xpath.c

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/*
 * contrib/xml2/xpath.c
 *
 * Parser interface for DOM-based parser (libxml) rather than
 * stream-based SAX-type parser
 */
#include "postgres.h"
 
#include "access/htup_details.h"
#include "executor/spi.h"
#include "fmgr.h"
#include "funcapi.h"
#include "lib/stringinfo.h"
#include "utils/builtins.h"
#include "utils/xml.h"
 
/* libxml includes */
 
#include <libxml/xpath.h>
#include <libxml/tree.h>
#include <libxml/xmlmemory.h>
#include <libxml/xmlerror.h>
#include <libxml/parserInternals.h>
 
PG_MODULE_MAGIC_EXT(
                    .name = "xml2",
                    .version = PG_VERSION
);
 
/* exported for use by xslt_proc.c */
 
PgXmlErrorContext *pgxml_parser_init(PgXmlStrictness strictness);
 
/* workspace for pgxml_xpath() */
 
typedef struct
{
    xmlDocPtr   doctree;
    xmlXPathContextPtr ctxt;
    xmlXPathObjectPtr res;
} xpath_workspace;
 
/* local declarations */
 
static xmlChar *pgxmlNodeSetToText(xmlNodeSetPtr nodeset,
                                   xmlChar *toptagname, xmlChar *septagname,
                                   xmlChar *plainsep);
 
static text *pgxml_result_to_text(xmlXPathObjectPtr res, xmlChar *toptag,
                                  xmlChar *septag, xmlChar *plainsep);
 
static xmlChar *pgxml_texttoxmlchar(text *textstring);
 
static xpath_workspace *pgxml_xpath(text *document, xmlChar *xpath,
                                    PgXmlErrorContext *xmlerrcxt);
 
static void cleanup_workspace(xpath_workspace *workspace);
 
 
/*
 * Initialize for xml parsing.
 *
 * As with the underlying pg_xml_init function, calls to this MUST be followed
 * by a PG_TRY block that guarantees that pg_xml_done is called.
 */
PgXmlErrorContext *
pgxml_parser_init(PgXmlStrictness strictness)
{
    PgXmlErrorContext *xmlerrcxt;
 
    /* Set up error handling (we share the core's error handler) */
    xmlerrcxt = pg_xml_init(strictness);
 
    /* Note: we're assuming an elog cannot be thrown by the following calls */
 
    /* Initialize libxml */
    xmlInitParser();
 
    return xmlerrcxt;
}
 
 
/* Encodes special characters (<, >, &, " and \r) as XML entities */
 
PG_FUNCTION_INFO_V1(xml_encode_special_chars);
 
Datum
xml_encode_special_chars(PG_FUNCTION_ARGS)
{
    text       *tin = PG_GETARG_TEXT_PP(0);
    text       *volatile tout = NULL;
    xmlChar    *volatile tt = NULL;
    PgXmlErrorContext *xmlerrcxt;
 
    xmlerrcxt = pg_xml_init(PG_XML_STRICTNESS_ALL);
 
    PG_TRY();
    {
        xmlChar    *ts;
 
        ts = pgxml_texttoxmlchar(tin);
 
        tt = xmlEncodeSpecialChars(NULL, ts);
        if (tt == NULL || pg_xml_error_occurred(xmlerrcxt))
            xml_ereport(xmlerrcxt, ERROR, ERRCODE_OUT_OF_MEMORY,
                        "could not allocate xmlChar");
        pfree(ts);
 
        tout = cstring_to_text((char *) tt);
    }
    PG_CATCH();
    {
        if (tt != NULL)
            xmlFree(tt);
 
        pg_xml_done(xmlerrcxt, true);
 
        PG_RE_THROW();
    }
    PG_END_TRY();
 
    if (tt != NULL)
        xmlFree(tt);
 
    pg_xml_done(xmlerrcxt, false);
 
