[PATCH] Re: Adding xpath_exists function - Mailing list pgsql-hackers

Bruce Momjian wrote:
> I have added this to the next commit-fest:
>
>     https://commitfest.postgresql.org/action/commitfest_view?id=6
>
>
Thanks Bruce. Attached is a revised patch which changes the code
slightly such that it uses an older version of the libxml library. I've
added comments to the code so that we remember why we didn't use the
latest function.

Regards,

--
Mike Fowler
Registered Linux user: 379787



*** a/src/backend/utils/adt/xml.c
--- b/src/backend/utils/adt/xml.c
***************
*** 3495,3497 **** xpath(PG_FUNCTION_ARGS)
--- 3495,3681 ----
      return 0;
  #endif
  }
+
+ /*
+  * Determines if the node specified by the supplied XPath exists
+  * in a given XML document, returning a boolean.
+  *
+  * It is up to the user to ensure that the XML passed is in fact
+  * an XML document - XPath doesn't work easily on fragments without
+  * a context node being known.
+  */
+ Datum
+ xpath_exists(PG_FUNCTION_ARGS)
+ {
+ #ifdef USE_LIBXML
+     text       *xpath_expr_text = PG_GETARG_TEXT_P(0);
+     xmltype    *data = PG_GETARG_XML_P(1);
+     ArrayType  *namespaces = PG_GETARG_ARRAYTYPE_P(2);
+     ArrayBuildState *astate = NULL;
+     xmlParserCtxtPtr ctxt = NULL;
+     xmlDocPtr    doc = NULL;
+     xmlXPathContextPtr xpathctx = NULL;
+     xmlXPathCompExprPtr xpathcomp = NULL;
+     xmlXPathObjectPtr xpathobj = NULL;
+     char       *datastr;
+     int32        len;
+     int32        xpath_len;
+     xmlChar    *string;
+     xmlChar    *xpath_expr;
+     int            i;
+     int            res_nitems;
+     int            ndim;
+     Datum       *ns_names_uris;
+     bool       *ns_names_uris_nulls;
+     int            ns_count;
+     int            result;
+
+     /*
+      * Namespace mappings are passed as text[].  If an empty array is passed
+      * (ndim = 0, "0-dimensional"), then there are no namespace mappings.
+      * Else, a 2-dimensional array with length of the second axis being equal
+      * to 2 should be passed, i.e., every subarray contains 2 elements, the
+      * first element defining the name, the second one the URI.  Example:
+      * ARRAY[ARRAY['myns', 'http://example.com'], ARRAY['myns2',
+      * 'http://example2.com']].
+      */
+     ndim = ARR_NDIM(namespaces);
+     if (ndim != 0)
+     {
+         int           *dims;
+
+         dims = ARR_DIMS(namespaces);
+
+         if (ndim != 2 || dims[1] != 2)
+             ereport(ERROR,
+                     (errcode(ERRCODE_DATA_EXCEPTION),
+                      errmsg("invalid array for XML namespace mapping"),
+                      errdetail("The array must be two-dimensional with length of the second axis equal to 2.")));
+
+         Assert(ARR_ELEMTYPE(namespaces) == TEXTOID);
+
+         deconstruct_array(namespaces, TEXTOID, -1, false, 'i',
+                           &ns_names_uris, &ns_names_uris_nulls,
+                           &ns_count);
+
+         Assert((ns_count % 2) == 0);    /* checked above */
+         ns_count /= 2;            /* count pairs only */
+     }
+     else
+     {
+         ns_names_uris = NULL;
+         ns_names_uris_nulls = NULL;
+         ns_count = 0;
+     }
+
+     datastr = VARDATA(data);
+     len = VARSIZE(data) - VARHDRSZ;
+     xpath_len = VARSIZE(xpath_expr_text) - VARHDRSZ;
+     if (xpath_len == 0)
+         ereport(ERROR,
+                 (errcode(ERRCODE_DATA_EXCEPTION),
+                  errmsg("empty XPath expression")));
+
+     string = (xmlChar *) palloc((len + 1) * sizeof(xmlChar));
+     memcpy(string, datastr, len);
+     string[len] = '\0';
+
+     xpath_expr = (xmlChar *) palloc((xpath_len + 1) * sizeof(xmlChar));
+     memcpy(xpath_expr, VARDATA(xpath_expr_text), xpath_len);
+     xpath_expr[xpath_len] = '\0';
+
+     pg_xml_init();
+     xmlInitParser();
+
+     PG_TRY();
+     {
+         /*
+          * redundant XML parsing (two parsings for the same value during one
