Fix OutgoingTransitionContext to match the description of TransitionMutators am: 33bec91aec

Original change: https://android-review.googlesource.com/c/platform/build/blueprint/+/2923530

Change-Id: I8f76a7cdc0694e0f8ea34fb1b17ade729a6cf412
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>

context.go

@@ -690,7 +690,7 @@ type IncomingTransitionContext interface {
 }
 
 type OutgoingTransitionContext interface {
-	// Module returns the target of the dependency edge for which the transition
+	// Module returns the source of the dependency edge for which the transition
 	// is being computed
 	Module() Module
 
@@ -699,7 +699,7 @@ type OutgoingTransitionContext interface {
 	DepTag() DependencyTag
 }
 
-// Transition mutators implement a top-down mechanism where a module tells its
+// TransitionMutator implements a top-down mechanism where a module tells its
 // direct dependencies what variation they should be built in but the dependency
 // has the final say.
 //
@@ -715,7 +715,7 @@ type OutgoingTransitionContext interface {
 // composition the outgoing transition of module A and the incoming transition
 // of module B.
 //
-// the outgoing transition should not take the properties of the dependency into
+// The outgoing transition should not take the properties of the dependency into
 // account, only those of the module that depends on it. For this reason, the
 // dependency is not even passed into it as an argument. Likewise, the incoming
 // transition should not take the properties of the depending module into
@@ -728,7 +728,7 @@ type OutgoingTransitionContext interface {
 // Soong makes sure that all modules are created in the desired variations and
 // that dependency edges are set up correctly. This ensures that "missing
 // variation" errors do not happen and allows for more flexible changes in the
-// value of the variation among dependency edges (as oppposed to bottom-up
+// value of the variation among dependency edges (as opposed to bottom-up
 // mutators where if module A in variation X depends on module B and module B
 // has that variation X, A must depend on variation X of B)
 //
@@ -757,27 +757,27 @@ type OutgoingTransitionContext interface {
 // thus some way is needed to change it state whereas Bazel creates each
 // configuration of a given configured target anew.
 type TransitionMutator interface {
-	// Returns the set of variations that should be created for a module no matter
+	// Split returns the set of variations that should be created for a module no matter
 	// who depends on it. Used when Make depends on a particular variation or when
 	// the module knows its variations just based on information given to it in
 	// the Blueprint file. This method should not mutate the module it is called
 	// on.
 	Split(ctx BaseModuleContext) []string
 
-	// Called on a module to determine which variation it wants from its direct
-	// dependencies. The dependency itself can override this decision. This method
-	// should not mutate the module itself.
+	// OutgoingTransition is called on a module to determine which variation it wants
+	// from its direct dependencies. The dependency itself can override this decision.
+	// This method should not mutate the module itself.
 	OutgoingTransition(ctx OutgoingTransitionContext, sourceVariation string) string
 
-	// Called on a module to determine which variation it should be in based on
-	// the variation modules that depend on it want. This gives the module a final
-	// say about its own variations. This method should not mutate the module
+	// IncomingTransition is called on a module to determine which variation it should
+	// be in based on the variation modules that depend on it want. This gives the module
+	// a final say about its own variations. This method should not mutate the module
 	// itself.
 	IncomingTransition(ctx IncomingTransitionContext, incomingVariation string) string
 
-	// Called after a module was split into multiple variations on each variation.
-	// It should not split the module any further but adding new dependencies is
-	// fine. Unlike all the other methods on TransitionMutator, this method is
+	// Mutate is called after a module was split into multiple variations on each
+	// variation.  It should not split the module any further but adding new dependencies
+	// is fine. Unlike all the other methods on TransitionMutator, this method is
 	// allowed to mutate the module.
 	Mutate(ctx BottomUpMutatorContext, variation string)
 }
@@ -841,15 +841,12 @@ func (t *transitionMutatorImpl) topDownMutator(mctx TopDownMutatorContext) {
 }
 
 type transitionContextImpl struct {
-	module Module
+	source Module
+	dep    Module
 	depTag DependencyTag
 	config interface{}
 }
 
-func (c *transitionContextImpl) Module() Module {
-	return c.module
-}
-
 func (c *transitionContextImpl) DepTag() DependencyTag {
 	return c.depTag
 }
@@ -858,11 +855,27 @@ func (c *transitionContextImpl) Config() interface{} {
 	return c.config
 }
 
+type outgoingTransitionContextImpl struct {
+	transitionContextImpl
+}
+
+func (c *outgoingTransitionContextImpl) Module() Module {
+	return c.source
+}
+
+type incomingTransitionContextImpl struct {
+	transitionContextImpl
+}
+
+func (c *incomingTransitionContextImpl) Module() Module {
+	return c.dep
+}
+
 func (t *transitionMutatorImpl) transition(mctx BaseMutatorContext) Transition {
 	return func(source Module, sourceVariation string, dep Module, depTag DependencyTag) string {
-		tc := &transitionContextImpl{module: dep, depTag: depTag, config: mctx.Config()}
-		outgoingVariation := t.mutator.OutgoingTransition(tc, sourceVariation)
-		finalVariation := t.mutator.IncomingTransition(tc, outgoingVariation)
+		tc := transitionContextImpl{source: source, dep: dep, depTag: depTag, config: mctx.Config()}
+		outgoingVariation := t.mutator.OutgoingTransition(&outgoingTransitionContextImpl{tc}, sourceVariation)
+		finalVariation := t.mutator.IncomingTransition(&incomingTransitionContextImpl{tc}, outgoingVariation)
 		return finalVariation
 	}
 }