Optimize filterArchStruct when nothing is filtered

Runtime-created struct types have limit on the name of the type,
which is the full text of the fields in the struct.  Avoid creating
runtime struct types when filtering produces an identical struct.
Also add tests and godoc for filterArchStruct.

Test: arch_test.go
Change-Id: If7eb27ea9e72073d5252e93f2110b08889ed76b1

3 files changed, 282 insertions, 22 deletions

diff --git a/Android.bp b/Android.bp
index 62e276ac..3215fa2b 100644
--- a/Android.bp
+++ b/Android.bp
@@ -71,6 +71,7 @@ bootstrap_go_package {
         "android/env.go",
     ],
     testSrcs: [
+        "android/arch_test.go",
         "android/config_test.go",
         "android/expand_test.go",
         "android/namespace_test.go",
diff --git a/android/arch.go b/android/arch.go
index de19dbe9..6aeccb0c 100644
--- a/android/arch.go
+++ b/android/arch.go
@@ -616,10 +616,10 @@ func decodeMultilib(base *ModuleBase, class OsClass) (multilib, extraMultilib st
 	}
 }
 
-func filterArchStructFields(fields []reflect.StructField) []reflect.StructField {
-	var ret []reflect.StructField
+func filterArchStructFields(fields []reflect.StructField) (filteredFields []reflect.StructField, filtered bool) {
 	for _, field := range fields {
 		if !proptools.HasTag(field, "android", "arch_variant") {
+			filtered = true
 			continue
 		}
 
@@ -637,15 +637,17 @@ func filterArchStructFields(fields []reflect.StructField) []reflect.StructField
 		// Recurse into structs
 		switch field.Type.Kind() {
 		case reflect.Struct:
-			var ok bool
-			field.Type, ok = filterArchStruct(field.Type)
-			if !ok {
+			var subFiltered bool
+			field.Type, subFiltered = filterArchStruct(field.Type)
+			filtered = filtered || subFiltered
+			if field.Type == nil {
 				continue
 			}
 		case reflect.Ptr:
 			if field.Type.Elem().Kind() == reflect.Struct {
-				nestedType, ok := filterArchStruct(field.Type.Elem())
-				if !ok {
+				nestedType, subFiltered := filterArchStruct(field.Type.Elem())
+				filtered = filtered || subFiltered
+				if nestedType == nil {
 					continue
 				}
 				field.Type = reflect.PtrTo(nestedType)
@@ -654,13 +656,17 @@ func filterArchStructFields(fields []reflect.StructField) []reflect.StructField
 			panic("Interfaces are not supported in arch_variant properties")
 		}
 
-		ret = append(ret, field)
+		filteredFields = append(filteredFields, field)
 	}
 
-	return ret
+	return filteredFields, filtered
 }
 
-func filterArchStruct(prop reflect.Type) (reflect.Type, bool) {
+// filterArchStruct takes a reflect.Type that is either a sturct or a pointer to a struct, and returns a reflect.Type
+// that only contains the fields in the original type that have an `android:"arch_variant"` struct tag, and a bool
+// that is true if the new struct type has fewer fields than the original type.  If there are no fields in the
+// original type with the struct tag it returns nil and true.
+func filterArchStruct(prop reflect.Type) (filteredProp reflect.Type, filtered bool) {
 	var fields []reflect.StructField
 
 	ptr := prop.Kind() == reflect.Ptr
@@ -672,13 +678,20 @@ func filterArchStruct(prop reflect.Type) (reflect.Type, bool) {
 		fields = append(fields, prop.Field(i))
 	}
 
-	fields = filterArchStructFields(fields)
+	filteredFields, filtered := filterArchStructFields(fields)
 
-	if len(fields) == 0 {
-		return nil, false
+	if len(filteredFields) == 0 {
+		return nil, true
+	}
+
+	if !filtered {
+		if ptr {
+			return reflect.PtrTo(prop), false
+		}
+		return prop, false
 	}
 
