Subject: [PATCH v3 1/3] remove PQfn

From: Nathan Bossart <[email-masked]>
Date: Mon, 1 Jun 2026 16:55:00 -0500

---
 doc/src/sgml/libpq.sgml        | 117 +--------------------------------
 src/backend/tcop/fastpath.c    |   4 +-
 src/include/tcop/dest.h        |   4 +-
 src/interfaces/libpq/fe-exec.c |  42 ++++++++++--
 src/interfaces/libpq/fe-lobj.c |  38 +++++------
 5 files changed, 60 insertions(+), 145 deletions(-)

diff --git a/doc/src/sgml/libpq.sgml b/doc/src/sgml/libpq.sgml
index 7d3c3bb66d8..123e7f03902 100644
--- a/doc/src/sgml/libpq.sgml
+++ b/doc/src/sgml/libpq.sgml
@@ -5887,7 +5887,7 @@ int PQflush(PGconn *conn);
     are permitted, command strings containing multiple SQL commands are
     disallowed, and so is <literal>COPY</literal>.
     Using synchronous command execution functions
-    such as <function>PQfn</function>,
+    such as
     <function>PQexec</function>,
     <function>PQexecParams</function>,
     <function>PQprepare</function>,
@@ -7046,121 +7046,6 @@ int PQrequestCancel(PGconn *conn);
   </sect2>
  </sect1>
 
- <sect1 id="libpq-fastpath">
-  <title>The Fast-Path Interface</title>
-
-  <indexterm zone="libpq-fastpath">
-   <primary>fast path</primary>
-  </indexterm>
-
-  <para>
-   <productname>PostgreSQL</productname> provides a fast-path interface
-   to send simple function calls to the server.
-  </para>
-
-  <warning>
-   <para>
-    This interface is unsafe and should not be used.  When
-    <parameter>result_is_int</parameter> is set to <literal>0</literal>,
-    <function>PQfn</function> may write data beyond the end of
-    <parameter>result_buf</parameter>, regardless of whether the buffer has
-    enough space for the requested number of bytes.  Furthermore, it is
-    obsolete, as one can achieve similar
-    performance and greater functionality by setting up a prepared
-    statement to define the function call.  Then, executing the statement
-    with binary transmission of parameters and results substitutes for a
-    fast-path function call.
-   </para>
-  </warning>
-
-  <para>
-   The function <function id="libpq-PQfn">PQfn</function><indexterm><primary>PQfn</primary></indexterm>
-   requests execution of a server function via the fast-path interface:
-<synopsis>
-PGresult *PQfn(PGconn *conn,
-               int fnid,
-               int *result_buf,
-               int *result_len,
-               int result_is_int,
-               const PQArgBlock *args,
-               int nargs);
-
-typedef struct
-{
-    int len;
-    int isint;
-    union
-    {
-        int *ptr;
-        int integer;
-    } u;
-} PQArgBlock;
-</synopsis>
-  </para>
-
-  <para>
-   The <parameter>fnid</parameter> argument is the OID of the function to be
-   executed.  <parameter>args</parameter> and <parameter>nargs</parameter> define the
-   parameters to be passed to the function; they must match the declared
-   function argument list.  When the <parameter>isint</parameter> field of a
-   parameter structure is true, the <parameter>u.integer</parameter> value is sent
-   to the server as an integer of the indicated length (this must be
-   2 or 4 bytes); proper byte-swapping occurs.  When <parameter>isint</parameter>
-   is false, the indicated number of bytes at <parameter>*u.ptr</parameter> are
-   sent with no processing; the data must be in the format expected by
-   the server for binary transmission of the function's argument data
-   type.  (The declaration of <parameter>u.ptr</parameter> as being of
-   type <type>int *</type> is historical; it would be better to consider
-   it <type>void *</type>.)
-   <parameter>result_buf</parameter> points to the buffer in which to place
-   the function's return value.  The caller must have allocated sufficient
-   space to store the return value.  (There is no check!) The actual result
-   length in bytes will be returned in the integer pointed to by
-   <parameter>result_len</parameter>.  If a 2- or 4-byte integer result
-   is expected, set <parameter>result_is_int</parameter> to 1, otherwise
-   set it to 0.  Setting <parameter>result_is_int</parameter> to 1 causes
-   <application>libpq</application> to byte-swap the value if necessary, so that it
-   is delivered as a proper <type>int</type> value for the client machine;
-   note that a 4-byte integer is delivered into <parameter>*result_buf</parameter>
-   for either allowed result size.
-   When <parameter>result_is_int</parameter> is 0, the binary-format byte string
-   sent by the server is returned unmodified. (In this case it's better
-   to consider <parameter>result_buf</parameter> as being of
-   type <type>void *</type>.)
-  </para>
-
-  <para>
-   <function>PQfn</function> always returns a valid
-   <structname>PGresult</structname> pointer, with
-   status <literal>PGRES_COMMAND_OK</literal> for success
-   or <literal>PGRES_FATAL_ERROR</literal> if some problem was encountered.
-   The result status should be
-   checked before the result is used.   The caller is responsible for
-   freeing  the  <structname>PGresult</structname>  with
-   <xref linkend="libpq-PQclear"/> when it is no longer needed.
-  </para>
-
-  <para>
-   To pass a NULL argument to the function, set
-   the <parameter>len</parameter> field of that parameter structure
-   to <literal>-1</literal>; the <parameter>isint</parameter>
-   and <parameter>u</parameter> fields are then irrelevant.
-  </para>
-
-  <para>
-   If the function returns NULL, <parameter>*result_len</parameter> is set
-   to <literal>-1</literal>, and <parameter>*result_buf</parameter> is not
-   modified.
-  </para>
-
-  <para>
-   Note that it is not possible to handle set-valued results when using
-   this interface.  Also, the function must be a plain function, not an
-   aggregate, window function, or procedure.
-  </para>
-
- </sect1>
-
  <sect1 id="libpq-notify">
   <title>Asynchronous Notification</title>
 
