Add flag to skip writing output from soong_zip.

We want to measure how much of build time is spent on writing large
zips to disk. We can do this by doing the rest of the work but
skipping the write itself.

Some local and trivial testing shows this is probably a lot of the
cost of these rules:

    $ python -m timeit -s "import os" -r 10 "os.system('soong_zip -write_if_changed -o foo.zip -D prebuilts/sdk')"
    1 loop, best of 10: 8.37 sec per loop
    $ python -m timeit -s "import os" -r 10 "os.system('soong_zip -o foo.zip -D prebuilts/sdk')"
    1 loop, best of 10: 5.39 sec per loop
    $ python -m timeit -s "import os" -r 10 "os.system('soong_zip -n -o foo.zip -D prebuilts/sdk')"
    1 loop, best of 10: 3.66 sec per loop

Bug: None
Test: treehugger
Change-Id: I43acdb08d0e00efaba9e5d7add972a7ec41646d2

2 files changed, 7 insertions, 2 deletions

diff --git a/zip/cmd/main.go b/zip/cmd/main.go
index a2ccc2011..5231faec9 100644
--- a/zip/cmd/main.go
+++ b/zip/cmd/main.go
@@ -173,6 +173,7 @@ func main() {
 	cpuProfile := flags.String("cpuprofile", "", "write cpu profile to file")
 	traceFile := flags.String("trace", "", "write trace to file")
 	sha256Checksum := flags.Bool("sha256", false, "add a zip header to each file containing its SHA256 digest")
+	doNotWrite := flags.Bool("n", false, "Nothing is written to disk -- all other work happens")
 
 	flags.Var(&rootPrefix{}, "P", "path prefix within the zip at which to place files")
 	flags.Var(&listFiles{}, "l", "file containing list of files to zip")
@@ -236,6 +237,7 @@ func main() {
 		StoreSymlinks:            *symlinks,
 		IgnoreMissingFiles:       *ignoreMissingFiles,
 		Sha256Checksum:           *sha256Checksum,
+		DoNotWrite:               *doNotWrite,
 	})
 	if err != nil {
 		fmt.Fprintln(os.Stderr, "error:", err.Error())
diff --git a/zip/zip.go b/zip/zip.go
index 5e1a10462..30a2ee762 100644
--- a/zip/zip.go
+++ b/zip/zip.go
@@ -282,6 +282,7 @@ type ZipArgs struct {
 	StoreSymlinks            bool
 	IgnoreMissingFiles       bool
 	Sha256Checksum           bool
+	DoNotWrite               bool
 
 	Stderr     io.Writer
 	Filesystem pathtools.FileSystem
@@ -400,7 +401,9 @@ func Zip(args ZipArgs) error {
 
 	var zipErr error
 
-	if !args.WriteIfChanged {
+	if args.DoNotWrite {
+		out = io.Discard
+	} else if !args.WriteIfChanged {
 		f, err := os.Create(args.OutputFilePath)
 		if err != nil {
 			return err
@@ -421,7 +424,7 @@ func Zip(args ZipArgs) error {
 		return zipErr
 	}
 
-	if args.WriteIfChanged {
+	if args.WriteIfChanged && !args.DoNotWrite {
 		err := pathtools.WriteFileIfChanged(args.OutputFilePath, buf.Bytes(), 0666)
 		if err != nil {
 			return err