New function '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.
(GnuTLS Cryptography, Format of GnuTLS Cryptography Inputs)
(Document Object Model): Fix indentation of the first paragraph.

* etc/NEWS (libxml-available-p): Mention libxml-available-p.

doc/lispref/text.texi

@@ -4536,9 +4536,9 @@ It should be somewhat more efficient on larger buffers than
 @cindex symmetric cipher
 @cindex cipher, symmetric
 
-If compiled with GnuTLS, Emacs offers built-in cryptographic support.
-Following the GnuTLS API terminology, the available tools are digests,
-MACs, symmetric ciphers, and AEAD ciphers.
+  If compiled with GnuTLS, Emacs offers built-in cryptographic
+support.  Following the GnuTLS API terminology, the available tools
+are digests, MACs, symmetric ciphers, and AEAD ciphers.
 
 The terms used herein, such as IV (Initialization Vector), require
 some familiarity with cryptography and will not be defined in detail.
@@ -4556,7 +4556,7 @@ structure of the GnuTLS library.
 @cindex format of gnutls cryptography inputs
 @cindex gnutls cryptography inputs format
 
-The inputs to GnuTLS cryptographic functions can be specified in
+  The inputs to GnuTLS cryptographic functions can be specified in
 several ways, both as primitive Emacs Lisp types or as lists.
 
 The list form is currently similar to how @code{md5} and
@@ -4723,8 +4723,15 @@ IV used.
 @section Parsing HTML and XML
 @cindex parsing html
 
-When Emacs is compiled with libxml2 support, the following functions
-are available to parse HTML or XML text into Lisp object trees.
+  Emacs can be compiled with built-in libxml2 support.
+
+@defun libxml-available-p
+This function returns non-@code{nil} if built-in libxml2 support is
+available in this Emacs session.
+@end defun
+
+When libxml2 support is available, the following functions can be used
+to parse HTML or XML text into Lisp object trees.
 
 @defun libxml-parse-html-region start end &optional base-url discard-comments
 This function parses the text between @var{start} and @var{end} as
@@ -4791,9 +4798,9 @@ about syntax).
 @cindex DOM
 @cindex Document Object Model
 
-The @acronym{DOM} returned by @code{libxml-parse-html-region} (and the
-other @acronym{XML} parsing functions) is a tree structure where each
-node has a node name (called a @dfn{tag}), and optional key/value
+  The @acronym{DOM} returned by @code{libxml-parse-html-region} (and
+the other @acronym{XML} parsing functions) is a tree structure where
+each node has a node name (called a @dfn{tag}), and optional key/value
 @dfn{attribute} list, and then a list of @dfn{child nodes}.  The child
 nodes are either strings or @acronym{DOM} objects.
 

etc/NEWS

@@ -39,6 +39,14 @@ text.
 +++
 ** New function 'logcount' calculates an integer's Hamming weight.
 
++++
+** New function 'libxml-available-p'.
+This function returns non-nil if libxml support is both compiled in
+and available at run time.  Lisp programs should use this function to
+detect built-in libxml support, instead of testing for that
+indirectly, e.g., by checking that functions like
+'libxml-parse-html-region' return nil.
+
 
 * Editing Changes in Emacs 27.1
 

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 ();

src/lisp.h

@@ -4391,9 +4391,9 @@ 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
 

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"
-
 
 #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 */
 
 
+
+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 */