    PG_RETURN_TEXT_P(tout);
}
 
/*
 * Function translates a nodeset into a text representation
 *
 * iterates over each node in the set and calls xmlNodeDump to write it to
 * an xmlBuffer -from which an xmlChar * string is returned.
 *
 * each representation is surrounded by <tagname> ... </tagname>
 *
 * plainsep is an ordinary (not tag) separator - if used, then nodes are
 * cast to string as output method
 */
static xmlChar *
pgxmlNodeSetToText(xmlNodeSetPtr nodeset,
                   xmlChar *toptagname,
                   xmlChar *septagname,
                   xmlChar *plainsep)
{
    volatile xmlBufferPtr buf = NULL;
    xmlChar    *volatile result = NULL;
    PgXmlErrorContext *xmlerrcxt;
 
    /* spin up some error handling */
    xmlerrcxt = pg_xml_init(PG_XML_STRICTNESS_ALL);
 
    PG_TRY();
    {
        buf = xmlBufferCreate();
 
        if (buf == NULL || pg_xml_error_occurred(xmlerrcxt))
            xml_ereport(xmlerrcxt, ERROR, ERRCODE_OUT_OF_MEMORY,
                        "could not allocate xmlBuffer");
 
        if ((toptagname != NULL) && (xmlStrlen(toptagname) > 0))
        {
            xmlBufferWriteChar(buf, "<");
            xmlBufferWriteCHAR(buf, toptagname);
            xmlBufferWriteChar(buf, ">");
        }
        if (nodeset != NULL)
        {
            for (int i = 0; i < nodeset->nodeNr; i++)
            {
                if (plainsep != NULL)
                {
                    xmlBufferWriteCHAR(buf,
                                       xmlXPathCastNodeToString(nodeset->nodeTab[i]));
 
                    /* If this isn't the last entry, write the plain sep. */
                    if (i < (nodeset->nodeNr) - 1)
                        xmlBufferWriteChar(buf, (char *) plainsep);
                }
                else
                {
                    if ((septagname != NULL) && (xmlStrlen(septagname) > 0))
                    {
                        xmlBufferWriteChar(buf, "<");
                        xmlBufferWriteCHAR(buf, septagname);
                        xmlBufferWriteChar(buf, ">");
                    }
                    xmlNodeDump(buf,
                                nodeset->nodeTab[i]->doc,
                                nodeset->nodeTab[i],
                                1, 0);
 
                    if ((septagname != NULL) && (xmlStrlen(septagname) > 0))
                    {
                        xmlBufferWriteChar(buf, "</");
                        xmlBufferWriteCHAR(buf, septagname);
                        xmlBufferWriteChar(buf, ">");
                    }
                }
            }
        }
 
        if ((toptagname != NULL) && (xmlStrlen(toptagname) > 0))
        {
            xmlBufferWriteChar(buf, "</");
            xmlBufferWriteCHAR(buf, toptagname);
            xmlBufferWriteChar(buf, ">");
        }
 
        result = xmlStrdup(xmlBufferContent(buf));
        if (result == NULL || pg_xml_error_occurred(xmlerrcxt))
            xml_ereport(xmlerrcxt, ERROR, ERRCODE_OUT_OF_MEMORY,
                        "could not allocate result");
    }
    PG_CATCH();
    {
        if (buf)
            xmlBufferFree(buf);
 
        pg_xml_done(xmlerrcxt, true);
 
        PG_RE_THROW();
    }
    PG_END_TRY();
 
    xmlBufferFree(buf);
    pg_xml_done(xmlerrcxt, false);
 
    return result;
}
 
 
/* Translate a PostgreSQL "varlena" -i.e. a variable length parameter
 * into the libxml2 representation
 */
static xmlChar *
pgxml_texttoxmlchar(text *textstring)
{
    return (xmlChar *) text_to_cstring(textstring);
}
 
/* Publicly visible XPath functions */
 
/*
 * This is a "raw" xpath function. Check that it returns child elements
 * properly
 */
PG_FUNCTION_INFO_V1(xpath_nodeset);
 
Datum
xpath_nodeset(PG_FUNCTION_ARGS)
{
    text       *document = PG_GETARG_TEXT_PP(0);
    text       *xpathsupp = PG_GETARG_TEXT_PP(1);   /* XPath expression */
    xmlChar    *toptag = pgxml_texttoxmlchar(PG_GETARG_TEXT_PP(2));
    xmlChar    *septag = pgxml_texttoxmlchar(PG_GETARG_TEXT_PP(3));
    xmlChar    *xpath;
    text       *volatile xpres = NULL;
    xpath_workspace *volatile workspace = NULL;
    PgXmlErrorContext *xmlerrcxt;
 
    xpath = pgxml_texttoxmlchar(xpathsupp);
    xmlerrcxt = pgxml_parser_init(PG_XML_STRICTNESS_LEGACY);
 
