Support dependencies on environment variables

Ninja can't depend on environment variables, so modifying build
behavior based on environment variables requires coordinating
between the soong script that invokes ninja and the soong_build
manifest generator.

Allow any module to call Config.Getenv to get the contents of an
environment variable while registering a dependency on it.
After all modules have been processed write out the state of
all used environment variables to a JSON file called
.soong.environment.  During the next build the soong script
will use the soong_env tool to compare the contents of
.soong.environment to the current environment, and force a
build manifest regeneration by deleting the .soong.environment
file if any variables have changed.

Change-Id: Id0d81933a857bc2fc1cd7a393a3c6cec73dc4824

10 files changed, 324 insertions, 23 deletions

diff --git a/Blueprints b/Blueprints
index c8442e7c..8a2d85f8 100644
--- a/Blueprints
+++ b/Blueprints
@@ -19,6 +19,7 @@ bootstrap_go_binary {
         "soong-cc",
         "soong-common",
         "soong-config",
+        "soong-env",
         "soong-genrule",
     ],
     srcs: [
@@ -28,6 +29,25 @@ bootstrap_go_binary {
 }
 
 bootstrap_go_binary {
+    name: "soong_env",
+    deps: [
+        "soong-env",
+    ],
+    srcs: [
+        "cmd/soong_env/soong_env.go",
+    ],
+}
+
+bootstrap_go_package {
+    name: "soong-env",
+    pkgPath: "android/soong/env",
+    srcs: [
+        "env/env.go",
+    ],
+}
+
+
+bootstrap_go_binary {
     name: "soong_glob",
     deps: [
         "soong-glob",
@@ -54,12 +74,14 @@ bootstrap_go_package {
     deps: [
         "blueprint",
         "blueprint-bootstrap",
+        "soong-env",
         "soong-glob",
     ],
     srcs: [
         "common/arch.go",
         "common/checkbuild.go",
         "common/defs.go",
+        "common/env.go",
         "common/glob.go",
         "common/module.go",
         "common/paths.go",
diff --git a/build.ninja.in b/build.ninja.in
index 2b802d43..4fb63250 100644
--- a/build.ninja.in
+++ b/build.ninja.in
@@ -53,7 +53,7 @@ rule g.bootstrap.link
 # Variant:
 # Type:    bootstrap_go_binary
 # Factory: github.com/google/blueprint/bootstrap.newGoBinaryModule
-# Defined: build/soong/Blueprints:123:1
+# Defined: build/soong/Blueprints:145:1
 
 build .bootstrap/androidmk/obj/androidmk.a: g.bootstrap.gc $
         ${g.bootstrap.srcDir}/build/soong/androidmk/cmd/androidmk/android.go $
@@ -79,7 +79,7 @@ default .bootstrap/bin/androidmk
 # Variant:
 # Type:    bootstrap_go_package
 # Factory: github.com/google/blueprint/bootstrap.newGoPackageModule
-# Defined: build/soong/Blueprints:136:1
+# Defined: build/soong/Blueprints:158:1
 
