Add Providers to Blueprint

Providers are a new concept for Blueprint, based on providers in Bazel:
https://docs.bazel.build/versions/master/skylark/rules.html#providers

Providers aim to simplify the interaction between modules by replacing
type asserting to arbitrary interfaces with requesting optional data
objects from modules.  This will also move Blueprint closer to supporting
incremental analysis by serializing the providers and only rerunning
the analysis phase on modules whose inputs have changed.

Change-Id: I39f5f78b372412a7dbf151ceccb3f917f6c874bf

Blueprints

@@ -17,6 +17,7 @@ bootstrap_go_package {
         "ninja_strings.go",
         "ninja_writer.go",
         "package_ctx.go",
+        "provider.go",
         "scope.go",
         "singleton_ctx.go",
     ],
@@ -26,6 +27,7 @@ bootstrap_go_package {
         "module_ctx_test.go",
         "ninja_strings_test.go",
         "ninja_writer_test.go",
+        "provider_test.go",
         "splice_modules_test.go",
         "visit_test.go",
     ],

context.go

@@ -117,6 +117,19 @@ type Context struct {
 	srcDir         string
 	fs             pathtools.FileSystem
 	moduleListFile string
+
+	// Mutators indexed by the ID of the provider associated with them.  Not all mutators will
+	// have providers, and not all providers will have a mutator, or if they do the mutator may
+	// not be registered in this Context.
+	providerMutators []*mutatorInfo
+
+	// The currently running mutator
+	startedMutator *mutatorInfo
+	// True for any mutators that have already run over all modules
+	finishedMutators map[*mutatorInfo]bool
+
+	// Can be set by tests to avoid invalidating Module values after mutators.
+	skipCloneModulesAfterMutators bool
 }
 
 // An Error describes a problem that was encountered that is related to a
@@ -254,6 +267,14 @@ type moduleInfo struct {
 
 	// set during PrepareBuildActions
 	actionDefs localBuildActions
+
+	providers []interface{}
+
+	startedMutator  *mutatorInfo
+	finishedMutator *mutatorInfo
+
+	startedGenerateBuildActions  bool
+	finishedGenerateBuildActions bool
 }
 
 type variant struct {
@@ -363,6 +384,7 @@ func newContext() *Context {
 		moduleInfo:         make(map[Module]*moduleInfo),
 		globs:              make(map[string]GlobPath),
 		fs:                 pathtools.OsFs,
+		finishedMutators:   make(map[*mutatorInfo]bool),
 		ninjaBuildDir:      nil,
 		requiredNinjaMajor: 1,
 		requiredNinjaMinor: 7,
@@ -1330,10 +1352,11 @@ func (c *Context) createVariations(origModule *moduleInfo, mutatorName string,
 
 		m := *origModule
 		newModule := &m
-		newModule.directDeps = append([]depInfo{}, origModule.directDeps...)
+		newModule.directDeps = append([]depInfo(nil), origModule.directDeps...)
 		newModule.logicModule = newLogicModule
 		newModule.variant = newVariant(origModule, mutatorName, variationName, local)
 		newModule.properties = newProperties
+		newModule.providers = append([]interface{}(nil), origModule.providers...)
 
 		newModules = append(newModules, newModule)
 
@@ -1518,6 +1541,8 @@ func (c *Context) ResolveDependencies(config interface{}) (deps []string, errs [
 
 func (c *Context) resolveDependencies(ctx context.Context, config interface{}) (deps []string, errs []error) {
 	pprof.Do(ctx, pprof.Labels("blueprint", "ResolveDependencies"), func(ctx context.Context) {
+		c.initProviders()
+
 		c.liveGlobals = newLiveTracker(config)
 
 		deps, errs = c.generateSingletonBuildActions(config, c.preSingletonInfo, c.liveGlobals)
@@ -1537,7 +1562,9 @@ func (c *Context) resolveDependencies(ctx context.Context, config interface{}) (
 		}
 		deps = append(deps, mutatorDeps...)
 
-		c.cloneModules()
+		if !c.skipCloneModulesAfterMutators {
+			c.cloneModules()
+		}
 
 		c.dependenciesReady = true
 	})
@@ -2419,6 +2446,8 @@ func (c *Context) runMutator(config interface{}, mutator *mutatorInfo,
 			pauseCh: pause,
 		}
 
+		module.startedMutator = mutator
+
 		func() {
 			defer func() {
 				if r := recover(); r != nil {
@@ -2434,6 +2463,8 @@ func (c *Context) runMutator(config interface{}, mutator *mutatorInfo,
 			direction.run(mutator, mctx)
 		}()
 
+		module.finishedMutator = mutator
+
 		if len(mctx.errs) > 0 {
 			errsCh <- mctx.errs
 			return true
@@ -2482,6 +2513,8 @@ func (c *Context) runMutator(config interface{}, mutator *mutatorInfo,
 		}
 	}()
 
