[PATCH v4 5/7] misc: syslog: Use fixed-sized buffer and remove memstream

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

A fixed-sized buffer is used instead of memstream for messages up to
1024 bytes to avoid the potential BUFSIZ (8K) malloc and free for
each syslog call.

Also, since the buffer size is know, memstream is replaced with a
malloced buffer for larger messages.

Checked on x86_64-linux-gnu.
---
 misc/syslog.c | 86 +++++++++++++++++++++++++++++++--------------------
 1 file changed, 52 insertions(+), 34 deletions(-)

diff --git a/misc/syslog.c b/misc/syslog.c
index 20f46a76a2..b184b15eea 100644
--- a/misc/syslog.c
+++ b/misc/syslog.c
@@ -123,12 +123,12 @@ void
 __vsyslog_internal(int pri, const char *fmt, va_list ap,
 		   unsigned int mode_flags)
 {
-  FILE *f;
-  char *buf = 0;
+  /* Try to use a static buffer as an optimization.  */
+  char bufs[1024];
+  char *buf = NULL;
   size_t bufsize = 0;
   int msgoff;
   int saved_errno = errno;
-  char failbuf[3 * sizeof (pid_t) + sizeof "out of memory []"];
 
 #define	INTERNALLOG LOG_ERR|LOG_CONS|LOG_PERROR|LOG_PID
   /* Check for invalid bits. */
@@ -152,43 +152,61 @@ __vsyslog_internal(int pri, const char *fmt, va_list ap,
   if ((pri & LOG_FACMASK) == 0)
     pri |= LogFacility;
 
-  /* Build the message in a memory-buffer stream.  */
-  f = __open_memstream (&buf, &bufsize);
-  if (f != NULL)
+  pid_t pid = LogStat & LOG_PID ? __getpid () : 0;
+
+  /* "%b %e %H:%M:%S "  */
+  char timestamp[sizeof "MMM DD hh:mm:ss "];
+  time_t now = time_now ();
+  struct tm now_tm;
+  __localtime_r (&now, &now_tm);
+  __strftime_l (timestamp, sizeof timestamp, "%b %e %T ", &now_tm,
+                _nl_C_locobj_ptr);
+
+#define SYSLOG_HEADER(__pri, __timestamp, __msgoff, pid) \
+  "<%d>%s %n%s%s%.0d%s: ",                               \
+  __pri, __timestamp, __msgoff,                          \
+  LogTag == NULL ? __progname : LogTag,                  \
+  "[" + (pid == 0), pid, "]" + (pid == 0)
+
+  int l = __snprintf (bufs, sizeof bufs,
+                      SYSLOG_HEADER (pri, timestamp, &msgoff, pid));
+  if (0 <= l && l < sizeof bufs)
     {
-      __fsetlocking (f, FSETLOCKING_BYCALLER);
-      /* "%b %e %H:%M:%S"  */
-      char timebuf[sizeof "MMM DD hh:mm:ss "];
-      time_t now = time_now ();
-      struct tm now_tm;
-      __localtime_r (&now, &now_tm);
-      __strftime_l (timebuf, sizeof (timebuf), "%b %e %T", &now_tm,
-		    _nl_C_locobj_ptr);
-
-      pid_t pid = LogStat & LOG_PID ? __getpid () : 0;
-
-      fprintf (f, "<%d>%s %n%s%s%.0d%s: ", pri, timebuf, &msgoff,
-               LogTag == NULL ? __progname : LogTag,
-               pid != 0 ? "[" : "", pid, pid != 0 ? "]" : "");
+      va_list apc;
+      va_copy (apc, ap);
+
       /* Restore errno for %m format.  */
       __set_errno (saved_errno);
 
-      /* We have the header.  Print the user's format into the buffer.  */
-      __vfprintf_internal (f, fmt, ap, mode_flags);
-
-      /* Close the memory stream; this will finalize the data into a malloc'd
-	 buffer in BUF.  */
-      fclose (f);
+      int vl = __vsnprintf_internal (bufs + l, sizeof bufs - l, fmt, apc,
+                                     mode_flags);
+      if (0 <= vl && vl < sizeof bufs - l)
+        {
+          buf = bufs;
+          bufsize = l + vl;
+        }
 
-      /* Tell the cancellation handler to free this buffer.  */
-      clarg.buf = buf;
+      va_end (apc);
     }
-  else
+
+  if (buf == NULL)
     {
-      /* Nothing much to do but emit an error message.  */
-      bufsize = __snprintf (failbuf, sizeof failbuf, "out of memory[%d]",
-                            __getpid ());
-      buf = failbuf;
+      buf = malloc (l * sizeof (char));
+      if (buf != NULL)
+	{
+	  /* Tell the cancellation handler to free this buffer.  */
+	  clarg.buf = buf;
+
+	  __snprintf (buf, sizeof buf,
+		      SYSLOG_HEADER (pri, timestamp, &msgoff, pid));
+	}
+      else
+        {
+	  /* Nothing much to do but emit an error message.  */
+          bufsize = __snprintf (bufs, sizeof bufs,
+                                "out of memory[%d]", __getpid ());
+          buf = bufs;
+        }
     }
 
   /* Output to stderr if requested. */
@@ -240,7 +258,7 @@ __vsyslog_internal(int pri, const char *fmt, va_list ap,
   __libc_cleanup_pop (0);
   __libc_lock_unlock (syslog_lock);
 
-  if (buf != failbuf)
+  if (buf != bufs)
     free (buf);
 }
 
-- 
2.32.0