[glibc/zack/y2038-preliminaries] misc: Use allocate_once in getmntent

[Zack Weinberg <zack at sourceware dot org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=35ffd20dbd76d3cb6b478c7a69bb40d8c827ed81

commit 35ffd20dbd76d3cb6b478c7a69bb40d8c827ed81
Author: Florian Weimer <fweimer@redhat.com>
Date:   Wed Aug 28 12:01:48 2019 +0200

    misc: Use allocate_once in getmntent
    
    Both the buffer and struct mntent are now allocated on the heap.
    This results in a slight reduction of RSS usage.
    
    Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>

Diff:
---
 ChangeLog     |  8 ++++++++
 misc/mntent.c | 37 +++++++++++++++++++++----------------
 2 files changed, 29 insertions(+), 16 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index a0c6a0e..37c2ff3 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,13 @@
 2019-08-28  Florian Weimer  <fweimer@redhat.com>
 
+	* misc/mntent.c (struct mntent_buffer): Define.
+	(mntent_buffer): Adjust type to void *.
+	(allocate): Adjust for allocate_once.
+	(deallocate): New function.
+	(getmntent): Call allocate_once.
+
+2019-08-28  Florian Weimer  <fweimer@redhat.com>
+
 	nptl: Move pthread_attr_setdetachstate implementation into libc.
 	* nptl/Makefile (routines): Add pthread_attr_setdetachstate.
 	(libpthread-routines): Remove pthread_attr_setdetachstate.
diff --git a/misc/mntent.c b/misc/mntent.c
index 980ea40..8fae606 100644
--- a/misc/mntent.c
+++ b/misc/mntent.c
@@ -18,36 +18,41 @@
 
 #include <mntent.h>
 #include <stdlib.h>
-#include <libc-lock.h>
+#include <allocate_once.h>
 
-/* We don't want to allocate the static buffer all the time since it
-   is not always used (in fact, rather infrequently).  Accept the
-   extra cost of a `malloc'.  */
-libc_freeres_ptr (static char *getmntent_buffer);
+struct mntent_buffer
+{
+  struct mntent m;
+  char buffer[4096];
+};
 
-/* This is the size of the buffer.  This is really big.  */
-#define BUFFER_SIZE	4096
+/* We don't want to allocate the static buffer all the time since it
+   is not always used (in fact, rather infrequently).  */
+libc_freeres_ptr (static void *mntent_buffer);
 
+static void *
+allocate (void *closure)
+{
+  return malloc (sizeof (struct mntent_buffer));
+}
 
 static void
-allocate (void)
+deallocate (void *closure, void *ptr)
 {
-  getmntent_buffer = (char *) malloc (BUFFER_SIZE);
+  free (ptr);
 }
 
-
 struct mntent *
 getmntent (FILE *stream)
 {
-  static struct mntent m;
-  __libc_once_define (static, once);
-  __libc_once (once, allocate);
-
-  if (getmntent_buffer == NULL)
+  struct mntent_buffer *buffer = allocate_once (&mntent_buffer,
+						allocate, deallocate, NULL);
+  if (buffer == NULL)
     /* If no core is available we don't have a chance to run the
        program successfully and so returning NULL is an acceptable
        result.  */
     return NULL;
 
-  return __getmntent_r (stream, &m, getmntent_buffer, BUFFER_SIZE);
+  return __getmntent_r (stream, &buffer->m,
+			buffer->buffer, sizeof (buffer->buffer));
 }