+	c.startedMutator = mutator
+
 	var visitErrs []error
 	if mutator.parallel {
 		visitErrs = parallelVisit(c.modulesSorted, direction.orderer(), parallelVisitLimit, visit)
@@ -2493,6 +2526,8 @@ func (c *Context) runMutator(config interface{}, mutator *mutatorInfo,
 		return nil, visitErrs
 	}
 
+	c.finishedMutators[mutator] = true
+
 	done <- true
 
 	if len(errs) > 0 {
@@ -2702,6 +2737,8 @@ func (c *Context) generateModuleBuildActions(config interface{},
 				handledMissingDeps: module.missingDeps == nil,
 			}
 
+			mctx.module.startedGenerateBuildActions = true
+
 			func() {
 				defer func() {
 					if r := recover(); r != nil {
@@ -2717,6 +2754,8 @@ func (c *Context) generateModuleBuildActions(config interface{},
 				mctx.module.logicModule.GenerateBuildActions(mctx)
 			}()
 
+			mctx.module.finishedGenerateBuildActions = true
+
 			if len(mctx.errs) > 0 {
 				errsCh <- mctx.errs
 				return true
@@ -3287,6 +3326,25 @@ func (c *Context) ModuleType(logicModule Module) string {
 	return module.typeName
 }
 
+// ModuleProvider returns the value, if any, for the provider for a module.  If the value for the
+// provider was not set it returns the zero value of the type of the provider, which means the
+// return value can always be type-asserted to the type of the provider.  The return value should
+// always be considered read-only.  It panics if called before the appropriate mutator or
+// GenerateBuildActions pass for the provider on the module.  The value returned may be a deep
+// copy of the value originally passed to SetProvider.
+func (c *Context) ModuleProvider(logicModule Module, provider ProviderKey) interface{} {
+	module := c.moduleInfo[logicModule]
+	value, _ := c.provider(module, provider)
+	return value
+}
+
+// ModuleHasProvider returns true if the provider for the given module has been set.
+func (c *Context) ModuleHasProvider(logicModule Module, provider ProviderKey) bool {
+	module := c.moduleInfo[logicModule]
+	_, ok := c.provider(module, provider)
+	return ok
+}
+
 func (c *Context) BlueprintFile(logicModule Module) string {
 	module := c.moduleInfo[logicModule]
 	return module.relBlueprintsFile

module_ctx.go

@@ -306,6 +306,31 @@ type BaseModuleContext interface {
 	// other words, it checks for the module AddReverseDependency would add a
 	// dependency on with the same argument.
 	OtherModuleReverseDependencyVariantExists(name string) bool
+
+	// OtherModuleProvider returns the value for a provider for the given module.  If the value is
+	// not set it returns the zero value of the type of the provider, so the return value can always
+	// be type asserted to the type of the provider.  The value returned may be a deep copy of the
+	// value originally passed to SetProvider.
+	OtherModuleProvider(m Module, provider ProviderKey) interface{}
+
+	// OtherModuleHasProvider returns true if the provider for the given module has been set.
+	OtherModuleHasProvider(m Module, provider ProviderKey) bool
+
+	// Provider returns the value for a provider for the current module.  If the value is
+	// not set it returns the zero value of the type of the provider, so the return value can always
+	// be type asserted to the type of the provider.  It panics if called before the appropriate
+	// mutator or GenerateBuildActions pass for the provider.  The value returned may be a deep
+	// copy of the value originally passed to SetProvider.
+	Provider(provider ProviderKey) interface{}
+
+	// HasProvider returns true if the provider for the current module has been set.
+	HasProvider(provider ProviderKey) bool
+
+	// SetProvider sets the value for a provider for the current module.  It panics if not called
+	// during the appropriate mutator or GenerateBuildActions pass for the provider, if the value
+	// is not of the appropriate type, or if the value has already been set.  The value should not
+	// be modified after being passed to SetProvider.
+	SetProvider(provider ProviderKey, value interface{})
 }
 
 type DynamicDependerModuleContext BottomUpMutatorContext
@@ -523,6 +548,32 @@ func (m *baseModuleContext) OtherModuleReverseDependencyVariantExists(name strin
 	return found != nil
 }
 