 build .bootstrap/androidmk-parser/pkg/android/soong/androidmk/parser.a: $
         g.bootstrap.gc $
@@ -295,12 +295,13 @@ build .bootstrap/soong-art/pkg/android/soong/art.a: g.bootstrap.gc $
         .bootstrap/blueprint-pathtools/pkg/github.com/google/blueprint/pathtools.a $
         .bootstrap/blueprint-deptools/pkg/github.com/google/blueprint/deptools.a $
         .bootstrap/blueprint-bootstrap/pkg/github.com/google/blueprint/bootstrap.a $
+        .bootstrap/soong-env/pkg/android/soong/env.a $
         .bootstrap/soong-glob/pkg/android/soong/glob.a $
         .bootstrap/soong-common/pkg/android/soong/common.a $
         .bootstrap/soong-config/pkg/android/soong/config.a $
         .bootstrap/soong-genrule/pkg/android/soong/genrule.a $
         .bootstrap/soong-cc/pkg/android/soong/cc.a
-    incFlags = -I .bootstrap/blueprint-parser/pkg -I .bootstrap/blueprint-proptools/pkg -I .bootstrap/blueprint/pkg -I .bootstrap/blueprint-pathtools/pkg -I .bootstrap/blueprint-deptools/pkg -I .bootstrap/blueprint-bootstrap/pkg -I .bootstrap/soong-glob/pkg -I .bootstrap/soong-common/pkg -I .bootstrap/soong-config/pkg -I .bootstrap/soong-genrule/pkg -I .bootstrap/soong-cc/pkg
+    incFlags = -I .bootstrap/blueprint-parser/pkg -I .bootstrap/blueprint-proptools/pkg -I .bootstrap/blueprint/pkg -I .bootstrap/blueprint-pathtools/pkg -I .bootstrap/blueprint-deptools/pkg -I .bootstrap/blueprint-bootstrap/pkg -I .bootstrap/soong-env/pkg -I .bootstrap/soong-glob/pkg -I .bootstrap/soong-common/pkg -I .bootstrap/soong-config/pkg -I .bootstrap/soong-genrule/pkg -I .bootstrap/soong-cc/pkg
     pkgPath = android/soong/art
 default .bootstrap/soong-art/pkg/android/soong/art.a
 
@@ -309,7 +310,7 @@ default .bootstrap/soong-art/pkg/android/soong/art.a
 # Variant:
 # Type:    bootstrap_go_package
 # Factory: github.com/google/blueprint/bootstrap.newGoPackageModule
-# Defined: build/soong/Blueprints:82:1
+# Defined: build/soong/Blueprints:104:1
 
 build .bootstrap/soong-cc/pkg/android/soong/cc.a: g.bootstrap.gc $
         ${g.bootstrap.srcDir}/build/soong/cc/builder.go $
@@ -327,11 +328,12 @@ build .bootstrap/soong-cc/pkg/android/soong/cc.a: g.bootstrap.gc $
         .bootstrap/blueprint-pathtools/pkg/github.com/google/blueprint/pathtools.a $
         .bootstrap/blueprint-deptools/pkg/github.com/google/blueprint/deptools.a $
         .bootstrap/blueprint-bootstrap/pkg/github.com/google/blueprint/bootstrap.a $
+        .bootstrap/soong-env/pkg/android/soong/env.a $
         .bootstrap/soong-glob/pkg/android/soong/glob.a $
         .bootstrap/soong-common/pkg/android/soong/common.a $
         .bootstrap/soong-config/pkg/android/soong/config.a $
         .bootstrap/soong-genrule/pkg/android/soong/genrule.a
-    incFlags = -I .bootstrap/blueprint-parser/pkg -I .bootstrap/blueprint-proptools/pkg -I .bootstrap/blueprint/pkg -I .bootstrap/blueprint-pathtools/pkg -I .bootstrap/blueprint-deptools/pkg -I .bootstrap/blueprint-bootstrap/pkg -I .bootstrap/soong-glob/pkg -I .bootstrap/soong-common/pkg -I .bootstrap/soong-config/pkg -I .bootstrap/soong-genrule/pkg
+    incFlags = -I .bootstrap/blueprint-parser/pkg -I .bootstrap/blueprint-proptools/pkg -I .bootstrap/blueprint/pkg -I .bootstrap/blueprint-pathtools/pkg -I .bootstrap/blueprint-deptools/pkg -I .bootstrap/blueprint-bootstrap/pkg -I .bootstrap/soong-env/pkg -I .bootstrap/soong-glob/pkg -I .bootstrap/soong-common/pkg -I .bootstrap/soong-config/pkg -I .bootstrap/soong-genrule/pkg
     pkgPath = android/soong/cc
 default .bootstrap/soong-cc/pkg/android/soong/cc.a
 
