Use hashed subdir for soong_config modules

This is to differentiate soong intermediate directories for soong config
modules. This will help incremental build across different
devices.

Test result of building panther, building cheetah, and building panther
again:

Before this change
- build time: 02:57
- # of tasks: 31044

After this change
- build time: 01:48
- # of tasks: 1694

Build time includes build.ninja generating time (which is more than 1
minute), so the overriden artifacts become far fewer.

And "NINJA_ARGS='-n -d explain' m" only gave 4 "command line changed"
nodes.

Bug: 279362051
Test: see above
Change-Id: I4891cbe823ae21628465e5c6eb26a4837ccdd202

android/module.go

@@ -15,6 +15,9 @@
 package android
 
 import (
+	"crypto/md5"
+	"encoding/hex"
+	"encoding/json"
 	"fmt"
 	"net/url"
 	"os"
@@ -714,6 +717,31 @@ func SortedUniqueNamedPaths(l NamedPaths) NamedPaths {
 	return l[:k+1]
 }
 
+// soongConfigTrace holds all references to VendorVars. Uses []string for blueprint:"mutated"
+type soongConfigTrace struct {
+	Bools   []string `json:",omitempty"`
+	Strings []string `json:",omitempty"`
+	IsSets  []string `json:",omitempty"`
+}
+
+func (c *soongConfigTrace) isEmpty() bool {
+	return len(c.Bools) == 0 && len(c.Strings) == 0 && len(c.IsSets) == 0
+}
+
+// Returns hash of serialized trace records (empty string if there's no trace recorded)
+func (c *soongConfigTrace) hash() string {
+	// Use MD5 for speed. We don't care collision or preimage attack
+	if c.isEmpty() {
+		return ""
+	}
+	j, err := json.Marshal(c)
+	if err != nil {
+		panic(fmt.Errorf("json marshal of %#v failed: %#v", *c, err))
+	}
+	hash := md5.Sum(j)
+	return hex.EncodeToString(hash[:])
+}
+
 type nameProperties struct {
 	// The name of the module.  Must be unique across all modules.
 	Name *string
@@ -958,6 +986,10 @@ type commonProperties struct {
 
 	// Bazel conversion status
 	BazelConversionStatus BazelConversionStatus `blueprint:"mutated"`
+
+	// SoongConfigTrace records accesses to VendorVars (soong_config)
+	SoongConfigTrace     soongConfigTrace `blueprint:"mutated"`
+	SoongConfigTraceHash string           `blueprint:"mutated"`
 }
 
 // CommonAttributes represents the common Bazel attributes from which properties
@@ -3160,6 +3192,10 @@ func (m *moduleContext) ModuleSubDir() string {
 	return m.bp.ModuleSubDir()
 }
 
+func (m *moduleContext) ModuleSoongConfigHash() string {
+	return m.module.base().commonProperties.SoongConfigTraceHash
+}
+
 func (b *baseModuleContext) Target() Target {
 	return b.target
 }
@@ -3744,6 +3780,8 @@ func (m *moduleContext) TargetRequiredModuleNames() []string {
 
 func init() {
 	RegisterParallelSingletonType("buildtarget", BuildTargetSingleton)
+	RegisterParallelSingletonType("soongconfigtrace", soongConfigTraceSingletonFunc)
+	FinalDepsMutators(registerSoongConfigTraceMutator)
 }
 
 func BuildTargetSingleton() Singleton {
@@ -3925,3 +3963,54 @@ func (d *installPathsDepSet) ToList() InstallPaths {
 	}
 	return d.depSet.ToList().(InstallPaths)
 }
+
+func registerSoongConfigTraceMutator(ctx RegisterMutatorsContext) {
+	ctx.BottomUp("soongconfigtrace", soongConfigTraceMutator).Parallel()
+}
+
+// soongConfigTraceMutator accumulates recorded soong_config trace from children. Also it normalizes
+// SoongConfigTrace to make it consistent.
+func soongConfigTraceMutator(ctx BottomUpMutatorContext) {
+	trace := &ctx.Module().base().commonProperties.SoongConfigTrace
+	ctx.VisitDirectDeps(func(m Module) {
+		childTrace := &m.base().commonProperties.SoongConfigTrace
+		trace.Bools = append(trace.Bools, childTrace.Bools...)
+		trace.Strings = append(trace.Strings, childTrace.Strings...)
+		trace.IsSets = append(trace.IsSets, childTrace.IsSets...)
+	})
+	trace.Bools = SortedUniqueStrings(trace.Bools)
+	trace.Strings = SortedUniqueStrings(trace.Strings)
+	trace.IsSets = SortedUniqueStrings(trace.IsSets)
+
+	ctx.Module().base().commonProperties.SoongConfigTraceHash = trace.hash()
+}
+
+// soongConfigTraceSingleton writes a map from each module's config hash value to trace data.
+func soongConfigTraceSingletonFunc() Singleton {
+	return &soongConfigTraceSingleton{}
+}
+
+type soongConfigTraceSingleton struct {
+}
+
+func (s *soongConfigTraceSingleton) GenerateBuildActions(ctx SingletonContext) {
+	outFile := PathForOutput(ctx, "soong_config_trace.json")
+
+	traces := make(map[string]*soongConfigTrace)
+	ctx.VisitAllModules(func(module Module) {
+		trace := &module.base().commonProperties.SoongConfigTrace
+		if !trace.isEmpty() {
+			hash := module.base().commonProperties.SoongConfigTraceHash
+			traces[hash] = trace
+		}
+	})
+
+	j, err := json.Marshal(traces)
+	if err != nil {
+		ctx.Errorf("json marshal to %q failed: %#v", outFile, err)
+		return
+	}
+
+	WriteFileRule(ctx, outFile, string(j))
+	ctx.Phony("soong_config_trace", outFile)
+}