+func (m *baseModuleContext) OtherModuleProvider(logicModule Module, provider ProviderKey) interface{} {
+	module := m.context.moduleInfo[logicModule]
+	value, _ := m.context.provider(module, provider)
+	return value
+}
+
+func (m *baseModuleContext) OtherModuleHasProvider(logicModule Module, provider ProviderKey) bool {
+	module := m.context.moduleInfo[logicModule]
+	_, ok := m.context.provider(module, provider)
+	return ok
+}
+
+func (m *baseModuleContext) Provider(provider ProviderKey) interface{} {
+	value, _ := m.context.provider(m.module, provider)
+	return value
+}
+
+func (m *baseModuleContext) HasProvider(provider ProviderKey) bool {
+	_, ok := m.context.provider(m.module, provider)
+	return ok
+}
+
+func (m *baseModuleContext) SetProvider(provider ProviderKey, value interface{}) {
+	m.context.setProvider(m.module, provider, value)
+}
+
 func (m *baseModuleContext) GetDirectDep(name string) (Module, DependencyTag) {
 	for _, dep := range m.module.directDeps {
 		if dep.module.Name() == name {
@@ -882,6 +933,14 @@ type BottomUpMutatorContext interface {
 	// be used to add dependencies on the toVariationName variant using the fromVariationName
 	// variant.
 	CreateAliasVariation(fromVariationName, toVariationName string)
+
+	// SetVariationProvider sets the value for a provider for the given newly created variant of
+	// the current module, i.e. one of the Modules returned by CreateVariations..  It panics if
+	// not called during the appropriate mutator or GenerateBuildActions pass for the provider,
+	// if the value is not of the appropriate type, or if the module is not a newly created
+	// variant of the current module.  The value should not be modified after being passed to
+	// SetVariationProvider.
+	SetVariationProvider(module Module, provider ProviderKey, value interface{})
 }
 
 // A Mutator function is called for each Module, and can use
@@ -925,6 +984,16 @@ func (mctx *mutatorContext) CreateLocalVariations(variationNames ...string) []Mo
 	return mctx.createVariations(variationNames, true)
 }
 
+func (mctx *mutatorContext) SetVariationProvider(module Module, provider ProviderKey, value interface{}) {
+	for _, variant := range mctx.newVariations {
+		if m := variant.module(); m != nil && m.logicModule == module {
+			mctx.context.setProvider(m, provider, value)
+			return
+		}
+	}
+	panic(fmt.Errorf("module %q is not a newly created variant of %q", module, mctx.module))
+}
+
 type pendingAlias struct {
 	fromVariant variant
 	target      *moduleInfo

