soong_ui kati.go: write more comments.

This CL also removes --warn_real_to_phony and --warn_phony_looks_real,
since --werror_real_to_phony and --wrror_phony_looks_real are already
specified.

Test: treehugger
Bug: 173474588

Change-Id: Icbb0f53182d1b912c62f8044f52bb79d985bf868

ui/build/kati.go

@@ -33,12 +33,18 @@ const katiBuildSuffix = ""
 const katiCleanspecSuffix = "-cleanspec"
 const katiPackageSuffix = "-package"
 
-// genKatiSuffix creates a suffix for kati-generated files so that we can cache
+// genKatiSuffix creates a filename suffix for kati-generated files so that we
-// them based on their inputs. So this should encode all common changes to Kati
+// can cache them based on their inputs. Such files include the generated Ninja
-// inputs. Currently that includes the TARGET_PRODUCT, kati-processed command
+// files and env.sh environment variable setup files.
-// line arguments, and the directories specified by mm/mmm.
+//
+// The filename suffix should encode all common changes to Kati inputs.
+// Currently that includes the TARGET_PRODUCT and kati-processed command line
+// arguments.
 func genKatiSuffix(ctx Context, config Config) {
+	// Construct the base suffix.
 	katiSuffix := "-" + config.TargetProduct()
+
+	// Append kati arguments to the suffix.
 	if args := config.KatiArgs(); len(args) > 0 {
 		katiSuffix += "-" + spaceSlashReplacer.Replace(strings.Join(args, "_"))
 	}
@@ -60,51 +66,101 @@ func genKatiSuffix(ctx Context, config Config) {
 	}
 }
 
+// Base function to construct and run the Kati command line with additional
+// arguments, and a custom function closure to mutate the environment Kati runs
+// in.
 func runKati(ctx Context, config Config, extraSuffix string, args []string, envFunc func(*Environment)) {
 	executable := config.PrebuiltBuildTool("ckati")
+	// cKati arguments.
 	args = append([]string{
+		// Instead of executing commands directly, generate a Ninja file.
 		"--ninja",
+		// Generate Ninja files in the output directory.
 		"--ninja_dir=" + config.OutDir(),
+		// Filename suffix of the generated Ninja file.
 		"--ninja_suffix=" + config.KatiSuffix() + extraSuffix,
+		// Remove common parts at the beginning of a Ninja file, like build_dir,
+		// local_pool and _kati_always_build_. Allows Kati to be run multiple
+		// times, with generated Ninja files combined in a single invocation
+		// using 'include'.
 		"--no_ninja_prelude",
+		// Support declaring phony outputs in AOSP Ninja.
 		"--use_ninja_phony_output",
+		// Support declaring symlink outputs in AOSP Ninja.
 		"--use_ninja_symlink_outputs",
+		// Regenerate the Ninja file if environment inputs have changed. e.g.
+		// CLI flags, .mk file timestamps, env vars, $(wildcard ..) and some
+		// $(shell ..) results.
 		"--regen",
+		// Skip '-include' directives starting with the specified path. Used to
+		// ignore generated .mk files.
 		"--ignore_optional_include=" + filepath.Join(config.OutDir(), "%.P"),
+		// Detect the use of $(shell echo ...).
 		"--detect_android_echo",
+		// Colorful ANSI-based warning and error messages.
 		"--color_warnings",
+		// Generate all targets, not just the top level requested ones.
 		"--gen_all_targets",
+		// Use the built-in emulator of GNU find for better file finding
+		// performance. Used with $(shell find ...).
 		"--use_find_emulator",
+		// Fail when the find emulator encounters problems.
 		"--werror_find_emulator",
+		// Do not provide any built-in rules.
 		"--no_builtin_rules",
+		// Fail when suffix rules are used.
 		"--werror_suffix_rules",
-		"--warn_real_to_phony",
+		// Fail when a real target depends on a phony target.
-		"--warn_phony_looks_real",
 		"--werror_real_to_phony",
-		"--werror_phony_looks_real",
+		// Makes real_to_phony checks assume that any top-level or leaf
-		"--werror_writable",
+		// dependencies that does *not* have a '/' in it is a phony target.
 		"--top_level_phony",
+		// Fail when a phony target contains slashes.
+		"--werror_phony_looks_real",
+		// Fail when writing to a read-only directory.
+		"--werror_writable",
+		// Print Kati's internal statistics, such as the number of variables,
+		// implicit/explicit/suffix rules, and so on.
 		"--kati_stats",
 	}, args...)
 
