Fix significant memory leak in contrib/xml2 functions.

Most of the functions that execute XPath queries leaked the data structures
created by libxml2.  This memory would not be recovered until end of
session, so it mounts up pretty quickly in any serious use of the feature.
Per report from Pavel Stehule, though this isn't his patch.

Back-patch to all supported branches.

contrib/xml2/xpath.c

@@ -35,6 +35,15 @@ Datum		xpath_table(PG_FUNCTION_ARGS);
 
 void		pgxml_parser_init(void);
 
+/* workspace for pgxml_xpath() */
+
+typedef struct
+{
+	xmlDocPtr	doctree;
+	xmlXPathContextPtr ctxt;
+	xmlXPathObjectPtr res;
+} xpath_workspace;
+
 /* local declarations */
 
 static xmlChar *pgxmlNodeSetToText(xmlNodeSetPtr nodeset,
@@ -46,7 +55,10 @@ static text *pgxml_result_to_text(xmlXPathObjectPtr res, xmlChar *toptag,
 
 static xmlChar *pgxml_texttoxmlchar(text *textstring);
 
-static xmlXPathObjectPtr pgxml_xpath(text *document, xmlChar *xpath);
+static xmlXPathObjectPtr pgxml_xpath(text *document, xmlChar *xpath,
+									 xpath_workspace *workspace);
+
+static void cleanup_workspace(xpath_workspace *workspace);
 
 #define GET_STR(textp) DatumGetCString(DirectFunctionCall1(textout, PointerGetDatum(textp)))
 
@@ -222,25 +234,22 @@ PG_FUNCTION_INFO_V1(xpath_nodeset);
 Datum
 xpath_nodeset(PG_FUNCTION_ARGS)
 {
-	xmlChar    *xpath,
-			   *toptag,
-			   *septag;
-	int32		pathsize;
-	text	   *xpathsupp,
-			   *xpres;
-
-	/* PG_GETARG_TEXT_P(0) is document buffer */
-	xpathsupp = PG_GETARG_TEXT_P(1);	/* XPath expression */
+	text	   *document = PG_GETARG_TEXT_P(0);
+	text	   *xpathsupp = PG_GETARG_TEXT_P(1);	/* XPath expression */
+	xmlChar    *toptag = pgxml_texttoxmlchar(PG_GETARG_TEXT_P(2));
+	xmlChar    *septag = pgxml_texttoxmlchar(PG_GETARG_TEXT_P(3));
+	xmlChar    *xpath;
+	text	   *xpres;
+	xmlXPathObjectPtr res;
+	xpath_workspace workspace;
 
-	toptag = pgxml_texttoxmlchar(PG_GETARG_TEXT_P(2));
-	septag = pgxml_texttoxmlchar(PG_GETARG_TEXT_P(3));
+	xpath = pgxml_texttoxmlchar(xpathsupp);
 
-	pathsize = VARSIZE(xpathsupp) - VARHDRSZ;
+	res = pgxml_xpath(document, xpath, &workspace);
 
-	xpath = pgxml_texttoxmlchar(xpathsupp);
+	xpres = pgxml_result_to_text(res, toptag, septag, NULL);
 
-	xpres = pgxml_result_to_text(pgxml_xpath(PG_GETARG_TEXT_P(0), xpath),
-								 toptag, septag, NULL);
+	cleanup_workspace(&workspace);
 
 	pfree(xpath);
 
@@ -258,23 +267,21 @@ PG_FUNCTION_INFO_V1(xpath_list);
 Datum
 xpath_list(PG_FUNCTION_ARGS)
 {
-	xmlChar    *xpath,
-			   *plainsep;
-	int32		pathsize;
-	text	   *xpathsupp,
-			   *xpres;
-
-	/* PG_GETARG_TEXT_P(0) is document buffer */
-	xpathsupp = PG_GETARG_TEXT_P(1);	/* XPath expression */
+	text	   *document = PG_GETARG_TEXT_P(0);
+	text	   *xpathsupp = PG_GETARG_TEXT_P(1);	/* XPath expression */
+	xmlChar    *plainsep = pgxml_texttoxmlchar(PG_GETARG_TEXT_P(2));
+	xmlChar    *xpath;
+	text	   *xpres;
+	xmlXPathObjectPtr res;
+	xpath_workspace workspace;
 
