[xml] [PATCH v2 (fix msg)] xmlcatalog: restore ability to query system catalog easily

[Jan Pokorný <jpokorny redhat com>]

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 :-)

Signed-off-by: Jan Pokorný <jpokorny redhat com>
---
 xmlcatalog.c | 12 ++++++++++--
 1 file 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;
     }
 
-- 
2.4.11