Automate dependency on inputs of genrule module type.

This change automates one of the best practices for
custom tools. The best practice says "Prefer to list
input files on the command line, otherwise we may not
know to re-run your command when a new input file is
added." [1]

Normally you'd reference these inputs with $(in) or
one of the forms of the $(location*) substituions on
the command line for the custom tool. However, if the
custom tool does not accept the list of inputs on the
command line, the build system could fail to re-run
the custom tool if the list changes.

This change adds a hash of the list of input names to
the sbox command that wraps the custom tool. If the
list of inputs change, the hash will change, and
therefore the sbox command will change, causing ninja
to re-run the custom tool. The hash is visible to
(but ignored by) the sbox command, and hidden from
your custom tool.

[1] https://android.googlesource.com/platform/build/soong/+/refs/heads/master/docs/best_practices.md#custom-build-tools

Test: TestGenruleHashInputs
Bug: 149397658
Change-Id: I18b547ea3c4296ee15bd6150a4778a8f376d80b7

cmd/sbox/sbox.go

@@ -36,6 +36,7 @@ var (
 	outputRoot    string
 	keepOutDir    bool
 	depfileOut    string
+	inputHash     string
 )
 
 func init() {
@@ -51,6 +52,8 @@ func init() {
 	flag.StringVar(&depfileOut, "depfile-out", "",
 		"file path of the depfile to generate. This value will replace '__SBOX_DEPFILE__' in the command and will be treated as an output but won't be added to __SBOX_OUT_FILES__")
 
+	flag.StringVar(&inputHash, "input-hash", "",
+		"This option is ignored. Typical usage is to supply a hash of the list of input names so that the module will be rebuilt if the list (and thus the hash) changes.")
 }
 
 func usageViolation(violation string) {
@@ -59,7 +62,7 @@ func usageViolation(violation string) {
 	}
 
 	fmt.Fprintf(os.Stderr,
-		"Usage: sbox -c <commandToRun> --sandbox-path <sandboxPath> --output-root <outputRoot> [--depfile-out depFile] <outputFile> [<outputFile>...]\n"+
+		"Usage: sbox -c <commandToRun> --sandbox-path <sandboxPath> --output-root <outputRoot> [--depfile-out depFile] [--input-hash hash] <outputFile> [<outputFile>...]\n"+
 			"\n"+
 			"Deletes <outputRoot>,"+
 			"runs <commandToRun>,"+

genrule/genrule.go

@@ -26,6 +26,7 @@ import (
 
 	"android/soong/android"
 	"android/soong/shared"
+	"crypto/sha256"
 	"path/filepath"
 )
 
@@ -309,6 +310,7 @@ func (g *Module) GenerateAndroidBuildActions(ctx android.ModuleContext) {
 			addLocationLabel(out.Rel(), []string{filepath.Join("__SBOX_OUT_DIR__", out.Rel())})
 		}
 
+		referencedIn := false
 		referencedDepfile := false
 
 		rawCommand, err := android.ExpandNinjaEscaped(task.cmd, func(name string) (string, bool, error) {
@@ -333,6 +335,7 @@ func (g *Module) GenerateAndroidBuildActions(ctx android.ModuleContext) {
 				}
 				return locationLabels[firstLabel][0], false, nil
 			case "in":
+				referencedIn = true
 				return "${in}", true, nil
 			case "out":
 				return "__SBOX_OUT_FILES__", false, nil
@@ -398,8 +401,16 @@ func (g *Module) GenerateAndroidBuildActions(ctx android.ModuleContext) {
 		// Escape the command for the shell
 		rawCommand = "'" + strings.Replace(rawCommand, "'", `'\''`, -1) + "'"
 		g.rawCommands = append(g.rawCommands, rawCommand)
-		sandboxCommand := fmt.Sprintf("rm -rf %s && $sboxCmd --sandbox-path %s --output-root %s -c %s %s $allouts",
-			task.genDir, sandboxPath, task.genDir, rawCommand, depfilePlaceholder)
+
+		sandboxCommand := fmt.Sprintf("rm -rf %s && $sboxCmd --sandbox-path %s --output-root %s",
+			task.genDir, sandboxPath, task.genDir)
+
+		if !referencedIn {
+			sandboxCommand = sandboxCommand + hashSrcFiles(srcFiles)
+		}
+
+		sandboxCommand = sandboxCommand + fmt.Sprintf(" -c %s %s $allouts",
+			rawCommand, depfilePlaceholder)
 
 		ruleParams := blueprint.RuleParams{
 			Command:     sandboxCommand,
@@ -463,6 +474,14 @@ func (g *Module) GenerateAndroidBuildActions(ctx android.ModuleContext) {
 
