From 1a12d032301b67d0b20bb93c80a819cb0f616833 Mon Sep 17 00:00:00 2001
From: Chris Parsons <cparsons@google.com>
Date: Mon, 6 Feb 2023 22:37:41 -0500
Subject: [PATCH] Avoid rewriting soong_build outputs if unchanged

This changes bp2build codegen, symlink forest generation, and
soong_build so that they do not rewrite output files if the contents are
unchanged.

Bug: 266983462
Test: m droid
Test: canonical_perf.sh benchmarking
Test: Manually verified that rerunning analysis did not regenerate
out/soong/workspace/prebuilts/sdk/BUILD.bazel unless contents changed

Change-Id: I5ec227df7a32b53c7fa0d741fb1403a51931024b
---
 android/paths.go           |  6 ++++--
 bp2build/symlink_forest.go | 23 ++++++-----------------
 2 files changed, 10 insertions(+), 19 deletions(-)

diff --git a/android/paths.go b/android/paths.go
index 2c5010473..6c3009f4c 100644
--- a/android/paths.go
+++ b/android/paths.go
@@ -16,7 +16,6 @@ package android
 
 import (
 	"fmt"
-	"io/ioutil"
 	"os"
 	"path/filepath"
 	"reflect"
@@ -2099,13 +2098,16 @@ func maybeRelErr(basePath string, targetPath string) (string, bool, error) {
 
 // Writes a file to the output directory.  Attempting to write directly to the output directory
 // will fail due to the sandbox of the soong_build process.
+// Only writes the file if the file doesn't exist or if it has different contents, to prevent
+// updating the timestamp if no changes would be made. (This is better for incremental
+// performance.)
 func WriteFileToOutputDir(path WritablePath, data []byte, perm os.FileMode) error {
 	absPath := absolutePath(path.String())
 	err := os.MkdirAll(filepath.Dir(absPath), 0777)
 	if err != nil {
 		return err
 	}
-	return ioutil.WriteFile(absPath, data, perm)
+	return pathtools.WriteFileIfChanged(absPath, data, perm)
 }
 
 func RemoveAllOutputDir(path WritablePath) error {
diff --git a/bp2build/symlink_forest.go b/bp2build/symlink_forest.go
index 183eb1205..aac5e7d2c 100644
--- a/bp2build/symlink_forest.go
+++ b/bp2build/symlink_forest.go
@@ -25,6 +25,7 @@ import (
 	"sync/atomic"
 
 	"android/soong/shared"
+	"github.com/google/blueprint/pathtools"
 )
 
 // A tree structure that describes what to do at each directory in the created
@@ -116,25 +117,13 @@ func mergeBuildFiles(output string, srcBuildFile string, generatedBuildFile stri
 		generatedBuildFileContent = packageDefaultVisibilityRegex.ReplaceAll(generatedBuildFileContent, []byte{})
 	}
 
-	outFile, err := os.Create(output)
-	if err != nil {
-		return err
+	newContents := generatedBuildFileContent
+	if newContents[len(newContents)-1] != '\n' {
+		newContents = append(newContents, '\n')
 	}
+	newContents = append(newContents, srcBuildFileContent...)
 
-	_, err = outFile.Write(generatedBuildFileContent)
-	if err != nil {
-		return err
-	}
-
-	if generatedBuildFileContent[len(generatedBuildFileContent)-1] != '\n' {
-		_, err = outFile.WriteString("\n")
-		if err != nil {
-			return err
-		}
-	}
-
-	_, err = outFile.Write(srcBuildFileContent)
-	return err
+	return pathtools.WriteFileIfChanged(output, newContents, 0666)
 }
 
 // Calls readdir() and returns it as a map from the basename of the files in dir