    PG_TRY();
    {
        workspace = pgxml_xpath(document, xpath, xmlerrcxt);
        xpres = pgxml_result_to_text(workspace->res, toptag, septag, NULL);
    }
    PG_CATCH();
    {
        if (workspace)
            cleanup_workspace(workspace);
 
        pg_xml_done(xmlerrcxt, true);
        PG_RE_THROW();
    }
    PG_END_TRY();
 
    cleanup_workspace(workspace);
    pg_xml_done(xmlerrcxt, false);
 
    pfree(xpath);
 
    if (xpres == NULL)
        PG_RETURN_NULL();
    PG_RETURN_TEXT_P(xpres);
}
 
/*
 * The following function is almost identical, but returns the elements in
 * a list.
 */
PG_FUNCTION_INFO_V1(xpath_list);
 
Datum
xpath_list(PG_FUNCTION_ARGS)
{
    text       *document = PG_GETARG_TEXT_PP(0);
    text       *xpathsupp = PG_GETARG_TEXT_PP(1);   /* XPath expression */
    xmlChar    *plainsep = pgxml_texttoxmlchar(PG_GETARG_TEXT_PP(2));
    xmlChar    *xpath;
    text       *volatile xpres = NULL;
    xpath_workspace *volatile workspace = NULL;
    PgXmlErrorContext *xmlerrcxt;
 
    xpath = pgxml_texttoxmlchar(xpathsupp);
    xmlerrcxt = pgxml_parser_init(PG_XML_STRICTNESS_LEGACY);
 
    PG_TRY();
    {
        workspace = pgxml_xpath(document, xpath, xmlerrcxt);
        xpres = pgxml_result_to_text(workspace->res, NULL, NULL, plainsep);
    }
    PG_CATCH();
    {
        if (workspace)
            cleanup_workspace(workspace);
 
        pg_xml_done(xmlerrcxt, true);
        PG_RE_THROW();
    }
    PG_END_TRY();
 
    cleanup_workspace(workspace);
    pg_xml_done(xmlerrcxt, false);
 
    pfree(xpath);
 
    if (xpres == NULL)
        PG_RETURN_NULL();
    PG_RETURN_TEXT_P(xpres);
}
 
 
PG_FUNCTION_INFO_V1(xpath_string);
 
Datum
xpath_string(PG_FUNCTION_ARGS)
{
    text       *document = PG_GETARG_TEXT_PP(0);
    text       *xpathsupp = PG_GETARG_TEXT_PP(1);   /* XPath expression */
    xmlChar    *xpath;
    int32       pathsize;
    text       *volatile xpres = NULL;
    xpath_workspace *volatile workspace = NULL;
    PgXmlErrorContext *xmlerrcxt;
 
    pathsize = VARSIZE_ANY_EXHDR(xpathsupp);
 
    /*
     * We encapsulate the supplied path with "string()" = 8 chars + 1 for NUL
     * at end
     */
    /* We could try casting to string using the libxml function? */
 
    xpath = (xmlChar *) palloc(pathsize + 9);
    memcpy(xpath, "string(", 7);
    memcpy(xpath + 7, VARDATA_ANY(xpathsupp), pathsize);
    xpath[pathsize + 7] = ')';
    xpath[pathsize + 8] = '\0';
 
    xmlerrcxt = pgxml_parser_init(PG_XML_STRICTNESS_LEGACY);
 
    PG_TRY();
    {
        workspace = pgxml_xpath(document, xpath, xmlerrcxt);
        xpres = pgxml_result_to_text(workspace->res, NULL, NULL, NULL);
    }
    PG_CATCH();
    {
        if (workspace)
            cleanup_workspace(workspace);
 
        pg_xml_done(xmlerrcxt, true);
        PG_RE_THROW();
    }
    PG_END_TRY();
 
    cleanup_workspace(workspace);
    pg_xml_done(xmlerrcxt, false);
 
    pfree(xpath);
 
    if (xpres == NULL)
        PG_RETURN_NULL();
    PG_RETURN_TEXT_P(xpres);
}
 
 
PG_FUNCTION_INFO_V1(xpath_number);
 