@@ -340,12 +342,13 @@ default .bootstrap/soong-cc/pkg/android/soong/cc.a
 # Variant:
 # Type:    bootstrap_go_package
 # Factory: github.com/google/blueprint/bootstrap.newGoPackageModule
-# Defined: build/soong/Blueprints:51:1
+# Defined: build/soong/Blueprints:71:1
 
 build .bootstrap/soong-common/pkg/android/soong/common.a: g.bootstrap.gc $
         ${g.bootstrap.srcDir}/build/soong/common/arch.go $
         ${g.bootstrap.srcDir}/build/soong/common/checkbuild.go $
         ${g.bootstrap.srcDir}/build/soong/common/defs.go $
+        ${g.bootstrap.srcDir}/build/soong/common/env.go $
         ${g.bootstrap.srcDir}/build/soong/common/glob.go $
         ${g.bootstrap.srcDir}/build/soong/common/module.go $
         ${g.bootstrap.srcDir}/build/soong/common/paths.go | $
@@ -356,8 +359,9 @@ build .bootstrap/soong-common/pkg/android/soong/common.a: g.bootstrap.gc $
         .bootstrap/blueprint-deptools/pkg/github.com/google/blueprint/deptools.a $
         .bootstrap/blueprint-pathtools/pkg/github.com/google/blueprint/pathtools.a $
         .bootstrap/blueprint-bootstrap/pkg/github.com/google/blueprint/bootstrap.a $
+        .bootstrap/soong-env/pkg/android/soong/env.a $
         .bootstrap/soong-glob/pkg/android/soong/glob.a
-    incFlags = -I .bootstrap/blueprint-parser/pkg -I .bootstrap/blueprint-proptools/pkg -I .bootstrap/blueprint/pkg -I .bootstrap/blueprint-deptools/pkg -I .bootstrap/blueprint-pathtools/pkg -I .bootstrap/blueprint-bootstrap/pkg -I .bootstrap/soong-glob/pkg
+    incFlags = -I .bootstrap/blueprint-parser/pkg -I .bootstrap/blueprint-proptools/pkg -I .bootstrap/blueprint/pkg -I .bootstrap/blueprint-deptools/pkg -I .bootstrap/blueprint-pathtools/pkg -I .bootstrap/blueprint-bootstrap/pkg -I .bootstrap/soong-env/pkg -I .bootstrap/soong-glob/pkg
     pkgPath = android/soong/common
 default .bootstrap/soong-common/pkg/android/soong/common.a
 
@@ -366,7 +370,7 @@ default .bootstrap/soong-common/pkg/android/soong/common.a
 # Variant:
 # Type:    bootstrap_go_package
 # Factory: github.com/google/blueprint/bootstrap.newGoPackageModule
-# Defined: build/soong/Blueprints:69:1
+# Defined: build/soong/Blueprints:91:1
 
 build .bootstrap/soong-config/pkg/android/soong/config.a: g.bootstrap.gc $
         ${g.bootstrap.srcDir}/build/soong/config/config.go | $
@@ -377,18 +381,31 @@ build .bootstrap/soong-config/pkg/android/soong/config.a: g.bootstrap.gc $
         .bootstrap/blueprint-deptools/pkg/github.com/google/blueprint/deptools.a $
         .bootstrap/blueprint-pathtools/pkg/github.com/google/blueprint/pathtools.a $
         .bootstrap/blueprint-bootstrap/pkg/github.com/google/blueprint/bootstrap.a $
+        .bootstrap/soong-env/pkg/android/soong/env.a $
         .bootstrap/soong-glob/pkg/android/soong/glob.a $
         .bootstrap/soong-common/pkg/android/soong/common.a
-    incFlags = -I .bootstrap/blueprint-parser/pkg -I .bootstrap/blueprint-proptools/pkg -I .bootstrap/blueprint/pkg -I .bootstrap/blueprint-deptools/pkg -I .bootstrap/blueprint-pathtools/pkg -I .bootstrap/blueprint-bootstrap/pkg -I .bootstrap/soong-glob/pkg -I .bootstrap/soong-common/pkg
+    incFlags = -I .bootstrap/blueprint-parser/pkg -I .bootstrap/blueprint-proptools/pkg -I .bootstrap/blueprint/pkg -I .bootstrap/blueprint-deptools/pkg -I .bootstrap/blueprint-pathtools/pkg -I .bootstrap/blueprint-bootstrap/pkg -I .bootstrap/soong-env/pkg -I .bootstrap/soong-glob/pkg -I .bootstrap/soong-common/pkg
     pkgPath = android/soong/config
 default .bootstrap/soong-config/pkg/android/soong/config.a
 
 # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #
+# Module:  soong-env
+# Variant:
+# Type:    bootstrap_go_package
+# Factory: github.com/google/blueprint/bootstrap.newGoPackageModule
+# Defined: build/soong/Blueprints:41:1
+
+build .bootstrap/soong-env/pkg/android/soong/env.a: g.bootstrap.gc $
+        ${g.bootstrap.srcDir}/build/soong/env/env.go | ${g.bootstrap.gcCmd}
+    pkgPath = android/soong/env
+default .bootstrap/soong-env/pkg/android/soong/env.a
+
+# # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #
 # Module:  soong-genrule
 # Variant:
 # Type:    bootstrap_go_package
 # Factory: github.com/google/blueprint/bootstrap.newGoPackageModule
-# Defined: build/soong/Blueprints:106:1
+# Defined: build/soong/Blueprints:128:1
 
 build .bootstrap/soong-genrule/pkg/android/soong/genrule.a: g.bootstrap.gc $
         ${g.bootstrap.srcDir}/build/soong/genrule/genrule.go | $
@@ -399,10 +416,11 @@ build .bootstrap/soong-genrule/pkg/android/soong/genrule.a: g.bootstrap.gc $
         .bootstrap/blueprint-pathtools/pkg/github.com/google/blueprint/pathtools.a $
         .bootstrap/blueprint-deptools/pkg/github.com/google/blueprint/deptools.a $
         .bootstrap/blueprint-bootstrap/pkg/github.com/google/blueprint/bootstrap.a $
+        .bootstrap/soong-env/pkg/android/soong/env.a $
         .bootstrap/soong-glob/pkg/android/soong/glob.a $
         .bootstrap/soong-common/pkg/android/soong/common.a $
         .bootstrap/soong-config/pkg/android/soong/config.a
-    incFlags = -I .bootstrap/blueprint-parser/pkg -I .bootstrap/blueprint-proptools/pkg -I .bootstrap/blueprint/pkg -I .bootstrap/blueprint-pathtools/pkg -I .bootstrap/blueprint-deptools/pkg -I .bootstrap/blueprint-bootstrap/pkg -I .bootstrap/soong-glob/pkg -I .bootstrap/soong-common/pkg -I .bootstrap/soong-config/pkg
+    incFlags = -I .bootstrap/blueprint-parser/pkg -I .bootstrap/blueprint-proptools/pkg -I .bootstrap/blueprint/pkg -I .bootstrap/blueprint-pathtools/pkg -I .bootstrap/blueprint-deptools/pkg -I .bootstrap/blueprint-bootstrap/pkg -I .bootstrap/soong-env/pkg -I .bootstrap/soong-glob/pkg -I .bootstrap/soong-common/pkg -I .bootstrap/soong-config/pkg
     pkgPath = android/soong/genrule
 default .bootstrap/soong-genrule/pkg/android/soong/genrule.a
 
