soong_zip: add --ignore_missing_files flag

soong_zip builds a list of files to zip early and then starts
zipping them all.  If a directory being zipped is concurrently
modified, a file that existed when soong_zip started may not
still exist.  Add a flag that continues when an expected file
does not exist.  Print a warning, since this should be rare
in normal usages but is a sign of a problem if it happens
regularly.

Test: zip_test.go
Test: m checkbuild
Test: m platform
Change-Id: I78426fe66fded8528ddd436c0f71a7442183cfeb

zip/cmd/main.go

@@ -136,7 +136,7 @@ func main() {
 	compLevel := flags.Int("L", 5, "deflate compression level (0-9)")
 	emulateJar := flags.Bool("jar", false, "modify the resultant .zip to emulate the output of 'jar'")
 	writeIfChanged := flags.Bool("write_if_changed", false, "only update resultant .zip if it has changed")
-
+	ignoreMissingFiles := flags.Bool("ignore_missing_files", false, "continue if a requested file does not exist")
 	symlinks := flags.Bool("symlinks", true, "store symbolic links in zip instead of following them")
 
 	parallelJobs := flags.Int("parallel", runtime.NumCPU(), "number of parallel threads to use")
@@ -200,6 +200,7 @@ func main() {
 		NonDeflatedFiles:         nonDeflatedFiles,
 		WriteIfChanged:           *writeIfChanged,
 		StoreSymlinks:            *symlinks,
+		IgnoreMissingFiles:       *ignoreMissingFiles,
 	})
 	if err != nil {
 		fmt.Fprintln(os.Stderr, "error:", err.Error())