Datum
xpath_number(PG_FUNCTION_ARGS)
{
    text       *document = PG_GETARG_TEXT_PP(0);
    text       *xpathsupp = PG_GETARG_TEXT_PP(1);   /* XPath expression */
    xmlChar    *xpath;
    volatile float4 fRes = 0.0;
    volatile bool isNull = false;
    xpath_workspace *volatile workspace = NULL;
    PgXmlErrorContext *xmlerrcxt;
 
    xpath = pgxml_texttoxmlchar(xpathsupp);
    xmlerrcxt = pgxml_parser_init(PG_XML_STRICTNESS_LEGACY);
 
    PG_TRY();
    {
        workspace = pgxml_xpath(document, xpath, xmlerrcxt);
        pfree(xpath);
 
        if (workspace->res == NULL)
            isNull = true;
        else
            fRes = xmlXPathCastToNumber(workspace->res);
    }
    PG_CATCH();
    {
        if (workspace)
            cleanup_workspace(workspace);
 
        pg_xml_done(xmlerrcxt, true);
        PG_RE_THROW();
    }
    PG_END_TRY();
 
    cleanup_workspace(workspace);
    pg_xml_done(xmlerrcxt, false);
 
    if (isNull || xmlXPathIsNaN(fRes))
        PG_RETURN_NULL();
 
    PG_RETURN_FLOAT4(fRes);
}
 
 
PG_FUNCTION_INFO_V1(xpath_bool);
 
Datum
xpath_bool(PG_FUNCTION_ARGS)
{
    text       *document = PG_GETARG_TEXT_PP(0);
    text       *xpathsupp = PG_GETARG_TEXT_PP(1);   /* XPath expression */
    xmlChar    *xpath;
    volatile int bRes = 0;
    xpath_workspace *volatile workspace = NULL;
    PgXmlErrorContext *xmlerrcxt;
 
    xpath = pgxml_texttoxmlchar(xpathsupp);
    xmlerrcxt = pgxml_parser_init(PG_XML_STRICTNESS_LEGACY);
 
    PG_TRY();
    {
        workspace = pgxml_xpath(document, xpath, xmlerrcxt);
        pfree(xpath);
 
        if (workspace->res == NULL)
            bRes = 0;
        else
            bRes = xmlXPathCastToBoolean(workspace->res);
    }
    PG_CATCH();
    {
        if (workspace)
            cleanup_workspace(workspace);
 
        pg_xml_done(xmlerrcxt, true);
        PG_RE_THROW();
    }
    PG_END_TRY();
 
    cleanup_workspace(workspace);
    pg_xml_done(xmlerrcxt, false);
 
    PG_RETURN_BOOL(bRes);
}
 
 
 
/* Core function to evaluate XPath query */
 
static xpath_workspace *
pgxml_xpath(text *document, xmlChar *xpath, PgXmlErrorContext *xmlerrcxt)
{
    int32       docsize = VARSIZE_ANY_EXHDR(document);
    xmlXPathCompExprPtr comppath;
    xpath_workspace *workspace = palloc0_object(xpath_workspace);
 
    workspace->doctree = NULL;
    workspace->ctxt = NULL;
    workspace->res = NULL;
 
    workspace->doctree = xmlReadMemory((char *) VARDATA_ANY(document),
                                       docsize, NULL, NULL,
                                       XML_PARSE_NOENT);
    if (workspace->doctree != NULL)
    {
        workspace->ctxt = xmlXPathNewContext(workspace->doctree);
        workspace->ctxt->node = xmlDocGetRootElement(workspace->doctree);
 
        /* compile the path */
        comppath = xmlXPathCtxtCompile(workspace->ctxt, xpath);
        if (comppath == NULL || pg_xml_error_occurred(xmlerrcxt))
            xml_ereport(xmlerrcxt, ERROR, ERRCODE_INVALID_ARGUMENT_FOR_XQUERY,
                        "XPath Syntax Error");
 