@@ -411,7 +429,7 @@ default .bootstrap/soong-genrule/pkg/android/soong/genrule.a
 # Variant:
 # Type:    bootstrap_go_package
 # Factory: github.com/google/blueprint/bootstrap.newGoPackageModule
-# Defined: build/soong/Blueprints:40:1
+# Defined: build/soong/Blueprints:60:1
 
 build .bootstrap/soong-glob/pkg/android/soong/glob.a: g.bootstrap.gc $
         ${g.bootstrap.srcDir}/build/soong/glob/glob.go | ${g.bootstrap.gcCmd} $
@@ -436,19 +454,20 @@ build .bootstrap/soong_build/obj/soong_build.a: g.bootstrap.gc $
         .bootstrap/blueprint-deptools/pkg/github.com/google/blueprint/deptools.a $
         .bootstrap/blueprint-pathtools/pkg/github.com/google/blueprint/pathtools.a $
         .bootstrap/blueprint-bootstrap/pkg/github.com/google/blueprint/bootstrap.a $
+        .bootstrap/soong-env/pkg/android/soong/env.a $
         .bootstrap/soong-glob/pkg/android/soong/glob.a $
         .bootstrap/soong-common/pkg/android/soong/common.a $
         .bootstrap/soong-config/pkg/android/soong/config.a $
         .bootstrap/soong-genrule/pkg/android/soong/genrule.a $
         .bootstrap/soong-cc/pkg/android/soong/cc.a $
         .bootstrap/soong-art/pkg/android/soong/art.a
-    incFlags = -I .bootstrap/blueprint-parser/pkg -I .bootstrap/blueprint-proptools/pkg -I .bootstrap/blueprint/pkg -I .bootstrap/blueprint-deptools/pkg -I .bootstrap/blueprint-pathtools/pkg -I .bootstrap/blueprint-bootstrap/pkg -I .bootstrap/soong-glob/pkg -I .bootstrap/soong-common/pkg -I .bootstrap/soong-config/pkg -I .bootstrap/soong-genrule/pkg -I .bootstrap/soong-cc/pkg -I .bootstrap/soong-art/pkg
+    incFlags = -I .bootstrap/blueprint-parser/pkg -I .bootstrap/blueprint-proptools/pkg -I .bootstrap/blueprint/pkg -I .bootstrap/blueprint-deptools/pkg -I .bootstrap/blueprint-pathtools/pkg -I .bootstrap/blueprint-bootstrap/pkg -I .bootstrap/soong-env/pkg -I .bootstrap/soong-glob/pkg -I .bootstrap/soong-common/pkg -I .bootstrap/soong-config/pkg -I .bootstrap/soong-genrule/pkg -I .bootstrap/soong-cc/pkg -I .bootstrap/soong-art/pkg
     pkgPath = soong_build
 default .bootstrap/soong_build/obj/soong_build.a
 
 build .bootstrap/soong_build/obj/a.out: g.bootstrap.link $
         .bootstrap/soong_build/obj/soong_build.a | ${g.bootstrap.linkCmd}
-    libDirFlags = -L .bootstrap/blueprint-parser/pkg -L .bootstrap/blueprint-proptools/pkg -L .bootstrap/blueprint/pkg -L .bootstrap/blueprint-deptools/pkg -L .bootstrap/blueprint-pathtools/pkg -L .bootstrap/blueprint-bootstrap/pkg -L .bootstrap/soong-glob/pkg -L .bootstrap/soong-common/pkg -L .bootstrap/soong-config/pkg -L .bootstrap/soong-genrule/pkg -L .bootstrap/soong-cc/pkg -L .bootstrap/soong-art/pkg
+    libDirFlags = -L .bootstrap/blueprint-parser/pkg -L .bootstrap/blueprint-proptools/pkg -L .bootstrap/blueprint/pkg -L .bootstrap/blueprint-deptools/pkg -L .bootstrap/blueprint-pathtools/pkg -L .bootstrap/blueprint-bootstrap/pkg -L .bootstrap/soong-env/pkg -L .bootstrap/soong-glob/pkg -L .bootstrap/soong-common/pkg -L .bootstrap/soong-config/pkg -L .bootstrap/soong-genrule/pkg -L .bootstrap/soong-cc/pkg -L .bootstrap/soong-art/pkg
 default .bootstrap/soong_build/obj/a.out
 
 build .bootstrap/bin/soong_build: g.bootstrap.cp $
@@ -456,11 +475,33 @@ build .bootstrap/bin/soong_build: g.bootstrap.cp $
 default .bootstrap/bin/soong_build
 
 # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #
+# Module:  soong_env
+# Variant:
+# Type:    bootstrap_go_binary
+# Factory: github.com/google/blueprint/bootstrap.newGoBinaryModule
+# Defined: build/soong/Blueprints:31:1
+
+build .bootstrap/soong_env/obj/soong_env.a: g.bootstrap.gc $
+        ${g.bootstrap.srcDir}/build/soong/cmd/soong_env/soong_env.go | $
+        ${g.bootstrap.gcCmd} .bootstrap/soong-env/pkg/android/soong/env.a
+    incFlags = -I .bootstrap/soong-env/pkg
+    pkgPath = soong_env
+default .bootstrap/soong_env/obj/soong_env.a
+
+build .bootstrap/soong_env/obj/a.out: g.bootstrap.link $
+        .bootstrap/soong_env/obj/soong_env.a | ${g.bootstrap.linkCmd}
+    libDirFlags = -L .bootstrap/soong-env/pkg
+default .bootstrap/soong_env/obj/a.out
+
+build .bootstrap/bin/soong_env: g.bootstrap.cp .bootstrap/soong_env/obj/a.out
+default .bootstrap/bin/soong_env
+
+# # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #
 # Module:  soong_glob
 # Variant:
 # Type:    bootstrap_go_binary
 # Factory: github.com/google/blueprint/bootstrap.newGoBinaryModule
-# Defined: build/soong/Blueprints:30:1
+# Defined: build/soong/Blueprints:50:1
 
 build .bootstrap/soong_glob/obj/soong_glob.a: g.bootstrap.gc $
         ${g.bootstrap.srcDir}/build/soong/cmd/soong_glob/soong_glob.go | $
@@ -498,7 +539,8 @@ rule s.bootstrap.minibp
 build .bootstrap/main.ninja.in: s.bootstrap.bigbp $
         ${g.bootstrap.srcDir}/Blueprints | .bootstrap/bin/androidmk $
         .bootstrap/bin/bpfmt .bootstrap/bin/bpmodify .bootstrap/bin/minibp $
-        .bootstrap/bin/soong_build .bootstrap/bin/soong_glob
+        .bootstrap/bin/soong_build .bootstrap/bin/soong_env $
+        .bootstrap/bin/soong_glob
 default .bootstrap/main.ninja.in
 build .bootstrap/notAFile: phony
 default .bootstrap/notAFile
diff --git a/cmd/soong_build/main.go b/cmd/soong_build/main.go
index b84b804c..cb579e9c 100644
--- a/cmd/soong_build/main.go
+++ b/cmd/soong_build/main.go
@@ -64,6 +64,7 @@ func main() {
 
 	// Singletons
 	ctx.RegisterSingletonType("checkbuild", common.CheckbuildSingleton)
+	ctx.RegisterSingletonType("env", common.EnvSingleton)
 