-	ret := reflect.StructOf(fields)
+	ret := reflect.StructOf(filteredFields)
 	if ptr {
 		ret = reflect.PtrTo(ret)
 	}
@@ -686,7 +699,13 @@ func filterArchStruct(prop reflect.Type) (reflect.Type, bool) {
 	return ret, true
 }
 
-func filterArchStructSharded(prop reflect.Type) ([]reflect.Type, bool) {
+// filterArchStruct takes a reflect.Type that is either a sturct or a pointer to a struct, and returns a list of
+// reflect.Type that only contains the fields in the original type that have an `android:"arch_variant"` struct tag,
+// and a bool that is true if the new struct type has fewer fields than the original type.  If there are no fields in
+// the original type with the struct tag it returns nil and true.  Each returned struct type will have a maximum of
+// 10 top level fields in it to attempt to avoid hitting the reflect.StructOf name length limit, although the limit
+// can still be reached with a single struct field with many fields in it.
+func filterArchStructSharded(prop reflect.Type) (filteredProp []reflect.Type, filtered bool) {
 	var fields []reflect.StructField
 
 	ptr := prop.Kind() == reflect.Ptr
@@ -698,24 +717,29 @@ func filterArchStructSharded(prop reflect.Type) ([]reflect.Type, bool) {
 		fields = append(fields, prop.Field(i))
 	}
 
-	fields = filterArchStructFields(fields)
+	fields, filtered = filterArchStructFields(fields)
+	if !filtered {
+		if ptr {
+			return []reflect.Type{reflect.PtrTo(prop)}, false
+		}
+		return []reflect.Type{prop}, false
+	}
 
 	if len(fields) == 0 {
-		return nil, false
+		return nil, true
 	}
 
 	shards := shardFields(fields, 10)
 
-	var ret []reflect.Type
 	for _, shard := range shards {
 		s := reflect.StructOf(shard)
 		if ptr {
 			s = reflect.PtrTo(s)
 		}
-		ret = append(ret, s)
+		filteredProp = append(filteredProp, s)
 	}
 
-	return ret, true
+	return filteredProp, true
 }
 
 func shardFields(fields []reflect.StructField, shardSize int) [][]reflect.StructField {
@@ -730,9 +754,12 @@ func shardFields(fields []reflect.StructField, shardSize int) [][]reflect.Struct
 	return ret
 }
 
+// createArchType takes a reflect.Type that is either a struct or a pointer to a struct, and returns a list of
+// reflect.Type that contains the arch-variant properties inside structs for each architecture, os, target, multilib,
+// etc.
 func createArchType(props reflect.Type) []reflect.Type {
-	propShards, ok := filterArchStructSharded(props)
-	if !ok {
+	propShards, _ := filterArchStructSharded(props)
+	if len(propShards) == 0 {
 		return nil
 	}
 
