Call TransitionMutator.IncomingTransition when adding dependencies later

Adding a dependency on a module that has already had a TransitionMutator
run on it may require adjusting the variation name based on the results
of IncomingTranstion.  Store the variants that existed before the
TransitionMutator ran, find one that is a subset of the requested
variant, and call TranstionMutator.IncomingTransition to update the
value.

Bug: 319288033
Test: TestPostTransitionDeps
Change-Id: I690357f9792401a3edbc5ae9fdcb666495954fbc

5 files changed, 277 insertions, 112 deletions

diff --git a/context.go b/context.go
index 0dbd223..738e4c3 100644
--- a/context.go
+++ b/context.go
@@ -91,6 +91,8 @@ type Context struct {
 	mutatorInfo         []*mutatorInfo
 	variantMutatorNames []string
 
+	transitionMutators []*transitionMutatorImpl
+
 	depsModified uint32 // positive if a mutator modified the dependencies
 
 	dependenciesReady bool // set to true on a successful ResolveDependencies
@@ -455,10 +457,11 @@ type singletonInfo struct {
 
 type mutatorInfo struct {
 	// set during RegisterMutator
-	topDownMutator  TopDownMutator
-	bottomUpMutator BottomUpMutator
-	name            string
-	parallel        bool
+	topDownMutator    TopDownMutator
+	bottomUpMutator   BottomUpMutator
+	name              string
+	parallel          bool
+	transitionMutator *transitionMutatorImpl
 }
 
 func newContext() *Context {
@@ -686,6 +689,8 @@ type MutatorHandle interface {
 	// method on the mutator context is thread-safe, but the mutator must handle synchronization
 	// for any modifications to global state or any modules outside the one it was invoked on.
 	Parallel() MutatorHandle
+
+	setTransitionMutator(impl *transitionMutatorImpl) MutatorHandle
 }
 
 func (mutator *mutatorInfo) Parallel() MutatorHandle {
@@ -693,6 +698,11 @@ func (mutator *mutatorInfo) Parallel() MutatorHandle {
 	return mutator
 }
 
+func (mutator *mutatorInfo) setTransitionMutator(impl *transitionMutatorImpl) MutatorHandle {
+	mutator.transitionMutator = impl
+	return mutator
+}
+
 // SetIgnoreUnknownModuleTypes sets the behavior of the context in the case
 // where it encounters an unknown module type while parsing Blueprints files. By
 // default, the context will report unknown module types as an error.  If this
@@ -1489,7 +1499,7 @@ func (c *Context) createVariations(origModule *moduleInfo, mutator *mutatorInfo,
 		var newLogicModule Module
 		var newProperties []interface{}
 
-		if i == 0 {
+		if i == 0 && mutator.transitionMutator == nil {
 			// Reuse the existing module for the first new variant
 			// This both saves creating a new module, and causes the insertion in c.moduleInfo below
 			// with logicModule as the key to replace the original entry in c.moduleInfo
@@ -1736,6 +1746,8 @@ func (c *Context) resolveDependencies(ctx context.Context, config interface{}) (
 		}
 		defer c.EndEvent("clone_modules")
 
+		c.clearTransitionMutatorInputVariants()
+
 		c.dependenciesReady = true
 	})
 
@@ -1772,8 +1784,8 @@ func blueprintDepsMutator(ctx BottomUpMutatorContext) {
 // findExactVariantOrSingle searches the moduleGroup for a module with the same variant as module,
 // and returns the matching module, or nil if one is not found.  A group with exactly one module
 // is always considered matching.
-func findExactVariantOrSingle(module *moduleInfo, possible *moduleGroup, reverse bool) *moduleInfo {
-	found, _ := findVariant(module, possible, nil, false, reverse)
+func (c *Context) findExactVariantOrSingle(module *moduleInfo, config any, possible *moduleGroup, reverse bool) *moduleInfo {
+	found, _ := c.findVariant(module, config, possible, nil, false, reverse)
 	if found == nil {
 		for _, moduleOrAlias := range possible.modules {
 			if m := moduleOrAlias.module(); m != nil {
@@ -1788,7 +1800,7 @@ func findExactVariantOrSingle(module *moduleInfo, possible *moduleGroup, reverse
 	return found
 }
 