 	configuration, err := config.New(srcDir)
 	if err != nil {
diff --git a/cmd/soong_env/soong_env.go b/cmd/soong_env/soong_env.go
new file mode 100644
index 00000000..933e525a
--- /dev/null
+++ b/cmd/soong_env/soong_env.go
@@ -0,0 +1,55 @@
+// Copyright 2015 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.
+
+// soong_glob is the command line tool that checks if the list of files matching a glob has
+// changed, and only updates the output file list if it has changed.  It is used to optimize
+// out build.ninja regenerations when non-matching files are added.  See
+// android/soong/common/glob.go for a longer description.
+package main
+
+import (
+	"flag"
+	"fmt"
+	"os"
+
+	"android/soong/env"
+)
+
+func usage() {
+	fmt.Fprintf(os.Stderr, "usage: soong_env env_file\n")
+	fmt.Fprintf(os.Stderr, "exits with success if the environment varibles in env_file match\n")
+	fmt.Fprintf(os.Stderr, "the current environment\n")
+	flag.PrintDefaults()
+	os.Exit(2)
+}
+
+func main() {
+	flag.Parse()
+
+	if flag.NArg() != 1 {
+		usage()
+	}
+
+	stale, err := env.StaleEnvFile(flag.Arg(0))
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "error: %s\n", err.Error())
+		os.Exit(1)
+	}
+
+	if stale {
+		os.Exit(1)
+	}
+
+	os.Exit(0)
+}
diff --git a/common/env.go b/common/env.go
new file mode 100644
index 00000000..e33a0258
--- /dev/null
+++ b/common/env.go
@@ -0,0 +1,47 @@
+// Copyright 2015 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 common
+
+import (
+	"android/soong/env"
+
+	"github.com/google/blueprint"
+)
+
+// This file supports dependencies on environment variables.  During build manifest generation,
+// any dependency on an environment variable is added to a list.  During the singleton phase
+// a JSON file is written containing the current value of all used environment variables.
+// The next time the top-level build script is run, it uses the soong_env executable to
+// compare the contents of the environment variables, rewriting the file if necessary to cause
+// a manifest regeneration.
+
+func EnvSingleton() blueprint.Singleton {
+	return &envSingleton{}
+}
+
+type envSingleton struct{}
+
+func (c *envSingleton) GenerateBuildActions(ctx blueprint.SingletonContext) {
+	envDeps := ctx.Config().(Config).EnvDeps()
+
+	envFile := ".soong.environment"
+
+	err := env.WriteEnvFile(envFile, envDeps)
+	if err != nil {
+		ctx.Errorf(err.Error())
+	}
+
+	ctx.AddNinjaFileDeps(envFile)
+}
diff --git a/common/module.go b/common/module.go
index 4b1200d3..3cb4c09d 100644
--- a/common/module.go
+++ b/common/module.go
@@ -20,6 +20,13 @@ import (
 	"github.com/google/blueprint"
 )
 
+type Config interface {
+	CpPreserveSymlinksFlags() string
+	SrcDir() string
+	Getenv(string) string
+	EnvDeps() map[string]string
+}
+
 var (
 	DeviceSharedLibrary = "shared_library"
 	DeviceStaticLibrary = "static_library"
diff --git a/common/paths.go b/common/paths.go
index bc75ea59..abe67bf5 100644
--- a/common/paths.go
+++ b/common/paths.go
@@ -20,11 +20,6 @@ import (
 	"github.com/google/blueprint"
 )
 
-type Config interface {
-	CpPreserveSymlinksFlags() string
-	SrcDir() string
-}
-
 // ModuleOutDir returns the path to the module-specific output directory.
 func ModuleOutDir(ctx AndroidModuleContext) string {
 	return filepath.Join(".intermediates", ctx.ModuleDir(), ctx.ModuleName(), ctx.ModuleSubDir())
diff --git a/config/config.go b/config/config.go
index 6cdc211e..6cb61e4b 100644
--- a/config/config.go
+++ b/config/config.go
@@ -41,7 +41,8 @@ func NewFileConfigurableOptions() FileConfigurableOptions {
 type Config struct {
 	FileConfigurableOptions
 
-	srcDir string // the path of the root source directory
+	srcDir  string // the path of the root source directory
+	envDeps map[string]string
 }
 
 // loads configuration options from a JSON file in the cwd.
@@ -103,7 +104,10 @@ func saveToConfigFile(config FileConfigurableOptions) error {
 // the root source directory. It also loads the config file, if found.
 func New(srcDir string) (*Config, error) {
 	// Make a config with default options
-	config := &Config{srcDir: srcDir}
+	config := &Config{
+		srcDir:  srcDir,
+		envDeps: make(map[string]string),
+	}
 