+          * command execution are possible)
+          */
+         ctxt = xmlNewParserCtxt();
+         if (ctxt == NULL)
+             xml_ereport(ERROR, ERRCODE_OUT_OF_MEMORY,
+                         "could not allocate parser context");
+         doc = xmlCtxtReadMemory(ctxt, (char *) string, len, NULL, NULL, 0);
+         if (doc == NULL)
+             xml_ereport(ERROR, ERRCODE_INVALID_XML_DOCUMENT,
+                         "could not parse XML document");
+         xpathctx = xmlXPathNewContext(doc);
+         if (xpathctx == NULL)
+             xml_ereport(ERROR, ERRCODE_OUT_OF_MEMORY,
+                         "could not allocate XPath context");
+         xpathctx->node = xmlDocGetRootElement(doc);
+         if (xpathctx->node == NULL)
+             xml_ereport(ERROR, ERRCODE_INTERNAL_ERROR,
+                         "could not find root XML element");
+
+         /* register namespaces, if any */
+         if (ns_count > 0)
+         {
+             for (i = 0; i < ns_count; i++)
+             {
+                 char       *ns_name;
+                 char       *ns_uri;
+
+                 if (ns_names_uris_nulls[i * 2] ||
+                     ns_names_uris_nulls[i * 2 + 1])
+                     ereport(ERROR,
+                             (errcode(ERRCODE_NULL_VALUE_NOT_ALLOWED),
+                       errmsg("neither namespace name nor URI may be null")));
+                 ns_name = TextDatumGetCString(ns_names_uris[i * 2]);
+                 ns_uri = TextDatumGetCString(ns_names_uris[i * 2 + 1]);
+                 if (xmlXPathRegisterNs(xpathctx,
+                                        (xmlChar *) ns_name,
+                                        (xmlChar *) ns_uri) != 0)
+                     ereport(ERROR,        /* is this an internal error??? */
+                             (errmsg("could not register XML namespace with name \"%s\" and URI \"%s\"",
+                                     ns_name, ns_uri)));
+             }
+         }
+
+         xpathcomp = xmlXPathCompile(xpath_expr);
+         if (xpathcomp == NULL)    /* TODO: show proper XPath error details */
+             xml_ereport(ERROR, ERRCODE_INTERNAL_ERROR,
+                         "invalid XPath expression");
+
+         /* Version 2.6.27 introduces a function named xmlXPathCompiledEvalToBoolean
+          * however we can derive the existence by whether any nodes are returned
+          * thereby preventing a library version upgrade */
+         xpathobj = xmlXPathCompiledEval(xpathcomp, xpathctx);
+         if (xpathobj == NULL)    /* TODO: reason? */
+             xml_ereport(ERROR, ERRCODE_INTERNAL_ERROR,
+                         "could not create XPath object");
+
+         if (xpathobj->nodesetval == NULL)
+             result = 0;
+         else
+             result = xpathobj->nodesetval->nodeNr;
+     }
+     PG_CATCH();
+     {
+         if (xpathobj)
+             xmlXPathFreeObject(xpathobj);
+         if (xpathcomp)
+             xmlXPathFreeCompExpr(xpathcomp);
+         if (xpathctx)
+             xmlXPathFreeContext(xpathctx);
+         if (doc)
+             xmlFreeDoc(doc);
+         if (ctxt)
+             xmlFreeParserCtxt(ctxt);
+         PG_RE_THROW();
+     }
+     PG_END_TRY();
+
+     xmlXPathFreeCompExpr(xpathcomp);
+     xmlXPathFreeContext(xpathctx);
+     xmlFreeDoc(doc);
+     xmlFreeParserCtxt(ctxt);
+
+     PG_RETURN_BOOL(result);
+ #else
+     NO_XML_SUPPORT();
+     return 0;
+ #endif
+ }
*** a/src/include/catalog/pg_proc.h
--- b/src/include/catalog/pg_proc.h
***************
*** 4385,4390 **** DESCR("evaluate XPath expression, with namespaces support");
--- 4385,4395 ----
  DATA(insert OID = 2932 (  xpath         PGNSP PGUID 14 1 0 0 f f f t f i 2 0 143 "25 142" _null_ _null_ _null_ _null_
"selectpg_catalog.xpath($1, $2, ''{}''::pg_catalog.text[])" _null_ _null_ _null_ )); 
  DESCR("evaluate XPath expression");

