bug#31852: Make memory-limit obsolete

[Paul Eggert <eggert@cs.ucla.edu>]

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Tags: patch

The memory-limit function has an implementation that is a relic of 
long-ago days when the heap was allocated via sbrk. This assumption is 
often no longer true now that ASLR is popular, and once we get portable 
dumping work it'll be true even less often. Since memory-limit returns 
nonsense so often and since nobody cares, we can and should mark 
memory-limit as obsolete. On its way out we can move it to a Lisp 
implementation and cause it to be at least somewhat more plausible.

Proposed patches attached. The first patch moves memory-limit to Lisp, 
the second one marks it obsolete. I didn't know where to put the Lisp 
implementation so I put it into lisp/subr.el.


[-- Attachment #2: 0001-Rewrite-memory-limit-in-Lisp.patch --]
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From 52f4585c1353c3c12079bc7bfe85ef44755134db Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Fri, 15 Jun 2018 18:05:28 -0700
Subject: [PATCH 1/2] Rewrite memory-limit in Lisp

Have it return Emacs virtual memory size, not the sbrk value
which is often useless newadays.
* doc/lispref/internals.texi (Garbage Collection):
* etc/NEWS: Document this.
* lisp/subr.el (memory-limit): New implementation in Lisp,
written in terms of process-attributes, and which returns
virtual memory size.
* src/alloc.c (Fmemory_limit): Remove C implementation.
---
 doc/lispref/internals.texi |  6 ++----
 etc/NEWS                   |  3 +++
 lisp/subr.el               |  4 ++++
 src/alloc.c                | 19 -------------------
 4 files changed, 9 insertions(+), 23 deletions(-)

diff --git a/doc/lispref/internals.texi b/doc/lispref/internals.texi
index 9cf1a4f9a3..faaf26f4f7 100644
--- a/doc/lispref/internals.texi
+++ b/doc/lispref/internals.texi
@@ -507,10 +507,8 @@ Garbage Collection
 memory Emacs is currently using.
 
 @defun memory-limit
-This function returns the address of the last byte Emacs has allocated,
-divided by 1024.  We divide the value by 1024 to make sure it fits in a
-Lisp integer.
-
+This function returns an estimate of the total amount of bytes of
+virtual memory that Emacs is currently using, divided by 1024.
 You can use this to get a general idea of how your actions affect the
 memory usage.
 @end defun
diff --git a/etc/NEWS b/etc/NEWS
index e89402db13..30409a89b5 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -656,6 +656,9 @@ socket has been passed to Emacs (Bug#24218).
 instead of just Microsoft platforms.  This fixes a 'get-free-disk-space'
 bug on OS X 10.8 and later (Bug#28639).
 
++++
+** 'memory-limit' now returns a better estimate of memory consumption.
+
 +++
 ** New macro 'combine-change-calls' arranges to call the change hooks
 ('before-change-functions' and 'after-change-functions') just once
diff --git a/lisp/subr.el b/lisp/subr.el
index 914112ccef..e4da2b2075 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -2182,6 +2182,10 @@ process-put
   (set-process-plist process
 		     (plist-put (process-plist process) propname value)))
 
+(defun memory-limit ()
+  "Return an estimate of Emacs virtual memory usage, divided by 1024."
+  (or (cdr (assq 'vsize (process-attributes (emacs-pid)))) 0))
+
 \f
 ;;;; Input and display facilities.
 
diff --git a/src/alloc.c b/src/alloc.c
index 7b2140501e..52620a54ac 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -7120,24 +7120,6 @@ or memory information can't be obtained, return nil.  */)
 
 /* Debugging aids.  */
 
-DEFUN ("memory-limit", Fmemory_limit, Smemory_limit, 0, 0, 0,
-       doc: /* Return the address of the last byte Emacs has allocated, divided by 1024.
-This may be helpful in debugging Emacs's memory usage.
-We divide the value by 1024 to make sure it fits in a Lisp integer.  */)
-  (void)
-{
-  Lisp_Object end;
-
-#if defined HAVE_NS || defined __APPLE__ || !HAVE_SBRK
-  /* Avoid warning.  sbrk has no relation to memory allocated anyway.  */
-  XSETINT (end, 0);
-#else
-  XSETINT (end, (intptr_t) (char *) sbrk (0) / 1024);
-#endif
-
-  return end;
-}
-
 DEFUN ("memory-use-counts", Fmemory_use_counts, Smemory_use_counts, 0, 0, 0,
        doc: /* Return a list of counters that measure how much consing there has been.
 Each of these counters increments for a certain kind of object.
@@ -7495,7 +7477,6 @@ The time is in seconds as a floating point value.  */);
   defsubr (&Smake_finalizer);
   defsubr (&Spurecopy);
   defsubr (&Sgarbage_collect);
-  defsubr (&Smemory_limit);
   defsubr (&Smemory_info);
   defsubr (&Smemory_use_counts);
   defsubr (&Ssuspicious_object);
-- 
2.17.1


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From 5645d547959780e21fe7c5eff12a0d0ae22702da Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Fri, 15 Jun 2018 18:21:10 -0700
Subject: [PATCH 2/2] Obsolete memory-limit

* doc/lispref/internals.texi (Garbage Collection):
Remove documentation of memory-limit.
* etc/NEWS: Say it's obsolete.
* lisp/subr.el (memory-limit): Declare that it's obsolete.
---
 doc/lispref/internals.texi | 7 -------
 etc/NEWS                   | 3 ++-
 lisp/subr.el               | 1 +
 3 files changed, 3 insertions(+), 8 deletions(-)

diff --git a/doc/lispref/internals.texi b/doc/lispref/internals.texi
index faaf26f4f7..e8fe26856c 100644
--- a/doc/lispref/internals.texi
+++ b/doc/lispref/internals.texi
@@ -506,13 +506,6 @@ Garbage Collection
 function @code{memory-limit} provides information on the total amount of
 memory Emacs is currently using.
 
-@defun memory-limit
-This function returns an estimate of the total amount of bytes of
-virtual memory that Emacs is currently using, divided by 1024.
-You can use this to get a general idea of how your actions affect the
-memory usage.
-@end defun
-
 @defvar memory-full
 This variable is @code{t} if Emacs is nearly out of memory for Lisp
 objects, and @code{nil} otherwise.
diff --git a/etc/NEWS b/etc/NEWS
index 30409a89b5..a1f853e8cd 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -657,7 +657,8 @@ instead of just Microsoft platforms.  This fixes a 'get-free-disk-space'
 bug on OS X 10.8 and later (Bug#28639).
 
 +++
-** 'memory-limit' now returns a better estimate of memory consumption.
+** The 'memory-limit' function is now obsolete.  You can use
+the 'vsize' component of (process-attributes (emacs-pid)) instead.
 
 +++
 ** New macro 'combine-change-calls' arranges to call the change hooks
diff --git a/lisp/subr.el b/lisp/subr.el
index e4da2b2075..6af32c95bc 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -2184,6 +2184,7 @@ process-put
 
 (defun memory-limit ()
   "Return an estimate of Emacs virtual memory usage, divided by 1024."
+  (declare (obsolete process-attributes "27.1"))
   (or (cdr (assq 'vsize (process-attributes (emacs-pid)))) 0))
 
 \f
-- 
2.17.1