[libxml2] xmlcatalog: restore ability to query system catalog easily

[Nick Wellnhofer <nwellnhof src gnome org>]

commit 6b780f65d83cb952142fd1b211eeb99c7957da49
Author: Jan Pokorný <jpokorny redhat com>
Date:   Thu Oct 19 13:27:29 2017 +0200

    xmlcatalog: restore ability to query system catalog easily
    
    I've noticed that easy way of locating a DocBook XSLT in a configure
    script of another project doesn't work anymore.  It is using something
    like: xmlcatalog "" ${DOCBOOK_XSL_URI}/${DOCBOOK_XSL_PATH}.  The script
    is then forced to a plain, suboptimal search using find utility.
    Indeed, I retrospectively realize that the check was working just
    by mere accident given that the window this presumably side-effect
    was applicable had not lasted long, some 8 months between
    
      054c716ea1bf001544127a4ab4f4346d1b9947e7
    
    introducing xmlInitializeCatalog in the LIBXML_TEST_VERSION-rooted
    call chain, and
    
      f65128f38289d77ff322d63aef2858cc0a819c34
    
    reverting that again.
    
    So while one can state /etc/xml/catalog constant explicitly, in
    some use cases (such as the mentioned one -- that's why I wanted
    to omit stating the full path in the first place), this is rather
    an implementation detail, perhaps subject to change from that POV.
    Therefore I propose to restore that behaviour in the targeted
    manner without global disruptions this time around.
    
    As a side-effect, this fixes indenting of the affected part.

 xmlcatalog.c |   12 ++++++++++--
 1 files changed, 10 insertions(+), 2 deletions(-)
---
diff --git a/xmlcatalog.c b/xmlcatalog.c
index 006f0cc..c02b97f 100644
--- a/xmlcatalog.c
+++ b/xmlcatalog.c
@@ -312,7 +312,8 @@ static void usage(const char *name) {
     /* split into 2 printf's to avoid overly long string (gcc warning) */
     printf("\
 Usage : %s [options] catalogfile entities...\n\
-\tParse the catalog file and query it for the entities\n\
+\tParse the catalog file (void specification possibly expressed as \"\"\n\
+\tappoints the default system one) and query it for the entities\n\
 \t--sgml : handle SGML Super catalogs for --add and --del\n\
 \t--shell : run a shell allowing interactive queries\n\
 \t--create : create a new catalog\n\
@@ -408,11 +409,18 @@ int main(int argc, char **argv) {
            continue;
        } else if (argv[i][0] == '-')
            continue;
-       filename = argv[i];
+
+       if (filename == NULL && argv[i][0] == '\0') {
+           /* Interpret empty-string catalog specification as
+              a shortcut for a default system catalog. */
+           xmlInitializeCatalog();
+       } else {
+           filename = argv[i];
            ret = xmlLoadCatalog(argv[i]);
            if ((ret < 0) && (create)) {
                xmlCatalogAdd(BAD_CAST "catalog", BAD_CAST argv[i], NULL);
            }
+       }
        break;
     }