-func (c *Context) addDependency(module *moduleInfo, tag DependencyTag, depName string) (*moduleInfo, []error) {
+func (c *Context) addDependency(module *moduleInfo, config any, tag DependencyTag, depName string) (*moduleInfo, []error) {
 	if _, ok := tag.(BaseDependencyTag); ok {
 		panic("BaseDependencyTag is not allowed to be used directly!")
 	}
@@ -1805,7 +1817,7 @@ func (c *Context) addDependency(module *moduleInfo, tag DependencyTag, depName s
 		return nil, c.discoveredMissingDependencies(module, depName, nil)
 	}
 
-	if m := findExactVariantOrSingle(module, possibleDeps, false); m != nil {
+	if m := c.findExactVariantOrSingle(module, config, possibleDeps, false); m != nil {
 		module.newDirectDeps = append(module.newDirectDeps, depInfo{m, tag})
 		atomic.AddUint32(&c.depsModified, 1)
 		return m, nil
@@ -1825,7 +1837,7 @@ func (c *Context) addDependency(module *moduleInfo, tag DependencyTag, depName s
 	}}
 }
 
-func (c *Context) findReverseDependency(module *moduleInfo, destName string) (*moduleInfo, []error) {
+func (c *Context) findReverseDependency(module *moduleInfo, config any, destName string) (*moduleInfo, []error) {
 	if destName == module.Name() {
 		return nil, []error{&BlueprintError{
 			Err: fmt.Errorf("%q depends on itself", destName),
@@ -1842,7 +1854,7 @@ func (c *Context) findReverseDependency(module *moduleInfo, destName string) (*m
 		}}
 	}
 
-	if m := findExactVariantOrSingle(module, possibleDeps, true); m != nil {
+	if m := c.findExactVariantOrSingle(module, config, possibleDeps, true); m != nil {
 		return m, nil
 	}
 
@@ -1860,7 +1872,33 @@ func (c *Context) findReverseDependency(module *moduleInfo, destName string) (*m
 	}}
 }
 
-func findVariant(module *moduleInfo, possibleDeps *moduleGroup, variations []Variation, far bool, reverse bool) (*moduleInfo, variationMap) {
+// applyIncomingTransitions takes a variationMap being used to add a dependency on a module in a moduleGroup
+// and applies the IncomingTransition method of each completed TransitionMutator to modify the requested variation.
+// It finds a variant that existed before the TransitionMutator ran that is a subset of the requested variant to
+// use as the module context for IncomingTransition.
+func (c *Context) applyIncomingTransitions(config any, group *moduleGroup, variant variationMap) {
+	for _, transitionMutator := range c.transitionMutators {
+		for _, inputVariant := range transitionMutator.inputVariants[group] {
+			if inputVariant.variant.variations.subsetOf(variant) {
+				sourceVariation := variant[transitionMutator.name]
+
+				ctx := &incomingTransitionContextImpl{
+					transitionContextImpl{context: c, source: nil, dep: inputVariant,
+						depTag: nil, config: config},
+				}
+
+				outgoingVariation := transitionMutator.mutator.IncomingTransition(ctx, sourceVariation)
+				variant[transitionMutator.name] = outgoingVariation
+				break
+			}
+		}
+	}
+
+}
+
+func (c *Context) findVariant(module *moduleInfo, config any,
+	possibleDeps *moduleGroup, variations []Variation, far bool, reverse bool) (*moduleInfo, variationMap) {
+
 	// We can't just append variant.Variant to module.dependencyVariant.variantName and
 	// compare the strings because the result won't be in mutator registration order.
 	// Create a new map instead, and then deep compare the maps.
@@ -1882,6 +1920,8 @@ func findVariant(module *moduleInfo, possibleDeps *moduleGroup, variations []Var
 		newVariant[v.Mutator] = v.Variation
 	}
 