        /* Now evaluate the path expression. */
        workspace->res = xmlXPathCompiledEval(comppath, workspace->ctxt);
 
        xmlXPathFreeCompExpr(comppath);
    }
 
    return workspace;
}
 
/* Clean up after processing the result of pgxml_xpath() */
static void
cleanup_workspace(xpath_workspace *workspace)
{
    if (workspace->res)
        xmlXPathFreeObject(workspace->res);
    workspace->res = NULL;
    if (workspace->ctxt)
        xmlXPathFreeContext(workspace->ctxt);
    workspace->ctxt = NULL;
    if (workspace->doctree)
        xmlFreeDoc(workspace->doctree);
    workspace->doctree = NULL;
}
 
static text *
pgxml_result_to_text(xmlXPathObjectPtr res,
                     xmlChar *toptag,
                     xmlChar *septag,
                     xmlChar *plainsep)
{
    xmlChar    *volatile xpresstr = NULL;
    text       *volatile xpres = NULL;
    PgXmlErrorContext *xmlerrcxt;
 
    if (res == NULL)
        return NULL;
 
    /* spin some error handling */
    xmlerrcxt = pg_xml_init(PG_XML_STRICTNESS_ALL);
 
    PG_TRY();
    {
        switch (res->type)
        {
            case XPATH_NODESET:
                xpresstr = pgxmlNodeSetToText(res->nodesetval,
                                              toptag,
                                              septag, plainsep);
                break;
 
            case XPATH_STRING:
                xpresstr = xmlStrdup(res->stringval);
                if (xpresstr == NULL || pg_xml_error_occurred(xmlerrcxt))
                    xml_ereport(xmlerrcxt, ERROR, ERRCODE_OUT_OF_MEMORY,
                                "could not allocate result");
                break;
 
            default:
                elog(NOTICE, "unsupported XQuery result: %d", res->type);
                xpresstr = xmlStrdup((const xmlChar *) "<unsupported/>");
                if (xpresstr == NULL || pg_xml_error_occurred(xmlerrcxt))
                    xml_ereport(xmlerrcxt, ERROR, ERRCODE_OUT_OF_MEMORY,
                                "could not allocate result");
        }
 
        /* Now convert this result back to text */
        xpres = cstring_to_text((char *) xpresstr);
    }
    PG_CATCH();
    {
        if (xpresstr != NULL)
            xmlFree(xpresstr);
 
        pg_xml_done(xmlerrcxt, true);
 
        PG_RE_THROW();
    }
    PG_END_TRY();
 
    /* Free various storage */
    xmlFree(xpresstr);
 
    pg_xml_done(xmlerrcxt, false);
 
    return xpres;
}
 
/*
 * xpath_table is a table function. It needs some tidying (as do the
 * other functions here!
 */
PG_FUNCTION_INFO_V1(xpath_table);
 
Datum
xpath_table(PG_FUNCTION_ARGS)
{
    /* Function parameters */
    char       *pkeyfield = text_to_cstring(PG_GETARG_TEXT_PP(0));
    char       *xmlfield = text_to_cstring(PG_GETARG_TEXT_PP(1));
    char       *relname = text_to_cstring(PG_GETARG_TEXT_PP(2));
    char       *xpathset = text_to_cstring(PG_GETARG_TEXT_PP(3));
    char       *condition = text_to_cstring(PG_GETARG_TEXT_PP(4));
 
    /* SPI (input tuple) support */
    SPITupleTable *tuptable;
    HeapTuple   spi_tuple;
    TupleDesc   spi_tupdesc;
 
 
    ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
    AttInMetadata *attinmeta;
 
    char      **values;
    xmlChar   **xpaths;
    char       *pos;
    const char *pathsep = "|";
 
    int         numpaths;
    int         ret;
    uint64      proc;
    int         j;
    int         rownr;          /* For issuing multiple rows from one original
                                 * document */
    bool        had_values;     /* To determine end of nodeset results */
    StringInfoData query_buf;
    PgXmlErrorContext *xmlerrcxt;
    volatile xmlDocPtr doctree = NULL;
 
    InitMaterializedSRF(fcinfo, MAT_SRF_USE_EXPECTED_DESC);
 
