			ClearDefaultArgs: thunk.ClearDefaultArgs,
+		}
+	}
+}
+
 // WithArgs sets the thunk's arg values.
 func (thunk Thunk) WithArgs(args []Value) Thunk {
 	thunk.Args = args

pkg/runtimes/buildkit.go

@@ -806,6 +806,9 @@ func (b *buildkitBuilder) llb(
 	if len(cmd.Args) == 0 {
 		if forceExec {
 			cmd.Args = append(ib.config.Entrypoint, ib.config.Cmd...)
+			if len(cmd.Args) == 0 {
+				return ib, fmt.Errorf("no command specified")
+			}
 		} else {
 			// no command; we're just overriding config
 			return ib, nil

pkg/runtimes/dagger.go

@@ -318,7 +318,7 @@ func (runtime *Dagger) container(ctx context.Context, thunk bass.Thunk, forceExe
 		ctr = ctr.WithSecretVariable(env.Name, secret)
 	}
 
-	if cmd.Args != nil {
+	if len(cmd.Args) > 0 {
 		ep, err := ctr.Entrypoint(ctx)
 		if err != nil {
 			return nil, err
@@ -337,14 +337,14 @@ func (runtime *Dagger) container(ctx context.Context, thunk bass.Thunk, forceExe
 			ctr = ctr.WithEntrypoint(ep)
 		}
 	} else if forceExec {
-		ctr = ctr.WithExec(nil)
+		ctr = ctr.WithExec(append(thunk.Entrypoint, thunk.DefaultArgs...))
 	}
 
-	if thunk.Entrypoint != nil {
+	if len(thunk.Entrypoint) > 0 || thunk.ClearEntrypoint {
 		ctr = ctr.WithEntrypoint(thunk.Entrypoint)
 	}
 
-	if thunk.DefaultArgs != nil {
+	if len(thunk.DefaultArgs) > 0 || thunk.ClearDefaultArgs {
 		ctr = ctr.WithDefaultArgs(dagger.ContainerWithDefaultArgsOpts{
 			Args: thunk.DefaultArgs,
 		})

pkg/runtimes/grpc_test.go

@@ -48,6 +48,7 @@ func TestGRPCRuntime(t *testing.T) {
 			{
 				Platform: bass.LinuxPlatform,
 				Runtime:  runtimes.BuildkitName,
+				Config:   config.Scope(),
 			},
 		},
 	})

pkg/runtimes/testdata/entrypoints.bass

@@ -15,38 +15,62 @@
       (read :raw)
       next))
 
-; but it is preserved
+; but it is preserved in the published image
 (assert = ["git"]
   (entrypoint not-respected))
 
 ; entrypoint *is* respected when running directly
+(run (linux/hello-world))
+
+; or reading
 (assert strings:includes?
   (-> (linux/hello-world)
       (read :raw)
       next)
   "Hello from Docker")
 
+; or accessing a path
+(def create-path
+  (-> (linux/alpine)
+      (with-entrypoint ["sh" "-c" "echo hello > index.html"])))
+
+(assert = "hello\n"
+  (-> create-path/index.html
+      (read :raw)
+      next))
+
+; entrypoint is removed when set to empty list
 (def unset-image
   (-> (linux/alpine/git :2.36.3)
       (with-entrypoint [])))
 
-; ; entrypoint is removed when set to empty list
 (assert null?
   (entrypoint unset-image))
 
-(def set-thunk
+; entrypoint can be set by thunk
+(def touch-and-run
   (-> (from (linux/busybox)
-        ($ touch index.html))
+        ($ sh -c "echo hello from index.html > index.html"))
+      (with-port :http 8000)
       (with-entrypoint ["httpd" "-f" "-p" "8000"])))
 
-; entrypoint can be set by thunk
 (assert = ["httpd" "-f" "-p" "8000"]
-  (entrypoint set-thunk))
+  (entrypoint touch-and-run))
 
+; entrypoint is preserved from thunk to thunk
 (def keeps-entrypoint
-  (from set-thunk
+  (from touch-and-run
     ($ echo "Hello, world!")))
 
-; entrypoint is preserved from thunk to thunk
 (assert = ["httpd" "-f" "-p" "8000"]
   (entrypoint keeps-entrypoint))
+
+; entrypoint is respected after running a command
+(def wget
+  (from (linux/alpine)
+    ($ wget -O- (addr touch-and-run :http "http://$host:$port/"))))
+
+(assert = "hello from index.html\n"
+  (-> wget
+      (read :raw)
+      next))