configs: Handle "dynamic" blocks as special during override merging

Previously we were treating "dynamic" blocks in configuration the same as
any other block type when merging config bodies, so that dynamic blocks
in the override would override any dynamic blocks present in the base,
without considering the dynamic block type.

It's more useful and intuitive for us to treat dynamic blocks as if they
are instances of their given block type for the purpose of overriding.
That means a foo block can be overridden by a dynamic "foo" block and
vice-versa, and dynamic blocks of different types do not interact at all
during overriding.

This requires us to recognize dynamic blocks and treat them specially
during decoding of a merged body. We leave them unexpanded here because
this package is not responsible for dynamic block expansion (that happens
in the sibling "lang" package) but we do decode them enough to recognize
their labels so we can treat them as if they were blocks of the labelled
type.

configs/module_merge_body.go

@@ -40,11 +40,12 @@ var _ hcl.Body = mergeBody{}
 
 func (b mergeBody) Content(schema *hcl.BodySchema) (*hcl.BodyContent, hcl.Diagnostics) {
 	var diags hcl.Diagnostics
-	oSchema := schemaForOverrides(schema)
+	baseSchema := schemaWithDynamic(schema)
+	overrideSchema := schemaWithDynamic(schemaForOverrides(schema))
 
-	baseContent, cDiags := b.Base.Content(schema)
+	baseContent, _, cDiags := b.Base.PartialContent(baseSchema)
 	diags = append(diags, cDiags...)
-	overrideContent, cDiags := b.Override.Content(oSchema)
+	overrideContent, _, cDiags := b.Override.PartialContent(overrideSchema)
 	diags = append(diags, cDiags...)
 
 	content := b.prepareContent(baseContent, overrideContent)
@@ -54,11 +55,12 @@ func (b mergeBody) Content(schema *hcl.BodySchema) (*hcl.BodyContent, hcl.Diagno
 
 func (b mergeBody) PartialContent(schema *hcl.BodySchema) (*hcl.BodyContent, hcl.Body, hcl.Diagnostics) {
 	var diags hcl.Diagnostics
-	oSchema := schemaForOverrides(schema)
+	baseSchema := schemaWithDynamic(schema)
+	overrideSchema := schemaWithDynamic(schemaForOverrides(schema))
 
-	baseContent, baseRemain, cDiags := b.Base.PartialContent(schema)
+	baseContent, baseRemain, cDiags := b.Base.PartialContent(baseSchema)
 	diags = append(diags, cDiags...)
-	overrideContent, overrideRemain, cDiags := b.Override.PartialContent(oSchema)
+	overrideContent, overrideRemain, cDiags := b.Override.PartialContent(overrideSchema)
 	diags = append(diags, cDiags...)
 
 	content := b.prepareContent(baseContent, overrideContent)
@@ -90,9 +92,21 @@ func (b mergeBody) prepareContent(base *hcl.BodyContent, override *hcl.BodyConte
 
 	overriddenBlockTypes := make(map[string]bool)
 	for _, block := range override.Blocks {
+		if block.Type == "dynamic" {
+			overriddenBlockTypes[block.Labels[0]] = true
+			continue
+		}
 		overriddenBlockTypes[block.Type] = true
 	}
 	for _, block := range base.Blocks {
+		// We skip over dynamic blocks whose type label is an overridden type
+		// but note that below we do still leave them as dynamic blocks in
+		// the result because expanding the dynamic blocks that are left is
+		// done much later during the core graph walks, where we can safely
+		// evaluate the expressions.
+		if block.Type == "dynamic" && overriddenBlockTypes[block.Labels[0]] {
+			continue
+		}
 		if overriddenBlockTypes[block.Type] {
 			continue
 		}