+	c.applyIncomingTransitions(config, possibleDeps, newVariant)
+
 	check := func(variant variationMap) bool {
 		if far {
 			return newVariant.subsetOf(variant)
@@ -1901,7 +1941,7 @@ func findVariant(module *moduleInfo, possibleDeps *moduleGroup, variations []Var
 	return foundDep, newVariant
 }
 
-func (c *Context) addVariationDependency(module *moduleInfo, variations []Variation,
+func (c *Context) addVariationDependency(module *moduleInfo, config any, variations []Variation,
 	tag DependencyTag, depName string, far bool) (*moduleInfo, []error) {
 	if _, ok := tag.(BaseDependencyTag); ok {
 		panic("BaseDependencyTag is not allowed to be used directly!")
@@ -1912,7 +1952,7 @@ func (c *Context) addVariationDependency(module *moduleInfo, variations []Variat
 		return nil, c.discoveredMissingDependencies(module, depName, nil)
 	}
 
-	foundDep, newVariant := findVariant(module, possibleDeps, variations, far, false)
+	foundDep, newVariant := c.findVariant(module, config, possibleDeps, variations, far, false)
 
 	if foundDep == nil {
 		if c.allowMissingDependencies {
@@ -2936,6 +2976,13 @@ func (c *Context) runMutator(config interface{}, mutator *mutatorInfo,
 
 	c.moduleInfo = newModuleInfo
 
+	isTransitionMutator := mutator.transitionMutator != nil
+
+	var transitionMutatorInputVariants map[*moduleGroup][]*moduleInfo
+	if isTransitionMutator {
+		transitionMutatorInputVariants = make(map[*moduleGroup][]*moduleInfo)
+	}
+
 	for _, group := range c.moduleGroups {
 		for i := 0; i < len(group.modules); i++ {
 			module := group.modules[i].module()
@@ -2946,6 +2993,10 @@ func (c *Context) runMutator(config interface{}, mutator *mutatorInfo,
 
 			// Update module group to contain newly split variants
 			if module.splitModules != nil {
+				if isTransitionMutator {
+					// For transition mutators, save the pre-split variant for reusing later in applyIncomingTransitions.
+					transitionMutatorInputVariants[group] = append(transitionMutatorInputVariants[group], module)
+				}
 				group.modules, i = spliceModules(group.modules, i, module.splitModules)
 			}
 
@@ -2996,6 +3047,11 @@ func (c *Context) runMutator(config interface{}, mutator *mutatorInfo,
 		}
 	}
 
+	if isTransitionMutator {
+		mutator.transitionMutator.inputVariants = transitionMutatorInputVariants
+		c.transitionMutators = append(c.transitionMutators, mutator.transitionMutator)
+	}
+
 	// Add in any new reverse dependencies that were added by the mutator
 	for module, deps := range reverseDeps {
 		sort.Sort(depSorter(deps))
@@ -3031,6 +3087,14 @@ func (c *Context) runMutator(config interface{}, mutator *mutatorInfo,
 	return deps, errs
 }
 