provider.go

@@ -0,0 +1,216 @@
+// Copyright 2020 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 blueprint
+
+import (
+	"fmt"
+	"reflect"
+)
+
+// This file implements Providers, modelled after Bazel
+// (https://docs.bazel.build/versions/master/skylark/rules.html#providers).
+// Each provider can be associated with a mutator, in which case the value for the provider for a
+// module can only be set during the mutator call for the module, and the value can only be
+// retrieved after the mutator call for the module. For providers not associated with a mutator, the
+// value can for the provider for a module can only be set during GenerateBuildActions for the
+// module, and the value can only be retrieved after GenerateBuildActions for the module.
+//
+// Providers are globally registered during init() and given a unique ID.  The value of a provider
+// for a module is stored in an []interface{} indexed by the ID.  If the value of a provider has
+// not been set, the value in the []interface{} will be nil.
+//
+// If the storage used by the provider value arrays becomes too large:
+//  sizeof([]interface) * number of providers * number of modules that have a provider value set
+// then the storage can be replaced with something like a bitwise trie.
+//
+// The purpose of providers is to provide a serializable checkpoint between modules to enable
+// Blueprint to skip parts of the analysis phase when inputs haven't changed.  To that end,
+// values passed to providers should be treated as immutable by callers to both the getters and
+// setters.  Go doesn't provide any way to enforce immutability on arbitrary types, so it may be
+// necessary for the getters and setters to make deep copies of the values, likely extending
+// proptools.CloneProperties to do so.
+
+type provider struct {
+	id      int
+	typ     reflect.Type
+	zero    interface{}
+	mutator string
+}
+
+type ProviderKey *provider
+
+var providerRegistry []ProviderKey
+
+// NewProvider returns a ProviderKey for the type of the given example value.  The example value
+// is otherwise unused.
+//
+// The returned ProviderKey can be used to set a value of the ProviderKey's type for a module
+// inside GenerateBuildActions for the module, and to get the value from GenerateBuildActions from
+// any module later in the build graph.
+//
+// Once Go has generics the exampleValue parameter will not be necessary:
+// NewProvider(type T)() ProviderKey(T)
+func NewProvider(exampleValue interface{}) ProviderKey {
+	return NewMutatorProvider(exampleValue, "")
+}
+
+// NewMutatorProvider returns a ProviderKey for the type of the given example value.  The example
+// value is otherwise unused.
+//
+// The returned ProviderKey can be used to set a value of the ProviderKey's type for a module inside
+// the given mutator for the module, and to get the value from GenerateBuildActions from any
+// module later in the build graph in the same mutator, or any module in a later mutator or during
+// GenerateBuildActions.
+//
+// Once Go has generics the exampleValue parameter will not be necessary:
+// NewMutatorProvider(type T)(mutator string) ProviderKey(T)
+func NewMutatorProvider(exampleValue interface{}, mutator string) ProviderKey {
+	checkCalledFromInit()
+
+	typ := reflect.TypeOf(exampleValue)
+	zero := reflect.Zero(typ).Interface()
+
+	provider := &provider{
+		id:      len(providerRegistry),
+		typ:     typ,
+		zero:    zero,
+		mutator: mutator,
+	}
+
+	providerRegistry = append(providerRegistry, provider)
+
+	return provider
+}
+
+// initProviders fills c.providerMutators with the *mutatorInfo associated with each provider ID,
+// if any.
+func (c *Context) initProviders() {
+	c.providerMutators = make([]*mutatorInfo, len(providerRegistry))
+	for _, provider := range providerRegistry {
+		for _, mutator := range c.mutatorInfo {
+			if mutator.name == provider.mutator {
+				c.providerMutators[provider.id] = mutator
+			}
+		}
+	}
+}
+
+// setProvider sets the value for a provider on a moduleInfo.  Verifies that it is called during the
+// appropriate mutator or GenerateBuildActions pass for the provider, and that the value is of the
+// appropriate type.  The value should not be modified after being passed to setProvider.
+//
+// Once Go has generics the value parameter can be typed:
+// setProvider(type T)(m *moduleInfo, provider ProviderKey(T), value T)
+func (c *Context) setProvider(m *moduleInfo, provider ProviderKey, value interface{}) {
+	if provider.mutator == "" {
+		if !m.startedGenerateBuildActions {
+			panic(fmt.Sprintf("Can't set value of provider %s before GenerateBuildActions started",
+				provider.typ))
+		} else if m.finishedGenerateBuildActions {
+			panic(fmt.Sprintf("Can't set value of provider %s after GenerateBuildActions finished",
+				provider.typ))
+		}
+	} else {
+		expectedMutator := c.providerMutators[provider.id]
+		if expectedMutator == nil {
+			panic(fmt.Sprintf("Can't set value of provider %s associated with unregistered mutator %s",
+				provider.typ, provider.mutator))
+		} else if c.mutatorFinishedForModule(expectedMutator, m) {
+			panic(fmt.Sprintf("Can't set value of provider %s after mutator %s finished",
+				provider.typ, provider.mutator))
+		} else if !c.mutatorStartedForModule(expectedMutator, m) {
+			panic(fmt.Sprintf("Can't set value of provider %s before mutator %s started",
+				provider.typ, provider.mutator))
+		}
+	}
+
+	if typ := reflect.TypeOf(value); typ != provider.typ {
+		panic(fmt.Sprintf("Value for provider has incorrect type, wanted %s, got %s",
+			provider.typ, typ))
+	}
+
+	if m.providers == nil {
+		m.providers = make([]interface{}, len(providerRegistry))
+	}
+
+	if m.providers[provider.id] != nil {
+		panic(fmt.Sprintf("Value of provider %s is already set", provider.typ))
+	}
+
+	m.providers[provider.id] = value
+}
+
+// provider returns the value, if any, for a given provider for a module.  Verifies that it is
+// called after the appropriate mutator or GenerateBuildActions pass for the provider on the module.
+// If the value for the provider was not set it returns the zero value of the type of the provider,
+// which means the return value can always be type-asserted to the type of the provider.  The return
+// value should always be considered read-only.
+//
+// Once Go has generics the return value can be typed and the type assert by callers can be dropped:
+// provider(type T)(m *moduleInfo, provider ProviderKey(T)) T
+func (c *Context) provider(m *moduleInfo, provider ProviderKey) (interface{}, bool) {
+	if provider.mutator == "" {
+		if !m.finishedGenerateBuildActions {
+			panic(fmt.Sprintf("Can't get value of provider %s before GenerateBuildActions finished",
+				provider.typ))
+		}
+	} else {
+		expectedMutator := c.providerMutators[provider.id]
+		if expectedMutator != nil && !c.mutatorFinishedForModule(expectedMutator, m) {
+			panic(fmt.Sprintf("Can't get value of provider %s before mutator %s finished",
+				provider.typ, provider.mutator))
+		}
+	}
+
+	if len(m.providers) > provider.id {
+		if p := m.providers[provider.id]; p != nil {
+			return p, true
+		}
+	}
+
+	return provider.zero, false
+}
+
+func (c *Context) mutatorFinishedForModule(mutator *mutatorInfo, m *moduleInfo) bool {
+	if c.finishedMutators[mutator] {
+		// mutator pass finished for all modules
+		return true
+	}
+
+	if c.startedMutator == mutator {
+		// mutator pass started, check if it is finished for this module
+		return m.finishedMutator == mutator
+	}
+
+	// mutator pass hasn't started
+	return false
+}
+
+func (c *Context) mutatorStartedForModule(mutator *mutatorInfo, m *moduleInfo) bool {
+	if c.finishedMutators[mutator] {
+		// mutator pass finished for all modules
+		return true
+	}
+
+	if c.startedMutator == mutator {
+		// mutator pass is currently running
+		if m.startedMutator == mutator {
+			// mutator has started for this module
+			return true
+		}
+	}
+
+	return false
+}

