fileio: tweak write_string_stream_ts() to write out trailing \n in one go even if buffering is off

This tweaks write_string_stream_ts() in one minor way: when stdio
buffering has been turned off, let's append the newline we shall append
to the buffer we write ourselves so that the kernel only gets one
syscall for the result. When buffering is enabled stdio will take care
of that anyway.

Follow-up for #7750.

src/basic/fileio.c

@@ -62,16 +62,28 @@ int write_string_stream_ts(
                 WriteStringFileFlags flags,
                 struct timespec *ts) {
 
+        bool needs_nl;
+
         assert(f);
         assert(line);
 
         if (ferror(f))
                 return -EIO;
 
+        needs_nl = !(flags & WRITE_STRING_FILE_AVOID_NEWLINE) && !endswith(line, "\n");
+
+        if (needs_nl && (flags & WRITE_STRING_FILE_DISABLE_BUFFER)) {
+                /* If STDIO buffering was disabled, then let's append the newline character to the string itself, so
+                 * that the write goes out in one go, instead of two */
+
+                line = strjoina(line, "\n");
+                needs_nl = false;
+        }
+
         if (fputs(line, f) == EOF)
                 return -errno;
 
-        if (!(flags & WRITE_STRING_FILE_AVOID_NEWLINE) && !endswith(line, "\n"))
+        if (needs_nl)
                 if (fputc('\n', f) == EOF)
                         return -errno;