 }
 
+func hashSrcFiles(srcFiles android.Paths) string {
+	h := sha256.New()
+	for _, src := range srcFiles {
+		h.Write([]byte(src.String()))
+	}
+	return fmt.Sprintf(" --input-hash %x", h.Sum(nil))
+}
+
 func (g *Module) generateSourceFile(ctx android.ModuleContext, task generateTask, rule blueprint.Rule) {
 	desc := "generate"
 	if len(task.out) == 0 {

genrule/genrule_test.go

@@ -502,6 +502,93 @@ func TestGenruleCmd(t *testing.T) {
 	}
 }
 
+func TestGenruleHashInputs(t *testing.T) {
+
+	// The basic idea here is to verify that the sbox command (which is
+	// in the Command field of the generate rule) contains a hash of the
+	// inputs, but only if $(in) is not referenced in the genrule cmd
+	// property.
+
+	// By including a hash of the inputs, we cause the rule to re-run if
+	// the list of inputs changes (because the sbox command changes).
+
+	// However, if the genrule cmd property already contains $(in), then
+	// the dependency is already expressed, so we don't need to include the
+	// hash in that case.
+
+	bp := `
+			genrule {
+				name: "hash0",
+				srcs: ["in1.txt", "in2.txt"],
+				out: ["out"],
+				cmd: "echo foo > $(out)",
+			}
+			genrule {
+				name: "hash1",
+				srcs: ["*.txt"],
+				out: ["out"],
+				cmd: "echo bar > $(out)",
+			}
+			genrule {
+				name: "hash2",
+				srcs: ["*.txt"],
+				out: ["out"],
+				cmd: "echo $(in) > $(out)",
+			}
+		`
+	testcases := []struct {
+		name         string
+		expectedHash string
+	}{
+		{
+			name: "hash0",
+			// sha256 value obtained from: echo -n 'in1.txtin2.txt' | sha256sum
+			expectedHash: "031097e11e0a8c822c960eb9742474f46336360a515744000d086d94335a9cb9",
+		},
+		{
+			name: "hash1",
+			// sha256 value obtained from: echo -n 'in1.txtin2.txtin3.txt' | sha256sum
+			expectedHash: "de5d22a4a7ab50d250cc59fcdf7a7e0775790d270bfca3a7a9e1f18a70dd996c",
+		},
+		{
+			name: "hash2",
+			// $(in) is present, option should not appear
+			expectedHash: "",
+		},
+	}
+
+	config := testConfig(bp, nil)
+	ctx := testContext(config)
+	_, errs := ctx.ParseFileList(".", []string{"Android.bp"})
+	if errs == nil {
+		_, errs = ctx.PrepareBuildActions(config)
+	}
+	if errs != nil {
+		t.Fatal(errs)
+	}
+
+	for _, test := range testcases {
+		t.Run(test.name, func(t *testing.T) {
+			gen := ctx.ModuleForTests(test.name, "")
+			command := gen.Rule("generator").RuleParams.Command
+
+			if len(test.expectedHash) > 0 {
+				// We add spaces before and after to make sure that
+				// this option doesn't abutt another sbox option.
+				expectedInputHashOption := " --input-hash " + test.expectedHash + " "
+
+				if !strings.Contains(command, expectedInputHashOption) {
+					t.Errorf("Expected command \"%s\" to contain \"%s\"", command, expectedInputHashOption)
+				}
+			} else {
+				if strings.Contains(command, "--input-hash") {
+					t.Errorf("Unexpected \"--input-hash\" found in command: \"%s\"", command)
+				}
+			}
+		})
+	}
+}
+
 func TestGenSrcs(t *testing.T) {
 	testcases := []struct {
 		name string