+// clearTransitionMutatorInputVariants removes the inputVariants field from every
+// TransitionMutator now that all dependencies have been resolved.
+func (c *Context) clearTransitionMutatorInputVariants() {
+	for _, mutator := range c.transitionMutators {
+		mutator.inputVariants = nil
+	}
+}
+
 // Replaces every build logic module with a clone of itself.  Prevents introducing problems where
 // a mutator sets a non-property member variable on a module, which works until a later mutator
 // creates variants of that module.
diff --git a/context_test.go b/context_test.go
index b946497..e69f5da 100644
--- a/context_test.go
+++ b/context_test.go
@@ -829,7 +829,8 @@ func Test_findVariant(t *testing.T) {
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			got, _ := findVariant(module, tt.possibleDeps, tt.variations, tt.far, tt.reverse)
+			ctx := NewContext()
+			got, _ := ctx.findVariant(module, nil, tt.possibleDeps, tt.variations, tt.far, tt.reverse)
 			if g, w := got == nil, tt.want == "nil"; g != w {
 				t.Fatalf("findVariant() got = %v, want %v", got, tt.want)
 			}
diff --git a/module_ctx.go b/module_ctx.go
index 6416a25..530eb5e 100644
--- a/module_ctx.go
+++ b/module_ctx.go
@@ -575,7 +575,7 @@ func (m *baseModuleContext) OtherModuleDependencyVariantExists(variations []Vari
 	if possibleDeps == nil {
 		return false
 	}
-	found, _ := findVariant(m.module, possibleDeps, variations, false, false)
+	found, _ := m.context.findVariant(m.module, m.config, possibleDeps, variations, false, false)
 	return found != nil
 }
 
@@ -584,7 +584,7 @@ func (m *baseModuleContext) OtherModuleFarDependencyVariantExists(variations []V
 	if possibleDeps == nil {
 		return false
 	}
-	found, _ := findVariant(m.module, possibleDeps, variations, true, false)
+	found, _ := m.context.findVariant(m.module, m.config, possibleDeps, variations, true, false)
 	return found != nil
 }
 
@@ -593,7 +593,7 @@ func (m *baseModuleContext) OtherModuleReverseDependencyVariantExists(name strin
 	if possibleDeps == nil {
 		return false
 	}
-	found, _ := findVariant(m.module, possibleDeps, nil, false, true)
+	found, _ := m.context.findVariant(m.module, m.config, possibleDeps, nil, false, true)
 	return found != nil
 }
 
@@ -1117,7 +1117,7 @@ func (mctx *mutatorContext) AddDependency(module Module, tag DependencyTag, deps
 	depInfos := make([]Module, 0, len(deps))
 	for _, dep := range deps {
 		modInfo := mctx.context.moduleInfo[module]
-		depInfo, errs := mctx.context.addDependency(modInfo, tag, dep)
+		depInfo, errs := mctx.context.addDependency(modInfo, mctx.config, tag, dep)
 		if len(errs) > 0 {
 			mctx.errs = append(mctx.errs, errs...)
 		}
@@ -1135,7 +1135,7 @@ func (mctx *mutatorContext) AddReverseDependency(module Module, tag DependencyTa
 		panic("BaseDependencyTag is not allowed to be used directly!")
 	}
 
-	destModule, errs := mctx.context.findReverseDependency(mctx.context.moduleInfo[module], destName)
+	destModule, errs := mctx.context.findReverseDependency(mctx.context.moduleInfo[module], mctx.config, destName)
 	if len(errs) > 0 {
 		mctx.errs = append(mctx.errs, errs...)
 		return
@@ -1152,7 +1152,7 @@ func (mctx *mutatorContext) AddVariationDependencies(variations []Variation, tag
 
 	depInfos := make([]Module, 0, len(deps))
 	for _, dep := range deps {
-		depInfo, errs := mctx.context.addVariationDependency(mctx.module, variations, tag, dep, false)
+		depInfo, errs := mctx.context.addVariationDependency(mctx.module, mctx.config, variations, tag, dep, false)
 		if len(errs) > 0 {
 			mctx.errs = append(mctx.errs, errs...)
 		}
@@ -1170,7 +1170,7 @@ func (mctx *mutatorContext) AddFarVariationDependencies(variations []Variation,
 