diff --git a/android/arch_test.go b/android/arch_test.go
new file mode 100644
index 00000000..0589e6c3
--- /dev/null
+++ b/android/arch_test.go
@@ -0,0 +1,232 @@
+// Copyright 2019 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package android
+
+import (
+	"reflect"
+	"testing"
+)
+
+type Named struct {
+	A *string `android:"arch_variant"`
+	B *string
+}
+
+type NamedAllFiltered struct {
+	A *string
+}
+
+type NamedNoneFiltered struct {
+	A *string `android:"arch_variant"`
+}
+
+func TestFilterArchStruct(t *testing.T) {
+	tests := []struct {
+		name     string
+		in       interface{}
+		out      interface{}
+		filtered bool
+	}{
+		// Property tests
+		{
+			name: "basic",
+			in: &struct {
+				A *string `android:"arch_variant"`
+				B *string
+			}{},
+			out: &struct {
+				A *string
+			}{},
+			filtered: true,
+		},
+		{
+			name: "all filtered",
+			in: &struct {
+				A *string
+			}{},
+			out:      nil,
+			filtered: true,
+		},
+		{
+			name: "none filtered",
+			in: &struct {
+				A *string `android:"arch_variant"`
+			}{},
+			out: &struct {
+				A *string `android:"arch_variant"`
+			}{},
+			filtered: false,
+		},
+
+		// Sub-struct tests
+		{
+			name: "substruct",
+			in: &struct {
+				A struct {
+					A *string `android:"arch_variant"`
+					B *string
+				} `android:"arch_variant"`
+			}{},
+			out: &struct {
+				A struct {
+					A *string
+				}
+			}{},
+			filtered: true,
+		},
+		{
+			name: "substruct all filtered",
+			in: &struct {
+				A struct {
+					A *string
+				} `android:"arch_variant"`
+			}{},
+			out:      nil,
+			filtered: true,
+		},
+		{
+			name: "substruct none filtered",
+			in: &struct {
+				A struct {
+					A *string `android:"arch_variant"`
+				} `android:"arch_variant"`
+			}{},
+			out: &struct {
+				A struct {
+					A *string `android:"arch_variant"`
+				} `android:"arch_variant"`
+			}{},
+			filtered: false,
+		},
+
+		// Named sub-struct tests
+		{
+			name: "named substruct",
+			in: &struct {
+				A Named `android:"arch_variant"`
+			}{},
+			out: &struct {
+				A struct {
+					A *string
+				}
+			}{},
+			filtered: true,
+		},
+		{
+			name: "substruct all filtered",
+			in: &struct {
+				A NamedAllFiltered `android:"arch_variant"`
+			}{},
+			out:      nil,
+			filtered: true,
+		},
+		{
+			name: "substruct none filtered",
+			in: &struct {
+				A NamedNoneFiltered `android:"arch_variant"`
+			}{},
+			out: &struct {
+				A NamedNoneFiltered `android:"arch_variant"`
+			}{},
+			filtered: false,
+		},
+
+		// Pointer to sub-struct tests
+		{
+			name: "pointer substruct",
+			in: &struct {
+				A *struct {
+					A *string `android:"arch_variant"`
+					B *string
+				} `android:"arch_variant"`
+			}{},
+			out: &struct {
+				A *struct {
+					A *string
+				}
+			}{},
+			filtered: true,
+		},
+		{
+			name: "pointer substruct all filtered",
+			in: &struct {
+				A *struct {
+					A *string
+				} `android:"arch_variant"`
+			}{},
+			out:      nil,
+			filtered: true,
+		},
+		{
+			name: "pointer substruct none filtered",
+			in: &struct {
+				A *struct {
+					A *string `android:"arch_variant"`
+				} `android:"arch_variant"`
+			}{},
+			out: &struct {
+				A *struct {
+					A *string `android:"arch_variant"`
+				} `android:"arch_variant"`
+			}{},
+			filtered: false,
+		},
+
+		// Pointer to named sub-struct tests
+		{
+			name: "pointer named substruct",
+			in: &struct {
+				A *Named `android:"arch_variant"`
+			}{},
+			out: &struct {
+				A *struct {
+					A *string
+				}
+			}{},
+			filtered: true,
+		},
+		{
+			name: "pointer substruct all filtered",
+			in: &struct {
+				A *NamedAllFiltered `android:"arch_variant"`
+			}{},
+			out:      nil,
+			filtered: true,
+		},
+		{
+			name: "pointer substruct none filtered",
+			in: &struct {
+				A *NamedNoneFiltered `android:"arch_variant"`
+			}{},
+			out: &struct {
+				A *NamedNoneFiltered `android:"arch_variant"`
+			}{},
+			filtered: false,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			out, filtered := filterArchStruct(reflect.TypeOf(test.in))
+			if filtered != test.filtered {
+				t.Errorf("expected filtered %v, got %v", test.filtered, filtered)
+			}
+			expected := reflect.TypeOf(test.out)
+			if out != expected {
+				t.Errorf("expected type %v, got %v", expected, out)
+			}
+		})
+	}
+}