chore: Avoid calling GCC for tests when C code is unchanged

build-template.ninja

@@ -55,7 +55,8 @@ build $milone_dotnet_release: $
 
 rule build_milone
   description = build_milone out=$out
-  command = MILONE_HOME=$$PWD $milone compile $verbosity MiloneLang >$out
+  command = MILONE_HOME=$$PWD $milone compile $verbosity MiloneLang | scripts/write-if-change $out
+  restat = 1
 
 rule build_cc
   description = build_cc in=$in
@@ -92,7 +93,8 @@ build target/milone_gen3.c: $
 
 rule diff
   description = diff out=$out
-  command = diff $in >$out
+  command = diff $in | scripts/write-if-change $out
+  restat = 1
 
 build target/milone_gen3.diff: $
   diff $
@@ -128,12 +130,14 @@ build test_self: $
 rule take_snapshot
   description = take_snapshot $project
   command = $
-    MILONE_HOME=$$PWD $milone compile -q $project >$out || $
-    echo 'milone-lang compile error.' >$out_file
+    MILONE_HOME=$$PWD $milone compile -q $project | scripts/write-if-change $out || $
+    echo 'milone-lang compile error.' | scripts/write-if-change $out_file
+  restat = 1
 
 # Print snapshot diff.
 rule git_diff_snapshot_stats
-  command = git diff --stat -- 'tests/**/*.c' >$out
+  command = git diff --stat -- 'tests/**/*.c' | scripts/write-if-change $out
+  restat = 1
 
 build snapshot_diff: $
   git_diff_snapshot_stats $
@@ -154,10 +158,13 @@ rule execute_test
   description = execute_test $c_file
   command = $
     test ! -z "$$(grep '#error' -- $c_file)" && $
-    echo 'milone-lang compile error.' >$out || { $
-      $in >$out ; $
-      echo '$$? = '$$? >>$out ; $
+    echo 'milone-lang compile error.' | scripts/write-if-change $out || { $
+      $in >$out.tmp ; $
+      echo '$$? = '$$? >>$out.tmp ; $
+      cat $out.tmp | scripts/write-if-change $out ; $
+      rm -f $out.tmp ; $
     }
+  restat = 1
 
 rule diff_test_output
   description = diff_test_output $test

scripts/write-if-change

@@ -0,0 +1,30 @@
+#!/bin/bash
+# USAGE:
+#   ... | write-if-change $FILENAME
+#
+# Write standard input to a file unless the input is same as current contents.
+# That is, timestamp changes only if its contents change.
+
+set -eu
+
+FILENAME="$1"
+
+if test -r "$FILENAME" && test ! -f "$FILENAME"
+then
+    echo "ERROR: $FILENAME is not a regular file."
+    exit 1
+fi
+
+if test ! -f "$FILENAME"
+then
+    cat >$FILENAME
+    exit 0
+fi
+
+OLD_CONTENTS="$(cat $FILENAME)"
+NEW_CONTENTS="$(cat)"
+
+if test "$OLD_CONTENTS" != "$NEW_CONTENTS"
+then
+    echo "$NEW_CONTENTS" >$FILENAME
+fi