Update message

pkg/resource/deploy/plan.go

@@ -188,9 +188,19 @@ func (rp *ResourcePlan) checkGoal(programGoal *resource.Goal) error {
 	}
 
 	// Check that the properties meet the constraints set in the plan.
-	if _, constrained := programGoal.Properties.ConstrainedTo(rp.Goal.Properties); !constrained {
+	if diff, constrained := programGoal.Properties.ConstrainedTo(rp.Goal.Properties); !constrained {
 		// TODO(pdg-plan): message!
-		return fmt.Errorf("properties changed")
+		var paths []string
+		for k := range diff.Adds {
+			paths = append(paths, "+"+string(k))
+		}
+		for k := range diff.Deletes {
+			paths = append(paths, "-"+string(k))
+		}
+		for k := range diff.Updates {
+			paths = append(paths, "~"+string(k))
+		}
+		return fmt.Errorf("properties changed: %v", strings.Join(paths, ", "))
 	}
 
 	// Check that the property dependencies match. Note that because it is legal for a property that is unknown in the

sdk/go/common/resource/properties_diff.go

@@ -74,6 +74,22 @@ func (diff *ObjectDiff) Keys() []PropertyKey {
 	return ks
 }
 
+func (diff *ObjectDiff) Paths() []PropertyPath {
+	var paths []PropertyPath
+	for k := range diff.Adds {
+		paths = append(paths, PropertyPath{string(k)})
+	}
+	for k := range diff.Adds {
+		paths = append(paths, PropertyPath{string(k)})
+	}
+	for k, d := range diff.Updates {
+		for _, child := range d.Paths() {
+			paths = append(paths, append(PropertyPath{string(k)}, child...))
+		}
+	}
+	return paths
+}
+
 // ValueDiff holds the results of diffing two property values.
 type ValueDiff struct {
 	Old    PropertyValue // the old value.
@@ -82,6 +98,17 @@ type ValueDiff struct {
 	Object *ObjectDiff   // the object's detailed diffs (only for objects).
 }
 
+func (diff ValueDiff) Paths() []PropertyPath {
+	switch {
+	case diff.Array != nil:
+		return diff.Array.Paths()
+	case diff.Object != nil:
+		return diff.Object.Paths()
+	default:
+		return nil
+	}
+}
+
 // ArrayDiff holds the results of diffing two arrays of property values.
 type ArrayDiff struct {
 	Adds    map[int]PropertyValue // elements added in the new.
@@ -90,6 +117,22 @@ type ArrayDiff struct {
 	Updates map[int]ValueDiff     // elements that have changed in the new.
 }
 
+func (diff *ArrayDiff) Paths() []PropertyPath {
+	var paths []PropertyPath
+	for k := range diff.Adds {
+		paths = append(paths, PropertyPath{k})
+	}
+	for k := range diff.Adds {
+		paths = append(paths, PropertyPath{k})
+	}
+	for k, d := range diff.Updates {
+		for _, child := range d.Paths() {
+			paths = append(paths, append(PropertyPath{k}, child...))
+		}
+	}
+	return paths
+}
+
 // Len computes the length of this array, taking into account adds, deletes, sames, and updates.
 func (diff *ArrayDiff) Len() int {
 	len := 0

sdk/go/common/resource/properties_path.go

@@ -1,6 +1,8 @@
 package resource
 
 import (
+	"bytes"
+	"fmt"
 	"strconv"
 	"strings"
 
@@ -291,3 +293,35 @@ func (p PropertyPath) Contains(other PropertyPath) bool {
 
 	return true
 }
+
+func requiresQuote(c rune) bool {
+	return !(c >= 'A' && c <= 'Z' || c >= 'a' && c <= 'z' || c >= '0' && c <= '9' || c == '_')
+}
+
+func (p PropertyPath) String() string {
+	var buf bytes.Buffer
+	for _, k := range p {
+		switch k := k.(type) {
+		case string:
+			var keyBuf bytes.Buffer
+			quoted := false
+			for _, c := range k {
+				if requiresQuote(c) {
+					quoted = true
+					if c == '"' {
+						keyBuf.WriteByte('\\')
+					}
+				}
+				keyBuf.WriteRune(c)
+			}
+			if !quoted {
+				fmt.Fprintf(&buf, ".%s", keyBuf.String())
+			} else {
+				fmt.Fprintf(&buf, `["%s"]`, keyBuf.String())
+			}
+		case int:
+			fmt.Fprintf(&buf, "[%d]", k)
+		}
+	}
+	return buf.String()
+}