libpq: Add support for Close on portals and statements

The following routines are added to libpq:
PGresult *PQclosePrepared(PGconn *conn, const char *stmt);
PGresult *PQclosePortal(PGconn *conn, const char *portal);
int PQsendClosePrepared(PGconn *conn, const char *stmt);
int PQsendClosePortal(PGconn *conn, const char *portal);

The "send" routines are non-blocking versions of the two others.

Close messages are part of the protocol but they did not have a libpq
implementation.  And, having these routines is for instance useful with
connection poolers as these can detect more easily Close messages
than DEALLOCATE queries.

The implementation takes advantage of what the Describe routines rely on
for portals and statements.  Some regression tests are added in
libpq_pipeline, for the four new routines, by closing portals and
statements created already by the tests.

Author: Jelte Fennema
Reviewed-by: Jian He, Michael Paquier
Discussion: https://postgr.es/m/CAGECzQTb4xFAopAVokudB+L62Kt44mNAL4Z9zZ7UTrs1TRFvWA@mail.gmail.com

Author: michaelpq

doc/src/sgml/libpq.sgml

@@ -3250,10 +3250,7 @@ PGresult *PQprepare(PGconn *conn,
 
     Prepared statements for use with <xref linkend="libpq-PQexecPrepared"/> can also
     be created by executing SQL <xref linkend="sql-prepare"/>
-    statements.  Also, although there is no <application>libpq</application>
-    function for deleting a prepared statement, the SQL <xref
-    linkend="sql-deallocate"/> statement
-    can be used for that purpose.
+    statements.
    </para>
 
    <para>
@@ -3360,6 +3357,66 @@ PGresult *PQdescribePortal(PGconn *conn, const char *portalName);
        </para>
       </listitem>
      </varlistentry>
+
+     <varlistentry id="libpq-PQclosePrepared">
+      <term><function>PQclosePrepared</function><indexterm><primary>PQclosePrepared</primary></indexterm></term>
+
+      <listitem>
+       <para>
+        Submits a request to close the specified prepared statement, and waits
+        for completion.
+<synopsis>
+PGresult *PQclosePrepared(PGconn *conn, const char *stmtName);
+</synopsis>
+       </para>
+
+       <para>
+        <xref linkend="libpq-PQclosePrepared"/> allows an application to close
+        a previously prepared statement. Closing a statement releases all
+        of its associated resources on the server and allows its name to be
+        reused.
+       </para>
+
+       <para>
+        <parameter>stmtName</parameter> can be <literal>""</literal> or
+        <symbol>NULL</symbol> to reference the unnamed statement. It is fine
+        if no statement exists with this name, in that case the operation is a
+        no-op. On success, a <structname>PGresult</structname> with
+        status <literal>PGRES_COMMAND_OK</literal> is returned.
+       </para>
+      </listitem>
+     </varlistentry>
+
+     <varlistentry id="libpq-PQclosePortal">
+      <term><function>PQclosePortal</function><indexterm><primary>PQclosePortal</primary></indexterm></term>
+
+      <listitem>
+       <para>
+        Submits a request to close the specified portal, and waits for
+        completion.
+<synopsis>
+PGresult *PQclosePortal(PGconn *conn, const char *portalName);
+</synopsis>
+       </para>
+
+       <para>
+        <xref linkend="libpq-PQclosePortal"/> allows an application to trigger
+        a close of a previously created portal. Closing a portal releases all
+        of its associated resources on the server and allows its name to be
+        reused. (<application>libpq</application> does not provide any
+        direct access to portals, but you can use this function to close a
+        cursor created with a <command>DECLARE CURSOR</command> SQL command.)
+       </para>
+
+       <para>
+        <parameter>portalName</parameter> can be <literal>""</literal> or
+        <symbol>NULL</symbol> to reference the unnamed portal. It is fine
+        if no portal exists with this name, in that case the operation is a
+        no-op. On success, a <structname>PGresult</structname> with status
+        <literal>PGRES_COMMAND_OK</literal> is returned.
+       </para>
+      </listitem>
+     </varlistentry>
     </variablelist>
    </para>
 