 	depInfos := make([]Module, 0, len(deps))
 	for _, dep := range deps {
-		depInfo, errs := mctx.context.addVariationDependency(mctx.module, variations, tag, dep, true)
+		depInfo, errs := mctx.context.addVariationDependency(mctx.module, mctx.config, variations, tag, dep, true)
 		if len(errs) > 0 {
 			mctx.errs = append(mctx.errs, errs...)
 		}
diff --git a/transition.go b/transition.go
index 31f785e..6a03e1d 100644
--- a/transition.go
+++ b/transition.go
@@ -144,8 +144,9 @@ type OutgoingTransitionContext interface {
 }
 
 type transitionMutatorImpl struct {
-	name    string
-	mutator TransitionMutator
+	name          string
+	mutator       TransitionMutator
+	inputVariants map[*moduleGroup][]*moduleInfo
 }
 
 // Adds each argument in items to l if it's not already there.
@@ -294,7 +295,7 @@ func (c *Context) RegisterTransitionMutator(name string, mutator TransitionMutat
 	impl := &transitionMutatorImpl{name: name, mutator: mutator}
 
 	c.RegisterTopDownMutator(name+"_deps", impl.topDownMutator).Parallel()
-	c.RegisterBottomUpMutator(name, impl.bottomUpMutator).Parallel()
+	c.RegisterBottomUpMutator(name, impl.bottomUpMutator).Parallel().setTransitionMutator(impl)
 	c.RegisterBottomUpMutator(name+"_mutate", impl.mutateMutator).Parallel()
 }
 
diff --git a/transition_test.go b/transition_test.go
index 128e7d5..dea818a 100644
--- a/transition_test.go
+++ b/transition_test.go
@@ -15,14 +15,48 @@
 package blueprint
 
 import (
+	"fmt"
 	"slices"
+	"strings"
 	"testing"
 )
 
-func TestTransition(t *testing.T) {
+func testTransition(bp string) (*Context, []error) {
 	ctx := newContext()
 	ctx.MockFileSystem(map[string][]byte{
-		"Android.bp": []byte(`
+		"Android.bp": []byte(bp),
+	})
+
+	ctx.RegisterBottomUpMutator("deps", depsMutator)
+	ctx.RegisterTransitionMutator("transition", transitionTestMutator{})
+	ctx.RegisterBottomUpMutator("post_transition_deps", postTransitionDepsMutator)
+
+	ctx.RegisterModuleType("transition_module", newTransitionModule)
+	_, errs := ctx.ParseBlueprintsFiles("Android.bp", nil)
+	if len(errs) > 0 {
+		return nil, errs
+	}
+
+	_, errs = ctx.ResolveDependencies(nil)
+	if len(errs) > 0 {
+		return nil, errs
+	}
+
+	return ctx, nil
+}
+
+func assertNoErrors(t *testing.T, errs []error) {
+	t.Helper()
+	if len(errs) > 0 {
+		t.Errorf("unexpected errors:")
+		for _, err := range errs {
+			t.Errorf("  %s", err)
+		}
+		t.FailNow()
+	}
+}
+
+const testTransitionBp = `
 			transition_module {
 			    name: "A",
 			    deps: ["B", "C"],
@@ -33,6 +67,7 @@ func TestTransition(t *testing.T) {
 				name: "B",
 				deps: ["C"],
 				outgoing: "c",
+				%s
 			}
 
 			transition_module {
@@ -43,103 +78,156 @@ func TestTransition(t *testing.T) {
 			transition_module {
 				name: "D",
 				incoming: "d",
+				deps: ["E"],
 			}
-		`),
-	})
 
-	ctx.RegisterBottomUpMutator("deps", depsMutator)
-	ctx.RegisterTransitionMutator("transition", transitionTestMutator{})
+			transition_module {
+				name: "E",
+			}
+		`
 