+	// Generate a minimal Ninja file.
+	//
+	// Used for build_test and multiproduct_kati, which runs Kati several
+	// hundred times for different configurations to test file generation logic.
+	// These can result in generating Ninja files reaching ~1GB or more,
+	// resulting in ~hundreds of GBs of writes.
+	//
+	// Since we don't care about executing the Ninja files in these test cases,
+	// generating the Ninja file content wastes time, so skip writing any
+	// information out with --empty_ninja_file.
+	//
+	// From https://github.com/google/kati/commit/87b8da7af2c8bea28b1d8ab17679453d859f96e5
 	if config.Environment().IsEnvTrue("EMPTY_NINJA_FILE") {
 		args = append(args, "--empty_ninja_file")
 	}
 
+	// Apply 'local_pool' to to all rules that don't specify a pool.
 	if config.UseRemoteBuild() {
 		args = append(args, "--default_pool=local_pool")
 	}
 
 	cmd := Command(ctx, config, "ckati", executable, args...)
+
+	// Set up the nsjail sandbox.
 	cmd.Sandbox = katiSandbox
+
+	// Set up stdout and stderr.
 	pipe, err := cmd.StdoutPipe()
 	if err != nil {
 		ctx.Fatalln("Error getting output pipe for ckati:", err)
 	}
 	cmd.Stderr = cmd.Stdout
 
+	// Apply the caller's function closure to mutate the environment variables.
 	envFunc(cmd.Environment)
 
+	// Pass on various build environment metadata to Kati.
 	if _, ok := cmd.Environment.Get("BUILD_USERNAME"); !ok {
 		username := "unknown"
 		if u, err := user.Current(); err == nil {
@@ -125,6 +181,8 @@ func runKati(ctx Context, config Config, extraSuffix string, args []string, envF
 	}
 
 	cmd.StartOrFatal()
+	// Set up the ToolStatus command line reader for Kati for a consistent UI
+	// for the user.
 	status.KatiReader(ctx.Status.StartTool(), pipe)
 	cmd.WaitOrFatal()
 }
@@ -134,35 +192,57 @@ func runKatiBuild(ctx Context, config Config) {
 	defer ctx.EndTrace()
 
 	args := []string{
+		// Mark the output directory as writable.
 		"--writable", config.OutDir() + "/",
+		// Fail when encountering implicit rules. e.g.
+		// %.foo: %.bar
+		//   cp $< $@
 		"--werror_implicit_rules",
+		// Entry point for the Kati Ninja file generation.
 		"-f", "build/make/core/main.mk",
 	}
 
 	if !config.BuildBrokenDupRules() {
+		// Fail when redefining / duplicating a target.
 		args = append(args, "--werror_overriding_commands")
 	}
 
 	args = append(args, config.KatiArgs()...)
 
 	args = append(args,
+		// Location of the Make vars .mk file generated by Soong.
 		"SOONG_MAKEVARS_MK="+config.SoongMakeVarsMk(),
+		// Location of the Android.mk file generated by Soong. This
+		// file contains Soong modules represented as Kati modules,
+		// allowing Kati modules to depend on Soong modules.
 		"SOONG_ANDROID_MK="+config.SoongAndroidMk(),
+		// Directory containing outputs for the target device.
 		"TARGET_DEVICE_DIR="+config.TargetDeviceDir(),
+		// Directory containing .mk files for packaging purposes, such as
+		// the dist.mk file, containing dist-for-goals data.
 		"KATI_PACKAGE_MK_DIR="+config.KatiPackageMkDir())
 
 	runKati(ctx, config, katiBuildSuffix, args, func(env *Environment) {})
 
