Allow n2 as a replacement for ninja in builds

If `SOONG_USE_N2=true` is set in the environment, then n2 is used in
place of ninja.  Some ninja features are not available in n2 at this
time, but this enables bringup efforts to happen in parallel.

Bug: 352368206
Test: manual
Change-Id: I8455cb24eb640a4651782ee76e48a7d3a9932b93

ui/build/config.go

@@ -121,6 +121,10 @@ type configImpl struct {
 	// There's quite a bit of overlap with module-info.json and soong module graph. We
 	// could consider merging them.
 	moduleDebugFile string
+
+	// Whether to use n2 instead of ninja.  This is controlled with the
+	// environment variable SOONG_USE_N2
+	useN2 bool
 }
 
 type NinjaWeightListSource uint
@@ -283,6 +287,10 @@ func NewConfig(ctx Context, args ...string) Config {
 		ret.moduleDebugFile, _ = filepath.Abs(shared.JoinPath(ret.SoongOutDir(), "soong-debug-info.json"))
 	}
 
+	if os.Getenv("SOONG_USE_N2") == "true" {
+		ret.useN2 = true
+	}
+
 	ret.environ.Unset(
 		// We're already using it
 		"USE_SOONG_UI",
@@ -339,6 +347,9 @@ func NewConfig(ctx Context, args ...string) Config {
 
 		// We read it here already, don't let others share in the fun
 		"GENERATE_SOONG_DEBUG",
+
+		// Use config.useN2 instead.
+		"SOONG_USE_N2",
 	)
 
 	if ret.UseGoma() || ret.ForceUseGoma() {

ui/build/ninja.go

@@ -56,6 +56,17 @@ func runNinjaForBuild(ctx Context, config Config) {
 		"-d", "stats",
 		"--frontend_file", fifo,
 	}
+	if config.useN2 {
+		executable = config.PrebuiltBuildTool("n2")
+		args = []string{
+			"-d", "trace",
+			// TODO: implement these features, or remove them.
+			//"-d", "keepdepfile",
+			//"-d", "keeprsp",
+			//"-d", "stats",
+			"--frontend-file", fifo,
+		}
+	}
 
 	args = append(args, config.NinjaArgs()...)
 
@@ -72,17 +83,21 @@ func runNinjaForBuild(ctx Context, config Config) {
 
 	args = append(args, "-f", config.CombinedNinjaFile())
 
-	args = append(args,
-		"-o", "usesphonyoutputs=yes",
-		"-w", "dupbuild=err",
-		"-w", "missingdepfile=err")
+	if !config.useN2 {
+		args = append(args,
+			"-o", "usesphonyoutputs=yes",
+			"-w", "dupbuild=err",
+			"-w", "missingdepfile=err")
+	}
 
 	if !config.BuildBrokenMissingOutputs() {
 		// Missing outputs will be treated as errors.
 		// BUILD_BROKEN_MISSING_OUTPUTS can be used to bypass this check.
-		args = append(args,
-			"-w", "missingoutfile=err",
-		)
+		if !config.useN2 {
+			args = append(args,
+				"-w", "missingoutfile=err",
+			)
+		}
 	}
 
 	cmd := Command(ctx, config, "ninja", executable, args...)
@@ -97,16 +112,22 @@ func runNinjaForBuild(ctx Context, config Config) {
 
 	switch config.NinjaWeightListSource() {
 	case NINJA_LOG:
-		cmd.Args = append(cmd.Args, "-o", "usesninjalogasweightlist=yes")
+		if !config.useN2 {
+			cmd.Args = append(cmd.Args, "-o", "usesninjalogasweightlist=yes")
+		}
 	case EVENLY_DISTRIBUTED:
 		// pass empty weight list means ninja considers every tasks's weight as 1(default value).
-		cmd.Args = append(cmd.Args, "-o", "usesweightlist=/dev/null")
+		if !config.useN2 {
+			cmd.Args = append(cmd.Args, "-o", "usesweightlist=/dev/null")
+		}
 	case EXTERNAL_FILE:
 		fallthrough
 	case HINT_FROM_SOONG:
 		// The weight list is already copied/generated.
-		ninjaWeightListPath := filepath.Join(config.OutDir(), ninjaWeightListFileName)
-		cmd.Args = append(cmd.Args, "-o", "usesweightlist="+ninjaWeightListPath)
+		if !config.useN2 {
+			ninjaWeightListPath := filepath.Join(config.OutDir(), ninjaWeightListFileName)
+			cmd.Args = append(cmd.Args, "-o", "usesweightlist="+ninjaWeightListPath)
+		}
 	}
 
 	// Allow both NINJA_ARGS and NINJA_EXTRA_ARGS, since both have been
@@ -206,11 +227,16 @@ func runNinjaForBuild(ctx Context, config Config) {
 			// We don't want this build broken flag to cause reanalysis, so allow it through to the
 			// actions.
 			"BUILD_BROKEN_INCORRECT_PARTITION_IMAGES",
+			"SOONG_USE_N2",
+			"RUST_BACKTRACE",
 		}, config.BuildBrokenNinjaUsesEnvVars()...)...)
 	}
 
 	cmd.Environment.Set("DIST_DIR", config.DistDir())
 	cmd.Environment.Set("SHELL", "/bin/bash")
+	if config.useN2 {
+		cmd.Environment.Set("RUST_BACKTRACE", "1")
+	}
 
 	// Print the environment variables that Ninja is operating in.
 	ctx.Verboseln("Ninja environment: ")

ui/build/soong.go

@@ -638,6 +638,22 @@ func runSoong(ctx Context, config Config) {
 			"--frontend_file", fifo,
 			"-f", filepath.Join(config.SoongOutDir(), "bootstrap.ninja"),
 		}
+		if config.useN2 {
+			ninjaArgs = []string{
+				// TODO: implement these features, or remove them.
+				//"-d", "keepdepfile",
+				//"-d", "stats",
+				//"-o", "usesphonyoutputs=yes",
+				//"-o", "preremoveoutputs=yes",
+				//"-w", "dupbuild=err",
+				//"-w", "outputdir=err",
+				//"-w", "missingoutfile=err",
+				"-v",
+				"-j", strconv.Itoa(config.Parallel()),
+				"--frontend-file", fifo,
+				"-f", filepath.Join(config.SoongOutDir(), "bootstrap.ninja"),
+			}
+		}
 
 		if extra, ok := config.Environment().Get("SOONG_UI_NINJA_ARGS"); ok {
 			ctx.Printf(`CAUTION: arguments in $SOONG_UI_NINJA_ARGS=%q, e.g. "-n", can make soong_build FAIL or INCORRECT`, extra)
@@ -645,8 +661,13 @@ func runSoong(ctx Context, config Config) {
 		}
 
 		ninjaArgs = append(ninjaArgs, targets...)
+		ninjaCmd := config.PrebuiltBuildTool("ninja")
+		if config.useN2 {
+			ninjaCmd = config.PrebuiltBuildTool("n2")
+		}
+
 		cmd := Command(ctx, config, "soong bootstrap",
-			config.PrebuiltBuildTool("ninja"), ninjaArgs...)
+			ninjaCmd, ninjaArgs...)
 
 		var ninjaEnv Environment