Sandbox soong_build by changing to root directory

This relands I12a0f907753fefd1997ab8b4ea2ac331234093cf along with
a fix to blueprint for absolute paths.

Store the current working directory and then change to the root
directory so that all file accesses must go through helpers in
the android package that properly track dependencies.

Change-Id: I24ac485677aa102eec1a2521d16820da6ee1ae77
Fixes: 146437378
Test: m checkbuild
Test: m OUT_DIR=/tmp/out nothing

android/paths.go

@@ -16,6 +16,8 @@ package android
 
 import (
 	"fmt"
+	"io/ioutil"
+	"os"
 	"path/filepath"
 	"reflect"
 	"sort"
@@ -25,10 +27,11 @@ import (
 	"github.com/google/blueprint/pathtools"
 )
 
+var absSrcDir string
+
 // PathContext is the subset of a (Module|Singleton)Context required by the
 // Path methods.
 type PathContext interface {
-	Fs() pathtools.FileSystem
 	Config() Config
 	AddNinjaFileDeps(deps ...string)
 }
@@ -390,7 +393,7 @@ func expandOneSrcPath(ctx ModuleContext, s string, expandedExcludes []string) (P
 		return PathsWithModuleSrcSubDir(ctx, paths, ""), nil
 	} else {
 		p := pathForModuleSrc(ctx, s)
-		if exists, _, err := ctx.Fs().Exists(p.String()); err != nil {
+		if exists, _, err := ctx.Config().fs.Exists(p.String()); err != nil {
 			reportPathErrorf(ctx, "%s: %s", p, err.Error())
 		} else if !exists {
 			reportPathErrorf(ctx, "module source path %q does not exist", p)
@@ -720,7 +723,7 @@ func existsWithDependencies(ctx PathContext, path SourcePath) (exists bool, err
 		var deps []string
 		// We cannot add build statements in this context, so we fall back to
 		// AddNinjaFileDeps
-		files, deps, err = pathtools.Glob(path.String(), nil, pathtools.FollowSymlinks)
+		files, deps, err = ctx.Config().fs.Glob(path.String(), nil, pathtools.FollowSymlinks)
 		ctx.AddNinjaFileDeps(deps...)
 	}
 
@@ -752,7 +755,7 @@ func PathForSource(ctx PathContext, pathComponents ...string) SourcePath {
 		if !exists {
 			modCtx.AddMissingDependencies([]string{path.String()})
 		}
-	} else if exists, _, err := ctx.Fs().Exists(path.String()); err != nil {
+	} else if exists, _, err := ctx.Config().fs.Exists(path.String()); err != nil {
 		reportPathErrorf(ctx, "%s: %s", path, err.Error())
 	} else if !exists {
 		reportPathErrorf(ctx, "source path %q does not exist", path)
@@ -1356,7 +1359,6 @@ type testPathContext struct {
 	config Config
 }
 
-func (x *testPathContext) Fs() pathtools.FileSystem   { return x.config.fs }
 func (x *testPathContext) Config() Config             { return x.config }
 func (x *testPathContext) AddNinjaFileDeps(...string) {}
 
@@ -1402,3 +1404,16 @@ func maybeRelErr(basePath string, targetPath string) (string, bool, error) {
 	}
 	return rel, true, nil
 }
+
+// Writes a file to the output directory.  Attempting to write directly to the output directory
+// will fail due to the sandbox of the soong_build process.
+func WriteFileToOutputDir(path WritablePath, data []byte, perm os.FileMode) error {
+	return ioutil.WriteFile(absolutePath(path.String()), data, perm)
+}
+
+func absolutePath(path string) string {
+	if filepath.IsAbs(path) {
+		return path
+	}
+	return filepath.Join(absSrcDir, path)
+}