allow additional_context to target another service

Signed-off-by: Nicolas De Loof <[email protected]>

loader/validate.go

@@ -28,7 +28,7 @@ import (
 
 // checkConsistency validate a compose model is consistent
 func checkConsistency(project *types.Project) error {
-	for _, s := range project.Services {
+	for name, s := range project.Services {
 		if s.Build == nil && s.Image == "" {
 			return fmt.Errorf("service %q has neither an image nor a build context specified: %w", s.Name, errdefs.ErrInvalid)
 		}
@@ -38,6 +38,18 @@ func checkConsistency(project *types.Project) error {
 				return fmt.Errorf("service %q declares mutualy exclusive dockerfile and dockerfile_inline: %w", s.Name, errdefs.ErrInvalid)
 			}
 
+			for add, c := range s.Build.AdditionalContexts {
+				if target, ok := strings.CutPrefix(c, types.ServicePrefix); ok {
+					t, err := project.GetService(target)
+					if err != nil {
+						return fmt.Errorf("service %q declares unknown service %q as additional contexts %s", name, target, add)
+					}
+					if t.Build == nil {
+						return fmt.Errorf("service %q declares non-buildable service %q as additional contexts %s", name, target, add)
+					}
+				}
+			}
+
 			if len(s.Build.Platforms) > 0 && s.Platform != "" {
 				var found bool
 				for _, platform := range s.Build.Platforms {

paths/context.go

@@ -16,13 +16,20 @@
 
 package paths
 
-import "strings"
+import (
+	"strings"
+
+	"github.com/compose-spec/compose-go/v2/types"
+)
 
 func (r *relativePathsResolver) absContextPath(value any) (any, error) {
 	v := value.(string)
 	if strings.Contains(v, "://") { // `docker-image://` or any builder specific context type
 		return v, nil
 	}
+	if strings.HasPrefix(v, types.ServicePrefix) { // `docker-image://` or any builder specific context type
+		return v, nil
+	}
 	if isRemoteContext(v) {
 		return v, nil
 	}