provider_test.go

@@ -0,0 +1,420 @@
+// Copyright 2020 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 blueprint
+
+import (
+	"fmt"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+type providerTestModule struct {
+	SimpleName
+	properties struct {
+		Deps []string
+	}
+
+	mutatorProviderValues              []string
+	generateBuildActionsProviderValues []string
+}
+
+func newProviderTestModule() (Module, []interface{}) {
+	m := &providerTestModule{}
+	return m, []interface{}{&m.properties, &m.SimpleName.Properties}
+}
+
+type providerTestMutatorInfo struct {
+	Values []string
+}
+
+type providerTestGenerateBuildActionsInfo struct {
+	Value string
+}
+
+type providerTestUnsetInfo string
+
+var providerTestMutatorInfoProvider = NewMutatorProvider(&providerTestMutatorInfo{}, "provider_mutator")
+var providerTestGenerateBuildActionsInfoProvider = NewProvider(&providerTestGenerateBuildActionsInfo{})
+var providerTestUnsetInfoProvider = NewMutatorProvider((providerTestUnsetInfo)(""), "provider_mutator")
+var providerTestUnusedMutatorProvider = NewMutatorProvider(&struct{ unused string }{}, "nonexistent_mutator")
+
+func (p *providerTestModule) GenerateBuildActions(ctx ModuleContext) {
+	unset := ctx.Provider(providerTestUnsetInfoProvider).(providerTestUnsetInfo)
+	if unset != "" {
+		panic(fmt.Sprintf("expected zero value for providerTestGenerateBuildActionsInfoProvider before it was set, got %q",
+			unset))
+	}
+
+	_ = ctx.Provider(providerTestUnusedMutatorProvider)
+
+	ctx.SetProvider(providerTestGenerateBuildActionsInfoProvider, &providerTestGenerateBuildActionsInfo{
+		Value: ctx.ModuleName(),
+	})
+
+	mp := ctx.Provider(providerTestMutatorInfoProvider).(*providerTestMutatorInfo)
+	if mp != nil {
+		p.mutatorProviderValues = mp.Values
+	}
+
+	ctx.VisitDirectDeps(func(module Module) {
+		gbap := ctx.OtherModuleProvider(module, providerTestGenerateBuildActionsInfoProvider).(*providerTestGenerateBuildActionsInfo)
+		if gbap != nil {
+			p.generateBuildActionsProviderValues = append(p.generateBuildActionsProviderValues, gbap.Value)
+		}
+	})
+}
+
+func providerTestDepsMutator(ctx BottomUpMutatorContext) {
+	if p, ok := ctx.Module().(*providerTestModule); ok {
+		ctx.AddDependency(ctx.Module(), nil, p.properties.Deps...)
+	}
+}
+
+func providerTestMutator(ctx BottomUpMutatorContext) {
+	values := []string{strings.ToLower(ctx.ModuleName())}
+
+	ctx.VisitDirectDeps(func(module Module) {
+		mp := ctx.OtherModuleProvider(module, providerTestMutatorInfoProvider).(*providerTestMutatorInfo)
+		if mp != nil {
+			values = append(values, mp.Values...)
+		}
+	})
+
+	ctx.SetProvider(providerTestMutatorInfoProvider, &providerTestMutatorInfo{
+		Values: values,
+	})
+}
+
+func providerTestAfterMutator(ctx BottomUpMutatorContext) {
+	_ = ctx.Provider(providerTestMutatorInfoProvider)
+}
+
+func TestProviders(t *testing.T) {
+	ctx := NewContext()
+	ctx.RegisterModuleType("provider_module", newProviderTestModule)
+	ctx.RegisterBottomUpMutator("provider_deps_mutator", providerTestDepsMutator)
+	ctx.RegisterBottomUpMutator("provider_mutator", providerTestMutator)
+	ctx.RegisterBottomUpMutator("provider_after_mutator", providerTestAfterMutator)
+
+	ctx.MockFileSystem(map[string][]byte{
+		"Blueprints": []byte(`
+			provider_module {
+				name: "A",
+				deps: ["B"],
+			}
+	
+			provider_module {
+				name: "B",
+				deps: ["C", "D"],
+			}
+	
+			provider_module {
+				name: "C",
+				deps: ["D"],
+			}
+	
+			provider_module {
+				name: "D",
+			}
+		`),
+	})
+
+	_, errs := ctx.ParseBlueprintsFiles("Blueprints", nil)
+	if len(errs) == 0 {
+		_, errs = ctx.ResolveDependencies(nil)
+	}
+	if len(errs) == 0 {
+		_, errs = ctx.PrepareBuildActions(nil)
+	}
+	if len(errs) > 0 {
+		t.Errorf("unexpected errors:")
+		for _, err := range errs {
+			t.Errorf("  %s", err)
+		}
+		t.FailNow()
+	}
+
+	aModule := ctx.moduleGroupFromName("A", nil).moduleByVariantName("").logicModule.(*providerTestModule)
+	if g, w := aModule.generateBuildActionsProviderValues, []string{"B"}; !reflect.DeepEqual(g, w) {
+		t.Errorf("expected A.generateBuildActionsProviderValues %q, got %q", w, g)
+	}
+	if g, w := aModule.mutatorProviderValues, []string{"a", "b", "c", "d", "d"}; !reflect.DeepEqual(g, w) {
+		t.Errorf("expected A.mutatorProviderValues %q, got %q", w, g)
+	}
+
+	bModule := ctx.moduleGroupFromName("B", nil).moduleByVariantName("").logicModule.(*providerTestModule)
+	if g, w := bModule.generateBuildActionsProviderValues, []string{"C", "D"}; !reflect.DeepEqual(g, w) {
+		t.Errorf("expected B.generateBuildActionsProviderValues %q, got %q", w, g)
+	}
+	if g, w := bModule.mutatorProviderValues, []string{"b", "c", "d", "d"}; !reflect.DeepEqual(g, w) {