-	ctx.RegisterModuleType("transition_module", newTransitionModule)
-	_, errs := ctx.ParseBlueprintsFiles("Android.bp", nil)
-	if len(errs) > 0 {
-		t.Errorf("unexpected parse errors:")
-		for _, err := range errs {
-			t.Errorf("  %s", err)
-		}
-		t.FailNow()
-	}
+func getTransitionModule(ctx *Context, name, variant string) *transitionModule {
+	group := ctx.moduleGroupFromName(name, nil)
+	module := group.moduleOrAliasByVariantName(variant).module()
+	return module.logicModule.(*transitionModule)
+}
 
-	_, errs = ctx.ResolveDependencies(nil)
-	if len(errs) > 0 {
-		t.Errorf("unexpected dep errors:")
-		for _, err := range errs {
-			t.Errorf("  %s", err)
-		}
-		t.FailNow()
+func checkTransitionVariants(t *testing.T, ctx *Context, name string, expectedVariants []string) {
+	t.Helper()
+	group := ctx.moduleGroupFromName(name, nil)
+	var gotVariants []string
+	for _, variant := range group.modules {
+		gotVariants = append(gotVariants, variant.moduleOrAliasVariant().variations["transition"])
+	}
+	if !slices.Equal(expectedVariants, gotVariants) {
+		t.Errorf("expected variants of %q to be %q, got %q", name, expectedVariants, gotVariants)
 	}
+}
 
-	getModule := func(name, variant string) *transitionModule {
-		group := ctx.moduleGroupFromName(name, nil)
-		module := group.moduleOrAliasByVariantName(variant).module()
-		return module.logicModule.(*transitionModule)
+func checkTransitionDeps(t *testing.T, ctx *Context, m Module, expected ...string) {
+	t.Helper()
+	var got []string
+	ctx.VisitDirectDeps(m, func(m Module) {
+		got = append(got, ctx.ModuleName(m)+"("+ctx.ModuleSubDir(m)+")")
+	})
+	if !slices.Equal(got, expected) {
+		t.Errorf("unexpected %q dependencies, got %q expected %q",
+			ctx.ModuleName(m), got, expected)
 	}
+}
 
-	checkVariants := func(name string, expectedVariants []string) {
-		t.Helper()
-		group := ctx.moduleGroupFromName(name, nil)
-		var gotVariants []string
-		for _, variant := range group.modules {
-			gotVariants = append(gotVariants, variant.moduleOrAliasVariant().variations["transition"])
-		}
-		if !slices.Equal(expectedVariants, gotVariants) {
-			t.Errorf("expected variants of %q to be %q, got %q", name, expectedVariants, gotVariants)
-		}
+func checkTransitionMutate(t *testing.T, m *transitionModule, variant string) {
+	t.Helper()
+	if m.properties.Mutated != variant {
+		t.Errorf("unexpected mutated property in %q, expected %q got %q", m.Name(), variant, m.properties.Mutated)
 	}
+}
+
+func TestTransition(t *testing.T) {
+	ctx, errs := testTransition(fmt.Sprintf(testTransitionBp, ""))
+	assertNoErrors(t, errs)
 
 	// Module A uses Split to create a and b variants
-	checkVariants("A", []string{"a", "b"})
+	checkTransitionVariants(t, ctx, "A", []string{"a", "b"})
 	// Module B inherits a and b variants from A
-	checkVariants("B", []string{"", "a", "b"})
+	checkTransitionVariants(t, ctx, "B", []string{"", "a", "b"})
 	// Module C inherits a and b variants from A, but gets an outgoing c variant from B
-	checkVariants("C", []string{"", "a", "b", "c"})
+	checkTransitionVariants(t, ctx, "C", []string{"", "a", "b", "c"})
 	// Module D always has incoming variant d
-	checkVariants("D", []string{"", "d"})
-
-	A_a := getModule("A", "a")
-	A_b := getModule("A", "b")
-	B_a := getModule("B", "a")
-	B_b := getModule("B", "b")
-	C_a := getModule("C", "a")
-	C_b := getModule("C", "b")
-	C_c := getModule("C", "c")
-	D_d := getModule("D", "d")
-
-	checkDeps := func(m Module, expected ...string) {
-		var got []string
-		ctx.VisitDirectDeps(m, func(m Module) {
-			got = append(got, ctx.ModuleName(m)+"("+ctx.ModuleSubDir(m)+")")
-		})
-		if !slices.Equal(got, expected) {
-			t.Errorf("unexpected %q dependencies, got %q expected %q",
-				ctx.ModuleName(m), got, expected)
-		}
-	}
+	checkTransitionVariants(t, ctx, "D", []string{"", "d"})
+	// Module E inherits d from D
+	checkTransitionVariants(t, ctx, "E", []string{"", "d"})
 