-	plainsep = pgxml_texttoxmlchar(PG_GETARG_TEXT_P(2));
+	xpath = pgxml_texttoxmlchar(xpathsupp);
 
-	pathsize = VARSIZE(xpathsupp) - VARHDRSZ;
+	res = pgxml_xpath(document, xpath, &workspace);
 
-	xpath = pgxml_texttoxmlchar(xpathsupp);
+	xpres = pgxml_result_to_text(res, NULL, NULL, plainsep);
 
-	xpres = pgxml_result_to_text(pgxml_xpath(PG_GETARG_TEXT_P(0), xpath),
-								 NULL, NULL, plainsep);
+	cleanup_workspace(&workspace);
 
 	pfree(xpath);
 
@@ -289,13 +296,13 @@ PG_FUNCTION_INFO_V1(xpath_string);
 Datum
 xpath_string(PG_FUNCTION_ARGS)
 {
+	text	   *document = PG_GETARG_TEXT_P(0);
+	text	   *xpathsupp = PG_GETARG_TEXT_P(1);	/* XPath expression */
 	xmlChar    *xpath;
 	int32		pathsize;
-	text	   *xpathsupp,
-			   *xpres;
-
-	/* PG_GETARG_TEXT_P(0) is document buffer */
-	xpathsupp = PG_GETARG_TEXT_P(1);	/* XPath expression */
+	text	   *xpres;
+	xmlXPathObjectPtr res;
+	xpath_workspace workspace;
 
 	pathsize = VARSIZE(xpathsupp) - VARHDRSZ;
 
@@ -306,13 +313,16 @@ xpath_string(PG_FUNCTION_ARGS)
 	/* We could try casting to string using the libxml function? */
 
 	xpath = (xmlChar *) palloc(pathsize + 9);
-	memcpy((char *) (xpath + 7), VARDATA(xpathsupp), pathsize);
 	strncpy((char *) xpath, "string(", 7);
+	memcpy((char *) (xpath + 7), VARDATA(xpathsupp), pathsize);
 	xpath[pathsize + 7] = ')';
 	xpath[pathsize + 8] = '\0';
 
-	xpres = pgxml_result_to_text(pgxml_xpath(PG_GETARG_TEXT_P(0), xpath),
-								 NULL, NULL, NULL);
+	res = pgxml_xpath(document, xpath, &workspace);
+
+	xpres = pgxml_result_to_text(res, NULL, NULL, NULL);
+
+	cleanup_workspace(&workspace);
 
 	pfree(xpath);
 
@@ -327,21 +337,17 @@ PG_FUNCTION_INFO_V1(xpath_number);
 Datum
 xpath_number(PG_FUNCTION_ARGS)
 {
+	text	   *document = PG_GETARG_TEXT_P(0);
+	text	   *xpathsupp = PG_GETARG_TEXT_P(1);	/* XPath expression */
 	xmlChar    *xpath;
-	int32		pathsize;
-	text	   *xpathsupp;
 	float4		fRes;
-
 	xmlXPathObjectPtr res;
-
-	/* PG_GETARG_TEXT_P(0) is document buffer */
-	xpathsupp = PG_GETARG_TEXT_P(1);	/* XPath expression */
-
-	pathsize = VARSIZE(xpathsupp) - VARHDRSZ;
+	xpath_workspace workspace;
 
 	xpath = pgxml_texttoxmlchar(xpathsupp);
 
-	res = pgxml_xpath(PG_GETARG_TEXT_P(0), xpath);
+	res = pgxml_xpath(document, xpath, &workspace);
+
 	pfree(xpath);
 
 	if (res == NULL)
@@ -349,6 +355,8 @@ xpath_number(PG_FUNCTION_ARGS)
 
 	fRes = xmlXPathCastToNumber(res);
 