+		t.Errorf("expected B.mutatorProviderValues %q, got %q", w, g)
+	}
+}
+
+type invalidProviderUsageMutatorInfo string
+type invalidProviderUsageGenerateBuildActionsInfo string
+
+var invalidProviderUsageMutatorInfoProvider = NewMutatorProvider(invalidProviderUsageMutatorInfo(""), "mutator_under_test")
+var invalidProviderUsageGenerateBuildActionsInfoProvider = NewProvider(invalidProviderUsageGenerateBuildActionsInfo(""))
+
+type invalidProviderUsageTestModule struct {
+	parent *invalidProviderUsageTestModule
+
+	SimpleName
+	properties struct {
+		Deps []string
+
+		Early_mutator_set_of_mutator_provider       bool
+		Late_mutator_set_of_mutator_provider        bool
+		Late_build_actions_set_of_mutator_provider  bool
+		Early_mutator_set_of_build_actions_provider bool
+
+		Early_mutator_get_of_mutator_provider       bool
+		Early_module_get_of_mutator_provider        bool
+		Early_mutator_get_of_build_actions_provider bool
+		Early_module_get_of_build_actions_provider  bool
+
+		Duplicate_set bool
+	}
+}
+
+func invalidProviderUsageDepsMutator(ctx BottomUpMutatorContext) {
+	if i, ok := ctx.Module().(*invalidProviderUsageTestModule); ok {
+		ctx.AddDependency(ctx.Module(), nil, i.properties.Deps...)
+	}
+}
+
+func invalidProviderUsageParentMutator(ctx TopDownMutatorContext) {
+	if i, ok := ctx.Module().(*invalidProviderUsageTestModule); ok {
+		ctx.VisitDirectDeps(func(module Module) {
+			module.(*invalidProviderUsageTestModule).parent = i
+		})
+	}
+}
+
+func invalidProviderUsageBeforeMutator(ctx BottomUpMutatorContext) {
+	if i, ok := ctx.Module().(*invalidProviderUsageTestModule); ok {
+		if i.properties.Early_mutator_set_of_mutator_provider {
+			// A mutator attempting to set the value of a provider associated with a later mutator.
+			ctx.SetProvider(invalidProviderUsageMutatorInfoProvider, invalidProviderUsageMutatorInfo(""))
+		}
+		if i.properties.Early_mutator_get_of_mutator_provider {
+			// A mutator attempting to get the value of a provider associated with a later mutator.
+			_ = ctx.Provider(invalidProviderUsageMutatorInfoProvider)
+		}
+	}
+}
+
+func invalidProviderUsageMutatorUnderTest(ctx TopDownMutatorContext) {
+	if i, ok := ctx.Module().(*invalidProviderUsageTestModule); ok {
+		if i.properties.Early_mutator_set_of_build_actions_provider {
+			// A mutator attempting to set the value of a non-mutator provider.
+			ctx.SetProvider(invalidProviderUsageGenerateBuildActionsInfoProvider, invalidProviderUsageGenerateBuildActionsInfo(""))
+		}
+		if i.properties.Early_mutator_get_of_build_actions_provider {
+			// A mutator attempting to get the value of a non-mutator provider.
+			_ = ctx.Provider(invalidProviderUsageGenerateBuildActionsInfoProvider)
+		}
+		if i.properties.Early_module_get_of_mutator_provider {
+			// A mutator attempting to get the value of a provider associated with this mutator on
+			// a module for which this mutator hasn't run.  This is a top down mutator so
+			// dependencies haven't run yet.
+			ctx.VisitDirectDeps(func(module Module) {
+				_ = ctx.OtherModuleProvider(module, invalidProviderUsageMutatorInfoProvider)
+			})
+		}
+	}
+}
+
+func invalidProviderUsageAfterMutator(ctx BottomUpMutatorContext) {
+	if i, ok := ctx.Module().(*invalidProviderUsageTestModule); ok {
+		if i.properties.Late_mutator_set_of_mutator_provider {
+			// A mutator trying to set the value of a provider associated with an earlier mutator.
+			ctx.SetProvider(invalidProviderUsageMutatorInfoProvider, invalidProviderUsageMutatorInfo(""))
+		}
+		if i.properties.Late_mutator_set_of_mutator_provider {
+			// A mutator trying to set the value of a provider associated with an earlier mutator.
+			ctx.SetProvider(invalidProviderUsageMutatorInfoProvider, invalidProviderUsageMutatorInfo(""))
+		}
+	}
+}
+
+func (i *invalidProviderUsageTestModule) GenerateBuildActions(ctx ModuleContext) {
+	if i.properties.Late_build_actions_set_of_mutator_provider {
+		// A GenerateBuildActions trying to set the value of a provider associated with a mutator.
+		ctx.SetProvider(invalidProviderUsageMutatorInfoProvider, invalidProviderUsageMutatorInfo(""))
+	}
+	if i.properties.Early_module_get_of_build_actions_provider {
+		// A GenerateBuildActions trying to get the value of a provider on a module for which
+		// GenerateBuildActions hasn't run.
+		_ = ctx.OtherModuleProvider(i.parent, invalidProviderUsageGenerateBuildActionsInfoProvider)
+	}
+	if i.properties.Duplicate_set {
+		ctx.SetProvider(invalidProviderUsageGenerateBuildActionsInfoProvider, invalidProviderUsageGenerateBuildActionsInfo(""))
