From 91dc2bf74dee032512f20070f33d37abeca74f38 Mon Sep 17 00:00:00 2001
From: Lennart Poettering <lennart@poettering.net>
Date: Sat, 30 Dec 2017 15:42:03 +0100
Subject: [PATCH] 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 | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/src/basic/fileio.c b/src/basic/fileio.c
index 71c404bdd0..7d71a3e985 100644
--- a/src/basic/fileio.c
+++ b/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;