+	// compress and dist the main build ninja file.
 	distGzipFile(ctx, config, config.KatiBuildNinjaFile())
 
+	// Cleanup steps.
 	cleanCopyHeaders(ctx, config)
 	cleanOldInstalledFiles(ctx, config)
 }
 
+// Clean out obsolete header files on the disk that were *not copied* during the
+// build with BUILD_COPY_HEADERS and LOCAL_COPY_HEADERS.
+//
+// These should be increasingly uncommon, as it's a deprecated feature and there
+// isn't an equivalent feature in Soong.
 func cleanCopyHeaders(ctx Context, config Config) {
 	ctx.BeginTrace("clean", "clean copy headers")
 	defer ctx.EndTrace()
 
+	// Read and parse the list of copied headers from a file in the product
+	// output directory.
 	data, err := ioutil.ReadFile(filepath.Join(config.ProductOut(), ".copied_headers_list"))
 	if err != nil {
 		if os.IsNotExist(err) {
@@ -178,6 +258,8 @@ func cleanCopyHeaders(ctx Context, config Config) {
 	headerDir := headers[0]
 	headers = headers[1:]
 
+	// Walk the tree and remove any headers that are not in the list of copied
+	// headers in the current build.
 	filepath.Walk(headerDir,
 		func(path string, info os.FileInfo, err error) error {
 			if err != nil {
@@ -196,6 +278,8 @@ func cleanCopyHeaders(ctx Context, config Config) {
 		})
 }
 
+// Clean out any previously installed files from the disk that are not installed
+// in the current build.
 func cleanOldInstalledFiles(ctx Context, config Config) {
 	ctx.BeginTrace("clean", "clean old installed files")
 	defer ctx.EndTrace()
@@ -213,18 +297,27 @@ func cleanOldInstalledFiles(ctx Context, config Config) {
 	cleanOldFiles(ctx, config.HostOut(), ".installable_test_files")
 }
 
+// Generate the Ninja file containing the packaging command lines for the dist
+// dir.
 func runKatiPackage(ctx Context, config Config) {
 	ctx.BeginTrace(metrics.RunKati, "kati package")
 	defer ctx.EndTrace()
 
 	args := []string{
+		// Mark the dist dir as writable.
 		"--writable", config.DistDir() + "/",
+		// Fail when encountering implicit rules. e.g.
 		"--werror_implicit_rules",
+		// Fail when redefining / duplicating a target.
 		"--werror_overriding_commands",
+		// Entry point.
 		"-f", "build/make/packaging/main.mk",
+		// Directory containing .mk files for packaging purposes, such as
+		// the dist.mk file, containing dist-for-goals data.
 		"KATI_PACKAGE_MK_DIR=" + config.KatiPackageMkDir(),
 	}
 
+	// Run Kati against a restricted set of environment variables.
 	runKati(ctx, config, katiPackageSuffix, args, func(env *Environment) {
 		env.Allow([]string{
 			// Some generic basics
@@ -251,16 +344,21 @@ func runKatiPackage(ctx Context, config Config) {
 		}
 	})
 
+	// Compress and dist the packaging Ninja file.
 	distGzipFile(ctx, config, config.KatiPackageNinjaFile())
 }
 
+// Run Kati on the cleanspec files to clean the build.
 func runKatiCleanSpec(ctx Context, config Config) {
 	ctx.BeginTrace(metrics.RunKati, "kati cleanspec")
 	defer ctx.EndTrace()
 
 	runKati(ctx, config, katiCleanspecSuffix, []string{
+		// Fail when encountering implicit rules. e.g.
 		"--werror_implicit_rules",
+		// Fail when redefining / duplicating a target.
 		"--werror_overriding_commands",
+		// Entry point.
 		"-f", "build/make/core/cleanbuild.mk",
 		"SOONG_MAKEVARS_MK=" + config.SoongMakeVarsMk(),
 		"TARGET_DEVICE_DIR=" + config.TargetDeviceDir(),