Subject: Re: bug#39529: 28.0.50; Metahelp does not contain help text

From: Paul Eggert <[email-masked]>
Date: Sat, 15 Feb 2020 15:25:44 -0800

On 2/9/20 11:34 AM, Eli Zaretskii wrote:

> I suspect the sxhash changes on Jan 7.  This problem started happening
> in that day's build.

Right you are. The problem was Emacs was using a hash table to unify identical 
Lisp compiled objects when purecopying them, even though their doc strings were 
not set up yet (and so were 0 placeholders that always compared equal). Formerly 
this bug was masked because sxhash simply returned the objects' hashed 
addresses, but the January 7 changes unveiled the bug. I install the attached 
patch to fix the problem.
>From 3480071dfab30eaca7f1d014600b864d2ea22f62 Mon Sep 17 00:00:00 2001
From: Paul Eggert <[email-masked]>
Date: Sat, 15 Feb 2020 15:12:34 -0800
Subject: [PATCH] Fix C-h C-h bug due to mutating a hash key
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Problem reported by Federico Tedin (Bug#39529).
The problem was that dumping uses a hash table based on 'equal'
when purecopying compiled objects, but then modifies the compiled
objects while they are keys in the table.  This no-no was uncovered
by the sxhash fixes in 2020-01-07T19:23:[email-masked].
Eli Zaretski pinpointed the patch that triggered the bug.
* src/lread.c (read1): When reading a compiled object, replace
its docstring with a unique negative integer instead of with 0,
so that purecopy doesn’t unify it with some other compiled object
that happens to have the same Lisp code.
---
 src/lread.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/src/lread.c b/src/lread.c
index c124d5a1d8..f39e81ae2c 100644
--- a/src/lread.c
+++ b/src/lread.c
@@ -2974,6 +2974,21 @@ read1 (Lisp_Object readcharfun, int *pch, bool first_in_list)
 	  vec = XVECTOR (tmp);
 	  if (vec->header.size == 0)
 	    invalid_syntax ("Empty byte-code object");
+
+	  if (COMPILED_DOC_STRING < vec->header.size
+	      && AREF (tmp, COMPILED_DOC_STRING) == make_fixnum (0))
+	    {
+	      /* read_list found a docstring like '(#$ . 5521)' and treated it
+		 as 0.  This placeholder 0 would lead to accidental sharing in
+		 purecopy's hash-consing, so replace it with a (hopefully)
+		 unique integer placeholder, which is negative so that it is
+		 not confused with a DOC file offset.  Eventually
+		 Snarf-documentation should replace the placeholder with the
+		 actual docstring.  */
+	      EMACS_UINT hash = XHASH (tmp) | (INTMASK - INTMASK / 2);
+	      ASET (tmp, COMPILED_DOC_STRING, make_ufixnum (hash));
+	    }
+
 	  make_byte_code (vec);
 	  return tmp;
 	}
-- 
2.17.1