-	checkDeps(A_a, "B(a)", "C(a)")
-	checkDeps(A_b, "B(b)", "C(b)")
-	checkDeps(B_a, "C(c)")
-	checkDeps(B_b, "C(c)")
-	checkDeps(C_a, "D(d)")
-	checkDeps(C_b, "D(d)")
-	checkDeps(C_c, "D(d)")
-	checkDeps(D_d)
-
-	checkMutate := func(m *transitionModule, variant string) {
-		t.Helper()
-		if m.properties.Mutated != variant {
-			t.Errorf("unexpected mutated property in %q, expected %q got %q", m.Name(), variant, m.properties.Mutated)
-		}
-	}
+	A_a := getTransitionModule(ctx, "A", "a")
+	A_b := getTransitionModule(ctx, "A", "b")
+	B_a := getTransitionModule(ctx, "B", "a")
+	B_b := getTransitionModule(ctx, "B", "b")
+	C_a := getTransitionModule(ctx, "C", "a")
+	C_b := getTransitionModule(ctx, "C", "b")
+	C_c := getTransitionModule(ctx, "C", "c")
+	D_d := getTransitionModule(ctx, "D", "d")
+	E_d := getTransitionModule(ctx, "E", "d")
+
+	checkTransitionDeps(t, ctx, A_a, "B(a)", "C(a)")
+	checkTransitionDeps(t, ctx, A_b, "B(b)", "C(b)")
+	checkTransitionDeps(t, ctx, B_a, "C(c)")
+	checkTransitionDeps(t, ctx, B_b, "C(c)")
+	checkTransitionDeps(t, ctx, C_a, "D(d)")
+	checkTransitionDeps(t, ctx, C_b, "D(d)")
+	checkTransitionDeps(t, ctx, C_c, "D(d)")
+	checkTransitionDeps(t, ctx, D_d, "E(d)")
+	checkTransitionDeps(t, ctx, E_d)
 