+		ctx.SetProvider(invalidProviderUsageGenerateBuildActionsInfoProvider, invalidProviderUsageGenerateBuildActionsInfo(""))
+	}
+}
+
+func TestInvalidProvidersUsage(t *testing.T) {
+	run := func(t *testing.T, module string, prop string, panicMsg string) {
+		t.Helper()
+		ctx := NewContext()
+		ctx.RegisterModuleType("invalid_provider_usage_test_module", func() (Module, []interface{}) {
+			m := &invalidProviderUsageTestModule{}
+			return m, []interface{}{&m.properties, &m.SimpleName.Properties}
+		})
+		ctx.RegisterBottomUpMutator("deps", invalidProviderUsageDepsMutator)
+		ctx.RegisterBottomUpMutator("before", invalidProviderUsageBeforeMutator)
+		ctx.RegisterTopDownMutator("mutator_under_test", invalidProviderUsageMutatorUnderTest)
+		ctx.RegisterBottomUpMutator("after", invalidProviderUsageAfterMutator)
+		ctx.RegisterTopDownMutator("parent", invalidProviderUsageParentMutator)
+
+		// Don't invalidate the parent pointer and before GenerateBuildActions.
+		ctx.skipCloneModulesAfterMutators = true
+
+		var parentBP, moduleUnderTestBP, childBP string
+
+		prop += ": true,"
+
+		switch module {
+		case "parent":
+			parentBP = prop
+		case "module_under_test":
+			moduleUnderTestBP = prop
+		case "child":
+			childBP = prop
+		}
+
+		bp := fmt.Sprintf(`
+			invalid_provider_usage_test_module {
+				name: "parent",
+				deps: ["module_under_test"],
+				%s
+			}
+	
+			invalid_provider_usage_test_module {
+				name: "module_under_test",
+				deps: ["child"],
+				%s
+			}
+	
+			invalid_provider_usage_test_module {
+				name: "child",
+				%s
+			}
+
+		`,
+			parentBP,
+			moduleUnderTestBP,
+			childBP)
+
+		ctx.MockFileSystem(map[string][]byte{
+			"Blueprints": []byte(bp),
+		})
+
+		_, errs := ctx.ParseBlueprintsFiles("Blueprints", nil)
+
+		if len(errs) == 0 {
+			_, errs = ctx.ResolveDependencies(nil)
+		}
+
+		if len(errs) == 0 {
+			_, errs = ctx.PrepareBuildActions(nil)
+		}
+
+		if len(errs) == 0 {
+			t.Fatal("expected an error")
+		}
+
+		if len(errs) > 1 {
+			t.Errorf("expected a single error, got %d:", len(errs))
+			for i, err := range errs {
+				t.Errorf("%d:  %s", i, err)
+			}
+			t.FailNow()
+		}
+
+		if panicErr, ok := errs[0].(panicError); ok {
+			if panicErr.panic != panicMsg {
+				t.Fatalf("expected panic %q, got %q", panicMsg, panicErr.panic)
+			}
+		} else {
+			t.Fatalf("expected a panicError, got %T: %s", errs[0], errs[0].Error())
+		}
+
+	}
+
+	tests := []struct {
+		prop   string
+		module string
+
+		panicMsg string
+		skip     string
+	}{
+		{
+			prop:     "early_mutator_set_of_mutator_provider",
+			module:   "module_under_test",
+			panicMsg: "Can't set value of provider blueprint.invalidProviderUsageMutatorInfo before mutator mutator_under_test started",
+		},
+		{
+			prop:     "late_mutator_set_of_mutator_provider",
+			module:   "module_under_test",
+			panicMsg: "Can't set value of provider blueprint.invalidProviderUsageMutatorInfo after mutator mutator_under_test finished",
+		},
+		{
+			prop:     "late_build_actions_set_of_mutator_provider",
+			module:   "module_under_test",
+			panicMsg: "Can't set value of provider blueprint.invalidProviderUsageMutatorInfo after mutator mutator_under_test finished",
+		},
+		{
+			prop:     "early_mutator_set_of_build_actions_provider",
+			module:   "module_under_test",
+			panicMsg: "Can't set value of provider blueprint.invalidProviderUsageGenerateBuildActionsInfo before GenerateBuildActions started",
+		},
+
+		{
+			prop:     "early_mutator_get_of_mutator_provider",
+			module:   "module_under_test",
+			panicMsg: "Can't get value of provider blueprint.invalidProviderUsageMutatorInfo before mutator mutator_under_test finished",
+		},
+		{
+			prop:     "early_module_get_of_mutator_provider",
+			module:   "module_under_test",
+			panicMsg: "Can't get value of provider blueprint.invalidProviderUsageMutatorInfo before mutator mutator_under_test finished",
+		},
+		{
+			prop:     "early_mutator_get_of_build_actions_provider",
+			module:   "module_under_test",
+			panicMsg: "Can't get value of provider blueprint.invalidProviderUsageGenerateBuildActionsInfo before GenerateBuildActions finished",
+		},
+		{
+			prop:     "early_module_get_of_build_actions_provider",
+			module:   "module_under_test",
+			panicMsg: "Can't get value of provider blueprint.invalidProviderUsageGenerateBuildActionsInfo before GenerateBuildActions finished",
+		},
+		{
+			prop:     "duplicate_set",
+			module:   "module_under_test",
+			panicMsg: "Value of provider blueprint.invalidProviderUsageGenerateBuildActionsInfo is already set",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.prop, func(t *testing.T) {
+			run(t, tt.module, tt.prop, tt.panicMsg)
+		})
+	}
+}