    /* must have at least one output column (for the pkey) */
    if (rsinfo->setDesc->natts < 1)
        ereport(ERROR,
                (errcode(ERRCODE_SYNTAX_ERROR),
                 errmsg("xpath_table must have at least one output column")));
 
    /*
     * At the moment we assume that the returned attributes make sense for the
     * XPath specified (i.e. we trust the caller). It's not fatal if they get
     * it wrong - the input function for the column type will raise an error
     * if the path result can't be converted into the correct binary
     * representation.
     */
 
    attinmeta = TupleDescGetAttInMetadata(rsinfo->setDesc);
 
    values = (char **) palloc(rsinfo->setDesc->natts * sizeof(char *));
    xpaths = (xmlChar **) palloc(rsinfo->setDesc->natts * sizeof(xmlChar *));
 
    /*
     * Split XPaths. xpathset is a writable CString.
     *
     * Note that we stop splitting once we've done all needed for tupdesc
     */
    numpaths = 0;
    pos = xpathset;
    while (numpaths < (rsinfo->setDesc->natts - 1))
    {
        xpaths[numpaths++] = (xmlChar *) pos;
        pos = strstr(pos, pathsep);
        if (pos != NULL)
        {
            *pos = '\0';
            pos++;
        }
        else
            break;
    }
 
    /* Now build query */
    initStringInfo(&query_buf);
 
    /* Build initial sql statement */
    appendStringInfo(&query_buf, "SELECT %s, %s FROM %s WHERE %s",
                     pkeyfield,
                     xmlfield,
                     relname,
                     condition);
 
    SPI_connect();
 
    if ((ret = SPI_exec(query_buf.data, 0)) != SPI_OK_SELECT)
        elog(ERROR, "xpath_table: SPI execution failed for query %s",
             query_buf.data);
 
    proc = SPI_processed;
    tuptable = SPI_tuptable;
    spi_tupdesc = tuptable->tupdesc;
 
    /*
     * Check that SPI returned correct result. If you put a comma into one of
     * the function parameters, this will catch it when the SPI query returns
     * e.g. 3 columns.
     */
    if (spi_tupdesc->natts != 2)
    {
        ereport(ERROR, (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                        errmsg("expression returning multiple columns is not valid in parameter list"),
                        errdetail("Expected two columns in SPI result, got %d.", spi_tupdesc->natts)));
    }
 
    /*
     * Setup the parser.  This should happen after we are done evaluating the
     * query, in case it calls functions that set up libxml differently.
     */
    xmlerrcxt = pgxml_parser_init(PG_XML_STRICTNESS_LEGACY);
 
    PG_TRY();
    {
        /* For each row i.e. document returned from SPI */
        uint64      i;
 
        for (i = 0; i < proc; i++)
        {
            char       *pkey;
            char       *xmldoc;
            xmlXPathContextPtr ctxt;
            xmlXPathObjectPtr res;
            xmlChar    *resstr;
            xmlXPathCompExprPtr comppath;
            HeapTuple   ret_tuple;
 
            /* Extract the row data as C Strings */
            spi_tuple = tuptable->vals[i];
            pkey = SPI_getvalue(spi_tuple, spi_tupdesc, 1);
            xmldoc = SPI_getvalue(spi_tuple, spi_tupdesc, 2);
 
            /*
             * Clear the values array, so that not-well-formed documents
             * return NULL in all columns.  Note that this also means that
             * spare columns will be NULL.
             */
            for (j = 0; j < rsinfo->setDesc->natts; j++)
                values[j] = NULL;
 
            /* Insert primary key */
            values[0] = pkey;
 
            /* Parse the document */
            if (xmldoc)
                doctree = xmlReadMemory(xmldoc, strlen(xmldoc),
                                        NULL, NULL,
                                        XML_PARSE_NOENT);
            else                /* treat NULL as not well-formed */
                doctree = NULL;
 
            if (doctree == NULL)
            {
                /* not well-formed, so output all-NULL tuple */
                ret_tuple = BuildTupleFromCStrings(attinmeta, values);
                tuplestore_puttuple(rsinfo->setResult, ret_tuple);
                heap_freetuple(ret_tuple);
            }
            else
            {
                /* New loop here - we have to deal with nodeset results */
                rownr = 0;
 
                do
                {
                    /* Now evaluate the set of xpaths. */
                    had_values = false;
                    for (j = 0; j < numpaths; j++)
                    {
                        ctxt = xmlXPathNewContext(doctree);
                        if (ctxt == NULL || pg_xml_error_occurred(xmlerrcxt))
                            xml_ereport(xmlerrcxt,
                                        ERROR, ERRCODE_OUT_OF_MEMORY,
                                        "could not allocate XPath context");
 
