provider.go

// 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
}