+ DATA(insert OID = 3037 (  xpath_exists     PGNSP PGUID 12 1 0 0 f f f t f i 3 0 16 "25 142 1009" _null_ _null_ _null_
_null_xpath_exists _null_ _null_ _null_ )); 
+ DESCR("evaluate XPath expression in a boolean context, with namespaces support");
+ DATA(insert OID = 3038 (  xpath_exists     PGNSP PGUID 14 1 0 0 f f f t f i 2 0 16 "25 142" _null_ _null_ _null_
_null_"select pg_catalog.xpath_exists($1, $2, ''{}''::pg_catalog.text[])" _null_ _null_ _null_ )); 
+ DESCR("evaluate XPath expression in a boolean context");
+
  /* uuid */
  DATA(insert OID = 2952 (  uuid_in           PGNSP PGUID 12 1 0 0 f f f t f i 1 0 2950 "2275" _null_ _null_ _null_
_null_uuid_in _null_ _null_ _null_ )); 
  DESCR("I/O");
*** a/src/include/utils/xml.h
--- b/src/include/utils/xml.h
***************
*** 37,42 **** extern Datum texttoxml(PG_FUNCTION_ARGS);
--- 37,43 ----
  extern Datum xmltotext(PG_FUNCTION_ARGS);
  extern Datum xmlvalidate(PG_FUNCTION_ARGS);
  extern Datum xpath(PG_FUNCTION_ARGS);
+ extern Datum xpath_exists(PG_FUNCTION_ARGS);

  extern Datum table_to_xml(PG_FUNCTION_ARGS);
  extern Datum query_to_xml(PG_FUNCTION_ARGS);
*** a/src/test/regress/expected/xml.out
--- b/src/test/regress/expected/xml.out
***************
*** 502,504 **** SELECT xpath('//b', '<a>one <b>two</b> three <b>etc</b></a>');
--- 502,545 ----
   {<b>two</b>,<b>etc</b>}
  (1 row)