                        ctxt->node = xmlDocGetRootElement(doctree);
 
                        /* compile the path */
                        comppath = xmlXPathCtxtCompile(ctxt, xpaths[j]);
                        if (comppath == NULL || pg_xml_error_occurred(xmlerrcxt))
                            xml_ereport(xmlerrcxt, ERROR,
                                        ERRCODE_INVALID_ARGUMENT_FOR_XQUERY,
                                        "XPath Syntax Error");
 
                        /* Now evaluate the path expression. */
                        res = xmlXPathCompiledEval(comppath, ctxt);
                        xmlXPathFreeCompExpr(comppath);
 
                        if (res != NULL)
                        {
                            switch (res->type)
                            {
                                case XPATH_NODESET:
                                    /* We see if this nodeset has enough nodes */
                                    if (res->nodesetval != NULL &&
                                        rownr < res->nodesetval->nodeNr)
                                    {
                                        resstr = xmlXPathCastNodeToString(res->nodesetval->nodeTab[rownr]);
                                        if (resstr == NULL || pg_xml_error_occurred(xmlerrcxt))
                                            xml_ereport(xmlerrcxt,
                                                        ERROR, ERRCODE_OUT_OF_MEMORY,
                                                        "could not allocate result");
                                        had_values = true;
                                    }
                                    else
                                        resstr = NULL;
 
                                    break;
 
                                case XPATH_STRING:
                                    resstr = xmlStrdup(res->stringval);
                                    if (resstr == NULL || pg_xml_error_occurred(xmlerrcxt))
                                        xml_ereport(xmlerrcxt,
                                                    ERROR, ERRCODE_OUT_OF_MEMORY,
                                                    "could not allocate result");
                                    break;
 
                                default:
                                    elog(NOTICE, "unsupported XQuery result: %d", res->type);
                                    resstr = xmlStrdup((const xmlChar *) "<unsupported/>");
                                    if (resstr == NULL || pg_xml_error_occurred(xmlerrcxt))
                                        xml_ereport(xmlerrcxt,
                                                    ERROR, ERRCODE_OUT_OF_MEMORY,
                                                    "could not allocate result");
                            }
 
                            /*
                             * Insert this into the appropriate column in the
                             * result tuple.
                             */
                            values[j + 1] = (char *) resstr;
                        }
                        xmlXPathFreeContext(ctxt);
                    }
 
                    /* Now add the tuple to the output, if there is one. */
                    if (had_values)
                    {
                        ret_tuple = BuildTupleFromCStrings(attinmeta, values);
                        tuplestore_puttuple(rsinfo->setResult, ret_tuple);
                        heap_freetuple(ret_tuple);
                    }
 
                    rownr++;
                } while (had_values);
            }
 
            if (doctree != NULL)
                xmlFreeDoc(doctree);
            doctree = NULL;
 
            if (pkey)
                pfree(pkey);
            if (xmldoc)
                pfree(xmldoc);
        }
    }
    PG_CATCH();
    {
        if (doctree != NULL)
            xmlFreeDoc(doctree);
 
        pg_xml_done(xmlerrcxt, true);
 
        PG_RE_THROW();
    }
    PG_END_TRY();
 
    if (doctree != NULL)
        xmlFreeDoc(doctree);
 
    pg_xml_done(xmlerrcxt, false);
 
    SPI_finish();
 
    /*
     * SFRM_Materialize mode expects us to return a NULL Datum. The actual
     * tuples are in our tuplestore and passed back through rsinfo->setResult.
     * rsinfo->setDesc is set to the tuple description that we actually used
     * to build our tuples with, so the caller can verify we did what it was
     * expecting.
     */
    return (Datum) 0;
}