@@ -4851,15 +4908,19 @@ unsigned char *PQunescapeBytea(const unsigned char *from, size_t *to_length);
    <xref linkend="libpq-PQsendQueryParams"/>,
    <xref linkend="libpq-PQsendPrepare"/>,
    <xref linkend="libpq-PQsendQueryPrepared"/>,
-   <xref linkend="libpq-PQsendDescribePrepared"/>, and
+   <xref linkend="libpq-PQsendDescribePrepared"/>,
    <xref linkend="libpq-PQsendDescribePortal"/>,
+   <xref linkend="libpq-PQsendClosePrepared"/>, and
+   <xref linkend="libpq-PQsendClosePortal"/>,
    which can be used with <xref linkend="libpq-PQgetResult"/> to duplicate
    the functionality of
    <xref linkend="libpq-PQexecParams"/>,
    <xref linkend="libpq-PQprepare"/>,
    <xref linkend="libpq-PQexecPrepared"/>,
-   <xref linkend="libpq-PQdescribePrepared"/>, and
+   <xref linkend="libpq-PQdescribePrepared"/>,
    <xref linkend="libpq-PQdescribePortal"/>
+   <xref linkend="libpq-PQclosePrepared"/>, and
+   <xref linkend="libpq-PQclosePortal"/>
    respectively.
 
    <variablelist>
@@ -5008,6 +5069,46 @@ int PQsendDescribePortal(PGconn *conn, const char *portalName);
      </listitem>
     </varlistentry>
 
+    <varlistentry id="libpq-PQsendClosePrepared">
+     <term><function>PQsendClosePrepared</function><indexterm><primary>PQsendClosePrepared</primary></indexterm></term>
+
+     <listitem>
+      <para>
+       Submits a request to close the specified prepared statement, without
+       waiting for completion.
+<synopsis>
+int PQsendClosePrepared(PGconn *conn, const char *stmtName);
+</synopsis>
+
+       This is an asynchronous version of <xref linkend="libpq-PQclosePrepared"/>:
+       it returns 1 if it was able to dispatch the request, and 0 if not.
+       After a successful call, call <xref linkend="libpq-PQgetResult"/> to
+       obtain the results.  The function's parameters are handled
+       identically to <xref linkend="libpq-PQclosePrepared"/>.
+      </para>
+     </listitem>
+    </varlistentry>
+
+    <varlistentry id="libpq-PQsendClosePortal">
+     <term><function>PQsendClosePortal</function><indexterm><primary>PQsendClosePortal</primary></indexterm></term>
+
+     <listitem>
+      <para>
+       Submits a request to close specified portal, without waiting for
+       completion.
+<synopsis>
+int PQsendClosePortal(PGconn *conn, const char *portalName);
+</synopsis>
+
+       This is an asynchronous version of <xref linkend="libpq-PQclosePortal"/>:
+       it returns 1 if it was able to dispatch the request, and 0 if not.
+       After a successful call, call <xref linkend="libpq-PQgetResult"/> to
+       obtain the results.  The function's parameters are handled
+       identically to <xref linkend="libpq-PQclosePortal"/>.
+      </para>
+     </listitem>
+    </varlistentry>
+
     <varlistentry id="libpq-PQgetResult">
      <term><function>PQgetResult</function><indexterm><primary>PQgetResult</primary></indexterm></term>
 
@@ -5019,7 +5120,9 @@ int PQsendDescribePortal(PGconn *conn, const char *portalName);
        <xref linkend="libpq-PQsendPrepare"/>,
        <xref linkend="libpq-PQsendQueryPrepared"/>,
        <xref linkend="libpq-PQsendDescribePrepared"/>,
-       <xref linkend="libpq-PQsendDescribePortal"/>, or
+       <xref linkend="libpq-PQsendDescribePortal"/>,
+       <xref linkend="libpq-PQsendClosePrepared"/>,
+       <xref linkend="libpq-PQsendClosePortal"/>, or
        <xref linkend="libpq-PQpipelineSync"/>
        call, and returns it.
        A null pointer is returned when the command is complete and there
@@ -5350,6 +5453,8 @@ int PQflush(PGconn *conn);
     <function>PQexecPrepared</function>,
     <function>PQdescribePrepared</function>,
     <function>PQdescribePortal</function>,
+    <function>PQclosePrepared</function>,
+    <function>PQclosePortal</function>,
     is an error condition.
     <function>PQsendQuery</function> is
     also disallowed, because it uses the simple query protocol.
@@ -5389,8 +5494,10 @@ int PQflush(PGconn *conn);
      establish a synchronization point in the pipeline,
      or when <xref linkend="libpq-PQflush"/> is called.
      The functions <xref linkend="libpq-PQsendPrepare"/>,
-     <xref linkend="libpq-PQsendDescribePrepared"/>, and
-     <xref linkend="libpq-PQsendDescribePortal"/> also work in pipeline mode.
+     <xref linkend="libpq-PQsendDescribePrepared"/>,
+     <xref linkend="libpq-PQsendDescribePortal"/>,
+     <xref linkend="libpq-PQsendClosePrepared"/>, and
+     <xref linkend="libpq-PQsendClosePortal"/> also work in pipeline mode.
      Result processing is described below.
     </para>
 

