From ac4f1dd36ce8c3b3c06652780d86056d78efa642 Mon Sep 17 00:00:00 2001
From: John Crispin <john@phrozen.org>
Date: Tue, 22 Sep 2026 12:39:37 +0200
Subject: [PATCH] build: write the build info files only when they change

buildversion, feedsversion and json_overview_image_info rewrite
version.buildinfo, feeds.buildinfo and profiles.json on every build. The
content is the same, but the new timestamp makes the checksum step hash
every image again, which takes 0.18 s for the OpenWrt One and grows with the
number of devices.

Write the three files to the temporary directory and copy them over the
result of the last build only when the content differs.

This takes the time of the index, image info and checksum step from 0.36 s
to 0.10 s, and the time of a full no-op build from 3.25 s to 3.00 s.

Signed-off-by: John Crispin <john@phrozen.org>
---
 Makefile | 12 +++++++++---
 rules.mk | 11 +++++++++++
 2 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/Makefile b/Makefile
index 02bfa514096..aec12743761 100644
--- a/Makefile
+++ b/Makefile
@@ -105,8 +105,10 @@ prereq: $(target/stamp-prereq) tmp/.prereq_packages
 
 $(BIN_DIR)/profiles.json: FORCE
 	$(if $(CONFIG_JSON_OVERVIEW_IMAGE_INFO), \
+		mkdir -p $(BIN_DIR) $(TMP_DIR); \
 		WORK_DIR=$(BUILD_DIR)/json_info_files \
-			$(SCRIPT_DIR)/json_overview_image_info.py $@ \
+			$(SCRIPT_DIR)/json_overview_image_info.py $(TMP_DIR)/.profiles.json && \
+		$(call cp_if_changed,$(TMP_DIR)/.profiles.json,$@) \
 	)
 
 json_overview_image_info: $(BIN_DIR)/profiles.json
@@ -115,10 +117,14 @@ checksum: FORCE
 	$(call sha256sums,$(BIN_DIR),$(CONFIG_BUILDBOT))
 
 buildversion: FORCE
-	$(SCRIPT_DIR)/getver.sh > $(BIN_DIR)/version.buildinfo
+	mkdir -p $(BIN_DIR) $(TMP_DIR)
+	$(SCRIPT_DIR)/getver.sh > $(TMP_DIR)/.version.buildinfo
+	$(call cp_if_changed,$(TMP_DIR)/.version.buildinfo,$(BIN_DIR)/version.buildinfo)
 
 feedsversion: FORCE
-	$(SCRIPT_DIR)/feeds list -fs > $(BIN_DIR)/feeds.buildinfo
+	mkdir -p $(BIN_DIR) $(TMP_DIR)
+	$(SCRIPT_DIR)/feeds list -fs > $(TMP_DIR)/.feeds.buildinfo
+	$(call cp_if_changed,$(TMP_DIR)/.feeds.buildinfo,$(BIN_DIR)/feeds.buildinfo)
 
 # diffconfig.sh runs the config parser over the whole tree twice, which takes
 # two seconds of every build. Skip it while neither .config nor a Kconfig file
diff --git a/rules.mk b/rules.mk
index d5b955a37f6..0fd2ccf5123 100644
--- a/rules.mk
+++ b/rules.mk
@@ -500,6 +500,17 @@ define file_copy
 	$(CP) $(1) $(2)
 endef
 
+##@
+# @brief Copy a file over another one only when the content differs, so that
+#        the destination keeps its timestamp.
+#
+# @param 1: Source file.
+# @param 2: Destination file.
+##
+define cp_if_changed
+	{ cmp -s $(1) $(2) || cp $(1) $(2); }
+endef
+
 ##@
 # @brief Calculate sha256sum of any plain file within a given directory.
 #