 	// Load any configurable options from the configuration file
 	err := loadFromConfigFile(config)
@@ -150,3 +154,17 @@ func (c *Config) CpPreserveSymlinksFlags() string {
 		return ""
 	}
 }
+
+func (c *Config) Getenv(key string) string {
+	var val string
+	var exists bool
+	if val, exists = c.envDeps[key]; !exists {
+		val = os.Getenv(key)
+		c.envDeps[key] = val
+	}
+	return val
+}
+
+func (c *Config) EnvDeps() map[string]string {
+	return c.envDeps
+}
diff --git a/env/env.go b/env/env.go
new file mode 100644
index 00000000..bf58a991
--- /dev/null
+++ b/env/env.go
@@ -0,0 +1,97 @@
+// Copyright 2015 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.
+
+// env implements the environment JSON file handling for the soong_env command line tool run before
+// the builder and for the env writer in the builder.
+package env
+
+import (
+	"encoding/json"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"sort"
+)
+
+type envFileEntry struct{ Key, Value string }
+type envFileData []envFileEntry
+
+func WriteEnvFile(filename string, envDeps map[string]string) error {
+	contents := make(envFileData, 0, len(envDeps))
+	for key, value := range envDeps {
+		contents = append(contents, envFileEntry{key, value})
+	}
+
+	sort.Sort(contents)
+
+	data, err := json.MarshalIndent(contents, "", "    ")
+	if err != nil {
+		return err
+	}
+
+	data = append(data, '\n')
+
+	err = ioutil.WriteFile(filename, data, 0664)
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func StaleEnvFile(filename string) (bool, error) {
+	data, err := ioutil.ReadFile(filename)
+	if err != nil {
+		return true, err
+	}
+
+	var contents envFileData
+
+	err = json.Unmarshal(data, &contents)
+	if err != nil {
+		return true, err
+	}
+
+	var changed []string
+	for _, entry := range contents {
+		key := entry.Key
+		old := entry.Value
+		cur := os.Getenv(key)
+		if old != cur {
+			changed = append(changed, fmt.Sprintf("%s (%q -> %q)", key, old, cur))
+		}
+	}
+
+	if len(changed) > 0 {
+		fmt.Printf("environment variables changed value:\n")
+		for _, s := range changed {
+			fmt.Printf("   %s\n", s)
+		}
+		return true, nil
+	}
+
+	return false, nil
+}
+
+func (e envFileData) Len() int {
+	return len(e)
+}
+
+func (e envFileData) Less(i, j int) bool {
+	return e[i].Key < e[j].Key
+}
+
+func (e envFileData) Swap(i, j int) {
+	e[i], e[j] = e[j], e[i]
+}
diff --git a/soong.bash b/soong.bash
index fc330d0b..fab15de5 100755
--- a/soong.bash
+++ b/soong.bash
@@ -27,4 +27,21 @@ fi
 # can regenerate the build manifest.
 export BLUEPRINT_NINJA_HAS_MULTIPASS=1
 
+# Ninja can't depend on environment variables, so do a manual comparison
+# of the relevant environment variables from the last build using the
+# soong_env tool and trigger a build manifest regeneration if necessary
+ENVFILE=${BUILDDIR}/.soong.environment
+ENVTOOL=${BUILDDIR}/.bootstrap/bin/soong_env
+if [ -f ${ENVFILE} ]; then
+    if [ -x ${ENVTOOL} ]; then
+        if ! ${ENVTOOL} ${ENVFILE}; then
+            echo "forcing build manifest regeneration"
+            rm -f ${ENVFILE}
+        fi
+    else
+        echo "Missing soong_env tool, forcing build manifest regeneration"
+        rm -f ${ENVFILE}
+    fi
+fi
+
 ${SRCDIR}/prebuilts/ninja/${PREBUILTOS}/ninja -C ${BUILDDIR} "$@"