src/interfaces/libpq/exports.txt

@@ -187,3 +187,7 @@ PQsetTraceFlags           184
 PQmblenBounded            185
 PQsendFlushRequest        186
 PQconnectionUsedGSSAPI    187
+PQclosePrepared           188
+PQclosePortal             189
+PQsendClosePrepared       190
+PQsendClosePortal         191

src/interfaces/libpq/fe-exec.c

@@ -77,8 +77,8 @@ static void parseInput(PGconn *conn);
 static PGresult *getCopyResult(PGconn *conn, ExecStatusType copytype);
 static bool PQexecStart(PGconn *conn);
 static PGresult *PQexecFinish(PGconn *conn);
-static int	PQsendDescribe(PGconn *conn, char desc_type,
-						   const char *desc_target);
+static int	PQsendTypedCommand(PGconn *conn, char command, char type,
+							   const char *target);
 static int	check_field_number(const PGresult *res, int field_num);
 static void pqPipelineProcessQueue(PGconn *conn);
 static int	pqPipelineFlush(PGconn *conn);
@@ -2422,7 +2422,7 @@ PQdescribePrepared(PGconn *conn, const char *stmt)
 {
 	if (!PQexecStart(conn))
 		return NULL;
-	if (!PQsendDescribe(conn, 'S', stmt))
+	if (!PQsendTypedCommand(conn, 'D', 'S', stmt))
 		return NULL;
 	return PQexecFinish(conn);
 }
@@ -2441,7 +2441,7 @@ PQdescribePortal(PGconn *conn, const char *portal)
 {
 	if (!PQexecStart(conn))
 		return NULL;
-	if (!PQsendDescribe(conn, 'P', portal))
+	if (!PQsendTypedCommand(conn, 'D', 'P', portal))
 		return NULL;
 	return PQexecFinish(conn);
 }
@@ -2456,7 +2456,7 @@ PQdescribePortal(PGconn *conn, const char *portal)
 int
 PQsendDescribePrepared(PGconn *conn, const char *stmt)
 {
-	return PQsendDescribe(conn, 'S', stmt);
+	return PQsendTypedCommand(conn, 'D', 'S', stmt);
 }
 
 /*
@@ -2469,26 +2469,96 @@ PQsendDescribePrepared(PGconn *conn, const char *stmt)
 int
 PQsendDescribePortal(PGconn *conn, const char *portal)
 {
-	return PQsendDescribe(conn, 'P', portal);
+	return PQsendTypedCommand(conn, 'D', 'P', portal);
 }
 