+ -- Test xpath_exists evaluation
+ INSERT INTO xmltest VALUES (4,
'<menu><beers><name>Budvar</name><cost>free</cost><name>Carling</name><cost>lots</cost></beers></menu>'::xml);
+ INSERT INTO xmltest VALUES (5,
'<menu><beers><name>Molson</name><cost>free</cost><name>Carling</name><cost>lots</cost></beers></menu>'::xml);
+ INSERT INTO xmltest VALUES (6, '<myns:menu
xmlns:myns="http://myns.com"><myns:beers><myns:name>Budvar</myns:name><myns:cost>free</myns:cost><myns:name>Carling</myns:name><myns:cost>lots</myns:cost></myns:beers></myns:menu>'::xml);
+ INSERT INTO xmltest VALUES (7, '<myns:menu
xmlns:myns="http://myns.com"><myns:beers><myns:name>Molson</myns:name><myns:cost>free</myns:cost><myns:name>Carling</myns:name><myns:cost>lots</myns:cost></myns:beers></myns:menu>'::xml);
+ SELECT COUNT(id) FROM xmltest WHERE xpath_exists('/menu/beer',data);
+  count
+ -------
+      0
+ (1 row)
+
+ SELECT COUNT(id) FROM xmltest WHERE xpath_exists('/menu/beers',data);
+  count
+ -------
+      2
+ (1 row)
+
+ SELECT COUNT(id) FROM xmltest WHERE xpath_exists('/menu/beers/name[text() = ''Molson'']',data);
+  count
+ -------
+      1
+ (1 row)
+
+ SELECT COUNT(id) FROM xmltest WHERE xpath_exists('/myns:menu/myns:beer',data,ARRAY[ARRAY['myns','http://myns.com']]);
+  count
+ -------
+      0
+ (1 row)
+
+ SELECT COUNT(id) FROM xmltest WHERE
xpath_exists('/myns:menu/myns:beers',data,ARRAY[ARRAY['myns','http://myns.com']]);
+  count
+ -------
+      2
+ (1 row)
+
+ SELECT COUNT(id) FROM xmltest WHERE xpath_exists('/myns:menu/myns:beers/myns:name[text() =
''Molson'']',data,ARRAY[ARRAY['myns','http://myns.com']]);
+  count
+ -------
+      1
+ (1 row)
+
*** a/src/test/regress/sql/xml.sql
--- b/src/test/regress/sql/xml.sql
***************
*** 163,165 **** SELECT xpath('', '<!-- error -->');
--- 163,178 ----
  SELECT xpath('//text()', '<local:data xmlns:local="http://127.0.0.1"><local:piece id="1">number
one</local:piece><local:pieceid="2" /></local:data>'); 
  SELECT xpath('//loc:piece/@id', '<local:data xmlns:local="http://127.0.0.1"><local:piece id="1">number
one</local:piece><local:pieceid="2" /></local:data>', ARRAY[ARRAY['loc', 'http://127.0.0.1']]); 
  SELECT xpath('//b', '<a>one <b>two</b> three <b>etc</b></a>');
+
+ -- Test xpath_exists evaluation
+ INSERT INTO xmltest VALUES (4,
'<menu><beers><name>Budvar</name><cost>free</cost><name>Carling</name><cost>lots</cost></beers></menu>'::xml);
+ INSERT INTO xmltest VALUES (5,
'<menu><beers><name>Molson</name><cost>free</cost><name>Carling</name><cost>lots</cost></beers></menu>'::xml);
+ INSERT INTO xmltest VALUES (6, '<myns:menu
xmlns:myns="http://myns.com"><myns:beers><myns:name>Budvar</myns:name><myns:cost>free</myns:cost><myns:name>Carling</myns:name><myns:cost>lots</myns:cost></myns:beers></myns:menu>'::xml);
+ INSERT INTO xmltest VALUES (7, '<myns:menu
xmlns:myns="http://myns.com"><myns:beers><myns:name>Molson</myns:name><myns:cost>free</myns:cost><myns:name>Carling</myns:name><myns:cost>lots</myns:cost></myns:beers></myns:menu>'::xml);
+
+ SELECT COUNT(id) FROM xmltest WHERE xpath_exists('/menu/beer',data);
+ SELECT COUNT(id) FROM xmltest WHERE xpath_exists('/menu/beers',data);
+ SELECT COUNT(id) FROM xmltest WHERE xpath_exists('/menu/beers/name[text() = ''Molson'']',data);
+ SELECT COUNT(id) FROM xmltest WHERE xpath_exists('/myns:menu/myns:beer',data,ARRAY[ARRAY['myns','http://myns.com']]);
+ SELECT COUNT(id) FROM xmltest WHERE
xpath_exists('/myns:menu/myns:beers',data,ARRAY[ARRAY['myns','http://myns.com']]);
+ SELECT COUNT(id) FROM xmltest WHERE xpath_exists('/myns:menu/myns:beers/myns:name[text() =
''Molson'']',data,ARRAY[ARRAY['myns','http://myns.com']]);