singleton_ctx.go

@@ -47,6 +47,16 @@ type SingletonContext interface {
 	// BlueprintFile returns the path of the Blueprint file that defined the given module.
 	BlueprintFile(module Module) string
 
+	// ModuleProvider returns the value, if any, for the provider for a module.  If the value for the
+	// provider was not set it returns the zero value of the type of the provider, which means the
+	// return value can always be type-asserted to the type of the provider.  The return value should
+	// always be considered read-only.  It panics if called before the appropriate mutator or
+	// GenerateBuildActions pass for the provider on the module.
+	ModuleProvider(module Module, provider ProviderKey) interface{}
+
+	// ModuleHasProvider returns true if the provider for the given module has been set.
+	ModuleHasProvider(m Module, provider ProviderKey) bool
+
 	// ModuleErrorf reports an error at the line number of the module type in the module definition.
 	ModuleErrorf(module Module, format string, args ...interface{})
 
@@ -188,6 +198,15 @@ func (s *singletonContext) ModuleType(logicModule Module) string {
 	return s.context.ModuleType(logicModule)
 }
 
+func (s *singletonContext) ModuleProvider(logicModule Module, provider ProviderKey) interface{} {
+	return s.context.ModuleProvider(logicModule, provider)
+}
+
+// ModuleHasProvider returns true if the provider for the given module has been set.
+func (s *singletonContext) ModuleHasProvider(logicModule Module, provider ProviderKey) bool {
+	return s.context.ModuleHasProvider(logicModule, provider)
+}
+
 func (s *singletonContext) BlueprintFile(logicModule Module) string {
 	return s.context.BlueprintFile(logicModule)
 }