configs/module_merge_test.go

@@ -136,3 +136,66 @@ func TestModuleOverrideModule(t *testing.T) {
 
 	assertResultDeepEqual(t, gotArgs, wantArgs)
 }
+
+func TestModuleOverrideDynamic(t *testing.T) {
+	schema := &hcl.BodySchema{
+		Blocks: []hcl.BlockHeaderSchema{
+			{Type: "foo"},
+			{Type: "dynamic", LabelNames: []string{"type"}},
+		},
+	}
+
+	t.Run("base is dynamic", func(t *testing.T) {
+		mod, diags := testModuleFromDir("test-fixtures/valid-modules/override-dynamic-block-base")
+		assertNoDiagnostics(t, diags)
+		if mod == nil {
+			t.Fatalf("module is nil")
+		}
+
+		if _, exists := mod.ManagedResources["test.foo"]; !exists {
+			t.Fatalf("no module 'example'")
+		}
+		if len(mod.ManagedResources) != 1 {
+			t.Fatalf("wrong number of managed resources in result %d; want 1", len(mod.ManagedResources))
+		}
+
+		body := mod.ManagedResources["test.foo"].Config
+		content, diags := body.Content(schema)
+		assertNoDiagnostics(t, diags)
+
+		if len(content.Blocks) != 1 {
+			t.Fatalf("wrong number of blocks in result %d; want 1", len(content.Blocks))
+		}
+		if got, want := content.Blocks[0].Type, "foo"; got != want {
+			t.Fatalf("wrong block type %q; want %q", got, want)
+		}
+	})
+	t.Run("override is dynamic", func(t *testing.T) {
+		mod, diags := testModuleFromDir("test-fixtures/valid-modules/override-dynamic-block-override")
+		assertNoDiagnostics(t, diags)
+		if mod == nil {
+			t.Fatalf("module is nil")
+		}
+
+		if _, exists := mod.ManagedResources["test.foo"]; !exists {
+			t.Fatalf("no module 'example'")
+		}
+		if len(mod.ManagedResources) != 1 {
+			t.Fatalf("wrong number of managed resources in result %d; want 1", len(mod.ManagedResources))
+		}
+
+		body := mod.ManagedResources["test.foo"].Config
+		content, diags := body.Content(schema)
+		assertNoDiagnostics(t, diags)
+
+		if len(content.Blocks) != 1 {
+			t.Fatalf("wrong number of blocks in result %d; want 1", len(content.Blocks))
+		}
+		if got, want := content.Blocks[0].Type, "dynamic"; got != want {
+			t.Fatalf("wrong block type %q; want %q", got, want)
+		}
+		if got, want := content.Blocks[0].Labels[0], "foo"; got != want {
+			t.Fatalf("wrong dynamic block label %q; want %q", got, want)
+		}
+	})
+}

configs/test-fixtures/valid-modules/override-dynamic-block-base/a_override.tf

@@ -0,0 +1,6 @@
+
+resource "test" "foo" {
+  foo {
+    from = "override"
+  }
+}

configs/test-fixtures/valid-modules/override-dynamic-block-base/base.tf

@@ -0,0 +1,9 @@
+
+resource "test" "foo" {
+  dynamic "foo" {
+    for_each = []
+    content {
+      from = "base"
+    }
+  }
+}

configs/test-fixtures/valid-modules/override-dynamic-block-override/a_override.tf

@@ -0,0 +1,9 @@
+
+resource "test" "foo" {
+  dynamic "foo" {
+    for_each = []
+    content {
+      from = "override"
+    }
+  }
+}

configs/test-fixtures/valid-modules/override-dynamic-block-override/base.tf

@@ -0,0 +1,6 @@
+
+resource "test" "foo" {
+  foo {
+    from = "base"
+  }
+}

configs/util.go

@@ -43,3 +43,21 @@ func schemaForOverrides(schema *hcl.BodySchema) *hcl.BodySchema {
 
 	return ret
 }
+
+// schemaWithDynamic takes a *hcl.BodySchema and produces a new one that
+// is equivalent except that it accepts an additional block type "dynamic" with
+// a single label, used to recognize usage of the HCL dynamic block extension.
+func schemaWithDynamic(schema *hcl.BodySchema) *hcl.BodySchema {
+	ret := &hcl.BodySchema{
+		Attributes: schema.Attributes,
+		Blocks:     make([]hcl.BlockHeaderSchema, len(schema.Blocks), len(schema.Blocks)+1),
+	}
+
+	copy(ret.Blocks, schema.Blocks)
+	ret.Blocks = append(ret.Blocks, hcl.BlockHeaderSchema{
+		Type:       "dynamic",
+		LabelNames: []string{"type"},
+	})
+
+	return ret
+}