Re: What's the right way to detect libxml2?

[Robert Pluim <rpluim@gmail.com>]

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Eli Zaretskii <eliz@gnu.org> writes:

> This is okay for the master branch, but please also add a NEWS entry
> about the new function, and the fact that from now on the detection of
> libxml2 availability should use that.

Attached, as I don't have commit privileges, and I don't know whether
my copyright assignment has finished being processed yet.

Regards

Robert


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From 7793e1feeac2714abee2bdf76ef7170e86821895 Mon Sep 17 00:00:00 2001
From: Robert Pluim <rpluim@gmail.com>
Date: Tue, 24 Oct 2017 19:01:28 +0200
Subject: [PATCH] Provide libxml-available-p

* src/emacs.c (main): Call syms_of_xml unconditionally

* src/lisp.h: Provide syms_of_xml prototype even when
HAVE_LIBXML2 is not defined

* src/xml.c (Flibxml_available_p): New function, cloned from
Fgnutls_available_p
(syms_of_xml): Provide Slibxml_available_p

* doc/lispref/text.texi (Parsing HTML/XML): Document libxml-available-p

* etc/NEWS (libxml-available-p): Document it
---
 doc/lispref/text.texi |  5 +++++
 etc/NEWS              |  5 +++++
 src/emacs.c           |  2 --
 src/lisp.h            |  2 +-
 src/xml.c             | 37 ++++++++++++++++++++++++++++++++-----
 5 files changed, 43 insertions(+), 8 deletions(-)

diff --git a/doc/lispref/text.texi b/doc/lispref/text.texi
index baa3c708e9..565d098069 100644
--- a/doc/lispref/text.texi
+++ b/doc/lispref/text.texi
@@ -4723,6 +4723,11 @@ Parsing HTML/XML
 @section Parsing HTML and XML
 @cindex parsing html
 
+@defun libxml-available-p
+This function returns non-@code{nil} if built-in libxml2 support is
+available.
+@end defun
+
 When Emacs is compiled with libxml2 support, the following functions
 are available to parse HTML or XML text into Lisp object trees.
 
diff --git a/etc/NEWS b/etc/NEWS
index 9ae36bdb03..dfe1dcd0d9 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -39,6 +39,11 @@ text.
 +++
 ** New function 'logcount' calculates an integer's Hamming weight.
 
++++
+** New function 'libxml-available-p' returns true if libxml support is
+   both compiled in and available.  It is defined unconditionally and
+   can thus be used to detect libxml support.
+
 \f
 * Editing Changes in Emacs 27.1
 
diff --git a/src/emacs.c b/src/emacs.c
index 0fe7d9113b..808abcd9aa 100644
--- a/src/emacs.c
+++ b/src/emacs.c
@@ -1542,9 +1542,7 @@ Using an Emacs configured with --with-x-toolkit=lucid does not have this problem
 #endif
 #endif /* HAVE_X_WINDOWS */
 
-#ifdef HAVE_LIBXML2
       syms_of_xml ();
-#endif
 
 #ifdef HAVE_LCMS2
       syms_of_lcms2 ();
diff --git a/src/lisp.h b/src/lisp.h
index 266370333f..cc8d90cbf1 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -4399,9 +4399,9 @@ extern void syms_of_xterm (void);
 extern char *x_get_keysym_name (int);
 #endif /* HAVE_WINDOW_SYSTEM */
 
-#ifdef HAVE_LIBXML2
 /* Defined in xml.c.  */
 extern void syms_of_xml (void);
+#ifdef HAVE_LIBXML2
 extern void xml_cleanup_parser (void);
 #endif
 
diff --git a/src/xml.c b/src/xml.c
index d087a34a5e..7afaa63c42 100644
--- a/src/xml.c
+++ b/src/xml.c
@@ -18,15 +18,15 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 
 #include <config.h>
 
+#include "lisp.h"
+#include "buffer.h"
+
 #ifdef HAVE_LIBXML2
 
 #include <libxml/tree.h>
 #include <libxml/parser.h>
 #include <libxml/HTMLparser.h>
 
-#include "lisp.h"
-#include "buffer.h"
-
 \f
 #ifdef WINDOWSNT
 
@@ -291,16 +291,43 @@ If DISCARD-COMMENTS is non-nil, all HTML comments are discarded. */)
     return parse_region (start, end, base_url, discard_comments, false);
   return Qnil;
 }
+#endif /* HAVE_LIBXML2 */
 
 \f
+
+DEFUN ("libxml-available-p", Flibxml_available_p, Slibxml_available_p, 0, 0, 0,
+       doc: /* Return t if libxml2 support is available in this instance of Emacs.*/)
+  (void)
+{
+#ifdef HAVE_LIBXML2
+# ifdef WINDOWSNT
+  Lisp_Object found = Fassq (Qlibxml2, Vlibrary_cache);
+  if (CONSP (found))
+    return XCDR (found);
+  else
+    {
+      Lisp_Object status;
+      status = init_libxml2_functions () ? Qt : Qnil;
+      Vlibrary_cache = Fcons (Fcons (Qlibxml2, status), Vlibrary_cache);
+      return status;
+    }
+# else
+  return Qt;
+# endif /* WINDOWSNT */
+#else
+  return Qnil;
+#endif	/* HAVE_LIBXML2 */
+}
+
 /***********************************************************************
 			    Initialization
  ***********************************************************************/
 void
 syms_of_xml (void)
 {
+#ifdef HAVE_LIBXML2
   defsubr (&Slibxml_parse_html_region);
   defsubr (&Slibxml_parse_xml_region);
+#endif
+  defsubr (&Slibxml_available_p);
 }
-
-#endif /* HAVE_LIBXML2 */
-- 
2.15.0.rc1