basic/fileio: extend atomic file writing with timestamp setting

There should be no functional change.

src/basic/fileio.c

@@ -51,7 +51,7 @@
 
 #define READ_FULL_BYTES_MAX (4U*1024U*1024U)
 
-int write_string_stream(FILE *f, const char *line, bool enforce_newline) {
+int write_string_stream_ts(FILE *f, const char *line, bool enforce_newline, struct timespec *ts) {
 
         assert(f);
         assert(line);
@@ -60,6 +60,13 @@ int write_string_stream(FILE *f, const char *line, bool enforce_newline) {
         if (enforce_newline && !endswith(line, "\n"))
                 fputc('\n', f);
 
+        if (ts) {
+                struct timespec twice[2] = {*ts, *ts};
+
+                if (futimens(fileno(f), twice) < 0)
+                        return -errno;
+        }
+
         return fflush_and_check(f);
 }
 
@@ -89,7 +96,7 @@ static int write_string_file_atomic(const char *fn, const char *line, bool enfor
         return r;
 }
 
-int write_string_file(const char *fn, const char *line, WriteStringFileFlags flags) {
+int write_string_file_ts(const char *fn, const char *line, WriteStringFileFlags flags, struct timespec *ts) {
         _cleanup_fclose_ FILE *f = NULL;
         int q, r;
 
@@ -104,7 +111,8 @@ int write_string_file(const char *fn, const char *line, WriteStringFileFlags fla
                         goto fail;
 
                 return r;
-        }
+        } else
+                assert(ts == NULL);
 
         if (flags & WRITE_STRING_FILE_CREATE) {
                 f = fopen(fn, "we");
@@ -131,7 +139,7 @@ int write_string_file(const char *fn, const char *line, WriteStringFileFlags fla
                 }
         }
 
-        r = write_string_stream(f, line, !(flags & WRITE_STRING_FILE_AVOID_NEWLINE));
+        r = write_string_stream_ts(f, line, !(flags & WRITE_STRING_FILE_AVOID_NEWLINE), ts);
         if (r < 0)
                 goto fail;