-	checkMutate(A_a, "a")
-	checkMutate(A_b, "b")
-	checkMutate(B_a, "a")
-	checkMutate(B_b, "b")
-	checkMutate(C_a, "a")
-	checkMutate(C_b, "b")
-	checkMutate(C_c, "c")
-	checkMutate(D_d, "d")
+	checkTransitionMutate(t, A_a, "a")
+	checkTransitionMutate(t, A_b, "b")
+	checkTransitionMutate(t, B_a, "a")
+	checkTransitionMutate(t, B_b, "b")
+	checkTransitionMutate(t, C_a, "a")
+	checkTransitionMutate(t, C_b, "b")
+	checkTransitionMutate(t, C_c, "c")
+	checkTransitionMutate(t, D_d, "d")
+	checkTransitionMutate(t, E_d, "d")
+}
+
+func TestPostTransitionDeps(t *testing.T) {
+	ctx, errs := testTransition(fmt.Sprintf(testTransitionBp,
+		`post_transition_deps: ["D:late", "E:d"],`))
+	assertNoErrors(t, errs)
+
+	// Module A uses Split to create a and b variants
+	checkTransitionVariants(t, ctx, "A", []string{"a", "b"})
+	// Module B inherits a and b variants from A
+	checkTransitionVariants(t, ctx, "B", []string{"", "a", "b"})
+	// Module C inherits a and b variants from A, but gets an outgoing c variant from B
+	checkTransitionVariants(t, ctx, "C", []string{"", "a", "b", "c"})
+	// Module D always has incoming variant d
+	checkTransitionVariants(t, ctx, "D", []string{"", "d"})
+	// Module E inherits d from D
+	checkTransitionVariants(t, ctx, "E", []string{"", "d"})
+
+	A_a := getTransitionModule(ctx, "A", "a")
+	A_b := getTransitionModule(ctx, "A", "b")
+	B_a := getTransitionModule(ctx, "B", "a")
+	B_b := getTransitionModule(ctx, "B", "b")
+	C_a := getTransitionModule(ctx, "C", "a")
+	C_b := getTransitionModule(ctx, "C", "b")
+	C_c := getTransitionModule(ctx, "C", "c")
+	D_d := getTransitionModule(ctx, "D", "d")
+	E_d := getTransitionModule(ctx, "E", "d")
+
+	checkTransitionDeps(t, ctx, A_a, "B(a)", "C(a)")
+	checkTransitionDeps(t, ctx, A_b, "B(b)", "C(b)")
+	checkTransitionDeps(t, ctx, B_a, "C(c)", "D(d)", "E(d)")
+	checkTransitionDeps(t, ctx, B_b, "C(c)", "D(d)", "E(d)")
+	checkTransitionDeps(t, ctx, C_a, "D(d)")
+	checkTransitionDeps(t, ctx, C_b, "D(d)")
+	checkTransitionDeps(t, ctx, C_c, "D(d)")
+	checkTransitionDeps(t, ctx, D_d, "E(d)")
+	checkTransitionDeps(t, ctx, E_d)
+
+	checkTransitionMutate(t, A_a, "a")
+	checkTransitionMutate(t, A_b, "b")
+	checkTransitionMutate(t, B_a, "a")
+	checkTransitionMutate(t, B_b, "b")
+	checkTransitionMutate(t, C_a, "a")
+	checkTransitionMutate(t, C_b, "b")
+	checkTransitionMutate(t, C_c, "c")
+	checkTransitionMutate(t, D_d, "d")
+	checkTransitionMutate(t, E_d, "d")
+}
+
+func TestPostTransitionDepsMissingVariant(t *testing.T) {
+	// TODO: eventually this will create the missing variant on demand
+	_, errs := testTransition(fmt.Sprintf(testTransitionBp,
+		`post_transition_deps: ["E:missing"],`))
+	expectedError := `Android.bp:8:4: dependency "E" of "B" missing variant:
+  transition:missing
+available variants:
+  transition:
+  transition:d`
+	if len(errs) != 1 || errs[0].Error() != expectedError {
+		t.Errorf("expected error %q, got %q", expectedError, errs)
+	}
 }
 
 type transitionTestMutator struct{}
@@ -172,10 +260,11 @@ func (transitionTestMutator) Mutate(ctx BottomUpMutatorContext, variation string
 type transitionModule struct {
 	SimpleName
 	properties struct {
-		Deps     []string
-		Split    []string
-		Outgoing *string
-		Incoming *string
+		Deps                 []string
+		Post_transition_deps []string
+		Split                []string
+		Outgoing             *string
+		Incoming             *string
 
 		Mutated string `blueprint:"mutated"`
 	}
@@ -196,3 +285,13 @@ func (f *transitionModule) Deps() []string {
 func (f *transitionModule) IgnoreDeps() []string {
 	return nil
 }
+
+func postTransitionDepsMutator(mctx BottomUpMutatorContext) {
+	if m, ok := mctx.Module().(*transitionModule); ok {
+		for _, dep := range m.properties.Post_transition_deps {
+			module, variation, _ := strings.Cut(dep, ":")
+			variations := []Variation{{"transition", variation}}
+			mctx.AddVariationDependencies(variations, walkerDepsTag{follow: true}, module)
+		}
+	}
+}