cty: Silently ignore refinements of cty.DynamicVal

For historical reasons we cannot permit any refinements on cty.DynamicVal,
because existing callers expect that (marks notwithstanding) cty.DynamicVal
is the only possible unknown value of an unknown type, and so adding
refinements would invalidate that assumption.

In the initial implementation of refinements it was treated as a caller
programming error to try to refine cty.DynamicVal. However, that violates
the convention that cty.DynamicVal is usable as a broad placeholder value
that supports any operation that would be valid on at least one possible
value.

Instead, we'll now compromise by just silently ignoring any attempt to
refine cty.DynamicVal. The refinement operation will run to completion
without panicking but it will also completely ignore all of the refinement
builder calls and eventually just return another plain cty.DynamicVal
as the "refined" result.

This is consistent with our typical expectation that refinements are
propagated on a best-effort basis but can be silently discarded (or lose
some detail) when passing through operations that don't or can't support
them.

This means that it should now be safe to call Refine on any value, but
refining can still panic if the specific refinements selected don't make
sense for whatever value is being refined.

This also includes some direct tests of the Value.Refine API, since it
was previously only tested indirectly through other operations that
consume or manipulate refinements.

Author: apparentlymart

CHANGELOG.md

@@ -1,5 +1,6 @@
 # 1.14.1 (Unreleased)
 
+* `cty`: It's now valid to use the `Refine` method on `cty.DynamicVal`, although all refinements will be silently discarded. This replaces the original behavior of panicking when trying to refine `cty.DynamicVal`.
 
 # 1.14.0 (August 30, 2023)
 

cty/unknown_refinement.go

@@ -44,7 +44,17 @@ func (v Value) Refine() *RefinementBuilder {
 	var wip unknownValRefinement
 	switch {
 	case ty == DynamicPseudoType && !v.IsKnown():
-		panic("cannot refine an unknown value of an unknown type")
+		// This case specifically matches DynamicVal, which is constrained
+		// by backward compatibility to be a singleton and so we cannot allow
+		// any refinements to it.
+		// To preserve the typical assumption that DynamicVal is a safe
+		// placeholder to use when no value is known at all, we silently
+		// ignore all attempts to refine this particular value and just
+		// always echo back a totally-unrefined DynamicVal.
+		return &RefinementBuilder{
+			orig:  DynamicVal,
+			marks: marks,
+		}
 	case ty == String:
 		wip = &refinementString{}
 	case ty == Number:
@@ -136,10 +146,27 @@ type RefinementBuilder struct {
 	wip   unknownValRefinement
 }
 
-func (b *RefinementBuilder) assertRefineable() {
+// refineable is an internal detail to help with two special situations
+// related to refinements:
+//   - If the refinement is to a value of a type that doesn't support any
+//     refinements at all, this function will immediately panic with a
+//     message reporting that, because it's a caller bug to try to refine
+//     a value in a way that's inappropriate for its known type.
+//   - If the refinement is to an unknown value of an unknown type
+//     (i.e. cty.DynamicVal) then it returns false, indicating that the
+//     caller should just silently ignore whatever refinement was requested.
+//   - In all other cases this function returns true, which means the direct
+//     caller should attempt to apply the requested refinement, and then
+//     panic itself if the requested refinement doesn't make sense for the
+//     specific value being refined.
+func (b *RefinementBuilder) refineable() bool {
+	if b.orig == DynamicVal {
+		return false
+	}
 	if b.wip == nil {
 		panic(fmt.Sprintf("cannot refine a %#v value", b.orig.Type()))
 	}
+	return true
 }
 
 // NotNull constrains the value as definitely not being null.
@@ -157,7 +184,9 @@ func (b *RefinementBuilder) assertRefineable() {
 // -- a value whose type is `cty.DynamicPseudoType` -- as being non-null.
 // An unknown value of an unknown type is always completely unconstrained.
 func (b *RefinementBuilder) NotNull() *RefinementBuilder {
-	b.assertRefineable()
+	if !b.refineable() {
+		return b
+	}
 
 	if b.orig.IsKnown() && b.orig.IsNull() {
 		panic("refining null value as non-null")
@@ -181,7 +210,9 @@ func (b *RefinementBuilder) NotNull() *RefinementBuilder {
 // value for each type constraint -- but this is here for symmetry with the
 // fact that a [ValueRange] can also represent that a value is definitely null.
 func (b *RefinementBuilder) Null() *RefinementBuilder {
-	b.assertRefineable()
+	if !b.refineable() {
+		return b
+	}
 
 	if b.orig.IsKnown() && !b.orig.IsNull() {
 		panic("refining non-null value as null")
@@ -209,7 +240,9 @@ func (b *RefinementBuilder) NumberRangeInclusive(min, max Value) *RefinementBuil
 // NumberRangeLowerBound constraints the lower bound of a number value, or
 // panics if this builder is not refining a number value.
 func (b *RefinementBuilder) NumberRangeLowerBound(min Value, inclusive bool) *RefinementBuilder {
-	b.assertRefineable()
+	if !b.refineable() {
+		return b
+	}
 
 	wip, ok := b.wip.(*refinementNumber)
 	if !ok {
@@ -258,7 +291,9 @@ func (b *RefinementBuilder) NumberRangeLowerBound(min Value, inclusive bool) *Re
 // NumberRangeUpperBound constraints the upper bound of a number value, or
 // panics if this builder is not refining a number value.
 func (b *RefinementBuilder) NumberRangeUpperBound(max Value, inclusive bool) *RefinementBuilder {
-	b.assertRefineable()
+	if !b.refineable() {
+		return b
+	}
 
 	wip, ok := b.wip.(*refinementNumber)
 	if !ok {
@@ -308,7 +343,9 @@ func (b *RefinementBuilder) NumberRangeUpperBound(max Value, inclusive bool) *Re
 // collection value, or panics if this builder is not refining a collection
 // value.
 func (b *RefinementBuilder) CollectionLengthLowerBound(min int) *RefinementBuilder {
-	b.assertRefineable()
+	if !b.refineable() {
+		return b
+	}
 
 	wip, ok := b.wip.(*refinementCollection)
 	if !ok {
@@ -340,7 +377,9 @@ func (b *RefinementBuilder) CollectionLengthLowerBound(min int) *RefinementBuild
 // The upper bound must be a known, non-null number or this function will
 // panic.
 func (b *RefinementBuilder) CollectionLengthUpperBound(max int) *RefinementBuilder {
-	b.assertRefineable()
+	if !b.refineable() {
+		return b
+	}
 
 	wip, ok := b.wip.(*refinementCollection)
 	if !ok {
@@ -419,7 +458,9 @@ func (b *RefinementBuilder) StringPrefix(prefix string) *RefinementBuilder {
 // Use [RefinementBuilder.StringPrefix] instead if an application cannot fully
 // control the final result to avoid violating this rule.
 func (b *RefinementBuilder) StringPrefixFull(prefix string) *RefinementBuilder {
-	b.assertRefineable()
+	if !b.refineable() {
+		return b
+	}
 
 	wip, ok := b.wip.(*refinementString)
 	if !ok {
@@ -487,7 +528,7 @@ func (b *RefinementBuilder) NewValue() (ret Value) {
 		ret = ret.WithMarks(b.marks)
 	}()
 
-	if b.orig.IsKnown() {
+	if b.orig.IsKnown() || b.orig == DynamicVal {
 		return b.orig
 	}