 /*
- * PQsendDescribe
- *	 Common code to send a Describe command
+ * PQclosePrepared
+ *	  Close a previously prepared statement
+ *
+ * If the query was not even sent, return NULL; conn->errorMessage is set to
+ * a relevant message.
+ * If the query was sent, a new PGresult is returned (which could indicate
+ * either success or failure).  On success, the PGresult contains status
+ * PGRES_COMMAND_OK. The user is responsible for freeing the PGresult via
+ * PQclear() when done with it.
+ */
+PGresult *
+PQclosePrepared(PGconn *conn, const char *stmt)
+{
+	if (!PQexecStart(conn))
+		return NULL;
+	if (!PQsendTypedCommand(conn, 'C', 'S', stmt))
+		return NULL;
+	return PQexecFinish(conn);
+}
+
+/*
+ * PQclosePortal
+ *	  Close a previously created portal
+ *
+ * This is exactly like PQclosePrepared, but for portals.  Note that at the
+ * moment, libpq doesn't really expose portals to the client; but this can be
+ * used with a portal created by a SQL DECLARE CURSOR command.
+ */
+PGresult *
+PQclosePortal(PGconn *conn, const char *portal)
+{
+	if (!PQexecStart(conn))
+		return NULL;
+	if (!PQsendTypedCommand(conn, 'C', 'P', portal))
+		return NULL;
+	return PQexecFinish(conn);
+}
+
+/*
+ * PQsendClosePrepared
+ *	 Submit a Close Statement command, but don't wait for it to finish
+ *
+ * Returns: 1 if successfully submitted
+ *			0 if error (conn->errorMessage is set)
+ */
+int
+PQsendClosePrepared(PGconn *conn, const char *stmt)
+{
+	return PQsendTypedCommand(conn, 'C', 'S', stmt);
+}
+
+/*
+ * PQsendClosePortal
+ *	 Submit a Close Portal command, but don't wait for it to finish
+ *
+ * Returns: 1 if successfully submitted
+ *			0 if error (conn->errorMessage is set)
+ */
+int
+PQsendClosePortal(PGconn *conn, const char *portal)
+{
+	return PQsendTypedCommand(conn, 'C', 'P', portal);
+}
+
+/*
+ * PQsendTypedCommand
+ *	 Common code to send a Describe or Close command
+ *
+ * Available options for "command" are
+ *	 'C' for Close; or
+ *	 'D' for Describe.
+ *
+ * Available options for "type" are
+ *	 'S' to run a command on a prepared statement; or
+ *	 'P' to run a command on a portal.
  *
- * Available options for desc_type are
- *	 'S' to describe a prepared statement; or
- *	 'P' to describe a portal.
  * Returns 1 on success and 0 on failure.
  */
 static int
-PQsendDescribe(PGconn *conn, char desc_type, const char *desc_target)
+PQsendTypedCommand(PGconn *conn, char command, char type, const char *target)
 {
 	PGcmdQueueEntry *entry = NULL;
 
-	/* Treat null desc_target as empty string */
-	if (!desc_target)
-		desc_target = "";
+	/* Treat null target as empty string */
+	if (!target)
+		target = "";
 
 	if (!PQsendQueryStart(conn, true))
 		return 0;
@@ -2497,10 +2567,10 @@ PQsendDescribe(PGconn *conn, char desc_type, const char *desc_target)
 	if (entry == NULL)
 		return 0;				/* error msg already set */
 
-	/* construct the Describe message */
-	if (pqPutMsgStart('D', conn) < 0 ||
-		pqPutc(desc_type, conn) < 0 ||
-		pqPuts(desc_target, conn) < 0 ||
+	/* construct the Close message */
+	if (pqPutMsgStart(command, conn) < 0 ||
+		pqPutc(type, conn) < 0 ||
+		pqPuts(target, conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
 		goto sendFailed;
 
@@ -2512,8 +2582,20 @@ PQsendDescribe(PGconn *conn, char desc_type, const char *desc_target)
 			goto sendFailed;
 	}
 
-	/* remember we are doing a Describe */
-	entry->queryclass = PGQUERY_DESCRIBE;
+	/* remember if we are doing a Close or a Describe */
+	if (command == 'C')
+	{
+		entry->queryclass = PGQUERY_CLOSE;
+	}
+	else if (command == 'D')
+	{
+		entry->queryclass = PGQUERY_DESCRIBE;
+	}
+	else
+	{
+		libpq_append_conn_error(conn, "unknown command type provided");
+		goto sendFailed;
+	}
 
 	/*
 	 * Give the data a push (in pipeline mode, only if we're past the size

src/interfaces/libpq/fe-protocol3.c

@@ -278,8 +278,25 @@ pqParseInput3(PGconn *conn)
 					}
 					break;
 				case '2':		/* Bind Complete */
+					/* Nothing to do for this message type */
+					break;
 				case '3':		/* Close Complete */
-					/* Nothing to do for these message types */
+					/* If we're doing PQsendClose, we're done; else ignore */
+					if (conn->cmd_queue_head &&
+						conn->cmd_queue_head->queryclass == PGQUERY_CLOSE)
+					{
+						if (!pgHavePendingResult(conn))
+						{
+							conn->result = PQmakeEmptyPGresult(conn,
+															   PGRES_COMMAND_OK);
+							if (!conn->result)
+							{
+								libpq_append_conn_error(conn, "out of memory");
+								pqSaveErrorResult(conn);
+							}
+						}
+						conn->asyncStatus = PGASYNC_READY;
+					}
 					break;
 				case 'S':		/* parameter status */
 					if (getParameterStatus(conn))