+	cleanup_workspace(&workspace);
+
 	if (xmlXPathIsNaN(fRes))
 		PG_RETURN_NULL();
 
@@ -361,21 +369,17 @@ PG_FUNCTION_INFO_V1(xpath_bool);
 Datum
 xpath_bool(PG_FUNCTION_ARGS)
 {
+	text	   *document = PG_GETARG_TEXT_P(0);
+	text	   *xpathsupp = PG_GETARG_TEXT_P(1);	/* XPath expression */
 	xmlChar    *xpath;
-	int32		pathsize;
-	text	   *xpathsupp;
 	int			bRes;
-
 	xmlXPathObjectPtr res;
-
-	/* PG_GETARG_TEXT_P(0) is document buffer */
-	xpathsupp = PG_GETARG_TEXT_P(1);	/* XPath expression */
-
-	pathsize = VARSIZE(xpathsupp) - VARHDRSZ;
+	xpath_workspace workspace;
 
 	xpath = pgxml_texttoxmlchar(xpathsupp);
 
-	res = pgxml_xpath(PG_GETARG_TEXT_P(0), xpath);
+	res = pgxml_xpath(document, xpath, &workspace);
+
 	pfree(xpath);
 
 	if (res == NULL)
@@ -383,6 +387,8 @@ xpath_bool(PG_FUNCTION_ARGS)
 
 	bRes = xmlXPathCastToBoolean(res);
 
+	cleanup_workspace(&workspace);
+
 	PG_RETURN_BOOL(bRes);
 }
 
@@ -391,49 +397,61 @@ xpath_bool(PG_FUNCTION_ARGS)
 /* Core function to evaluate XPath query */
 
 static xmlXPathObjectPtr
-pgxml_xpath(text *document, xmlChar *xpath)
+pgxml_xpath(text *document, xmlChar *xpath, xpath_workspace *workspace)
 {
-	xmlDocPtr	doctree;
-	xmlXPathContextPtr ctxt;
+	int32		docsize = VARSIZE(document) - VARHDRSZ;
 	xmlXPathObjectPtr res;
 	xmlXPathCompExprPtr comppath;
-	int32		docsize;
 
-	docsize = VARSIZE(document) - VARHDRSZ;
+	workspace->doctree = NULL;
+	workspace->ctxt = NULL;
+	workspace->res = NULL;
 
 	pgxml_parser_init();
 
-	doctree = xmlParseMemory((char *) VARDATA(document), docsize);
-	if (doctree == NULL)
+	workspace->doctree = xmlParseMemory((char *) VARDATA(document), docsize);
+	if (workspace->doctree == NULL)
 		return NULL;			/* not well-formed */
 
-	ctxt = xmlXPathNewContext(doctree);
-	ctxt->node = xmlDocGetRootElement(doctree);
+	workspace->ctxt = xmlXPathNewContext(workspace->doctree);
+	workspace->ctxt->node = xmlDocGetRootElement(workspace->doctree);
 
 	/* compile the path */
 	comppath = xmlXPathCompile(xpath);
 	if (comppath == NULL)
 	{
-		xmlFreeDoc(doctree);
+		cleanup_workspace(workspace);
 		xml_ereport(ERROR, ERRCODE_EXTERNAL_ROUTINE_EXCEPTION,
 					"XPath Syntax Error");
 	}
 
 	/* Now evaluate the path expression. */
-	res = xmlXPathCompiledEval(comppath, ctxt);
+	res = xmlXPathCompiledEval(comppath, workspace->ctxt);
+	workspace->res = res;
+
 	xmlXPathFreeCompExpr(comppath);
 
 	if (res == NULL)
-	{
-		xmlXPathFreeContext(ctxt);
-		xmlFreeDoc(doctree);
+		cleanup_workspace(workspace);
 
-		return NULL;
-	}
-	/* xmlFreeDoc(doctree); */
 	return res;
 }
 
+/* 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,