[PATCH v3 5/7] misc: syslog: Use fixed-sized buffer

[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.  The memstream is still used as fallback for
larger messages.

Checked on x86_64-linux-gnu.
---
 misc/syslog.c | 102 ++++++++++++++++++++++++++++++++------------------
 1 file changed, 65 insertions(+), 37 deletions(-)

diff --git a/misc/syslog.c b/misc/syslog.c
index e8b1dfe9b8..7852441615 100644
--- a/misc/syslog.c
+++ b/misc/syslog.c
@@ -123,12 +123,11 @@ void
 __vsyslog_internal(int pri, const char *fmt, va_list ap,
 		   unsigned int mode_flags)
 {
-  FILE *f;
-  char *buf = 0;
+  char *buf = NULL;
   size_t bufsize = 0;
+  bool buf_malloced = false;
   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. */
@@ -152,46 +151,75 @@ __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;
+
+  enum
+    {
+      timestamp_size = sizeof "MMM DD hh:mm:ss ",
+      bufs_size = 1024
+    };
+
+  /* "%h %e %H:%M:%S "  */
+  char timestamp[timestamp_size];
+  time_t now = time_now ();
+  struct tm now_tm;
+  __localtime_r (&now, &now_tm);
+  __strftime_l (timestamp, sizeof timestamp, "%h %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 ? "]" : ""
+
+  /* Try to use a static buffer as an optimization.  */
+  char bufs[bufs_size];
+  int l = __snprintf (bufs, sizeof bufs,
+                      SYSLOG_HEADER (pri, timestamp, &msgoff, pid));
+  if (l < sizeof (bufs))
     {
-      __fsetlocking (f, FSETLOCKING_BYCALLER);
-      /* "%h %e %H:%M:%S"  */
-      char timebuf[3+1               /* "%h "  */
-                   + 2+1             /* "%e "  */
-                   + 2+1 + 2+1 + 2+1 /* "%T"  */];
-      time_t now = time_now ();
-      struct tm now_tm;
-      __localtime_r (&now, &now_tm);
-      __strftime_l (timebuf, sizeof (timebuf), "%h %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);
+      int vl = __vsnprintf_internal (bufs + l, sizeof bufs - l, fmt, apc,
+                                     mode_flags);
+      if (l + vl < sizeof bufs)
+        {
+          buf = bufs;
+          bufsize = l + vl;
+        }
 
-      /* Close the memory stream; this will finalize the data into a malloc'd
-	 buffer in BUF.  */
-      fclose (f);
-
-      /* Tell the cancellation handler to free this buffer.  */
-      clarg.buf = buf;
+      va_end (apc);
     }
-  else
+
+  /* If the required size is larger than buffer size fallbacks to
+     open_memstream.  */
+  if (buf == NULL)
     {
-      /* We cannot get a stream.  There is not much we can do but emitting an
-         error messages.  */
-      bufsize = __snprintf (failbuf, sizeof failbuf, "out of memory[%d]",
-                            __getpid ());
-      buf = failbuf;
+      FILE *f = __open_memstream (&buf, &bufsize);
+      if (f != NULL)
+        {
+          __fsetlocking (f, FSETLOCKING_BYCALLER);
+          fprintf (f, SYSLOG_HEADER (pri, timestamp, &msgoff, pid));
+          /* Restore errno for %m format.  */
+          __set_errno (saved_errno);
+          __vfprintf_internal (f, fmt, ap, mode_flags);
+          fclose (f);
+
+          /* Tell the cancellation handler to free this buffer.  */
+          buf_malloced = true;
+          clarg.buf = buf;
+        }
+      else
+        {
+          bufsize = __snprintf (bufs, sizeof bufs,
+                                "out of memory[%d]", __getpid ());
+          buf = bufs;
+        }
     }
 
   /* Output to stderr if requested. */
@@ -243,7 +271,7 @@ __vsyslog_internal(int pri, const char *fmt, va_list ap,
   __libc_cleanup_pop (0);
   __libc_lock_unlock (syslog_lock);
 
-  if (buf != failbuf)
+  if (buf_malloced)
     free (buf);
 }
 
-- 
2.32.0