diff --git a/src/backend/tcop/fastpath.c b/src/backend/tcop/fastpath.c
index 52772bc90a8..5379e4ad9f5 100644
--- a/src/backend/tcop/fastpath.c
+++ b/src/backend/tcop/fastpath.c
@@ -11,7 +11,9 @@
  *	  src/backend/tcop/fastpath.c
  *
  * NOTES
- *	  This cruft is the server side of PQfn.
+ *	  This cruft is the server side of PQfn.  libpq's PQfn() was retired in
+ *	  v20 and now always errors, but the server code is retained for the
+ *	  benefit of older clients.
  *
  *-------------------------------------------------------------------------
  */
diff --git a/src/include/tcop/dest.h b/src/include/tcop/dest.h
index 103f27fc3cb..507414421ec 100644
--- a/src/include/tcop/dest.h
+++ b/src/include/tcop/dest.h
@@ -12,8 +12,8 @@
  *
  *	  - a remote process is the destination when we are
  *		running a backend with a frontend and the frontend executes
- *		PQexec() or PQfn().  In this case, the results are sent
- *		to the frontend via the functions in backend/libpq.
+ *		PQexec().  In this case, the results are sent to the frontend via
+ *		the functions in backend/libpq.
  *
  *	  - DestNone is the destination when the system executes
  *		a query internally.  The results are discarded.
diff --git a/src/interfaces/libpq/fe-exec.c b/src/interfaces/libpq/fe-exec.c
index 7b8edacbfde..2f034d70e65 100644
--- a/src/interfaces/libpq/fe-exec.c
+++ b/src/interfaces/libpq/fe-exec.c
@@ -2986,10 +2986,10 @@ PQendcopy(PGconn *conn)
  *		nargs			: # of arguments in args array.
  *
  * RETURNS
- *		PGresult with status = PGRES_COMMAND_OK if successful.
- *			*result_len is > 0 if there is a return value, 0 if not.
- *		PGresult with status = PGRES_FATAL_ERROR if backend returns an error.
- *		NULL on communications failure.  conn->errorMessage will be set.
+ *		This function was unsafe and is no longer supported, so it now always
+ *		sets *result_len to 0 and returns a PGresult with status set to
+ *		PGRES_FATAL_ERROR (unless the connection is in the wrong state, in
+ *		which case it returns NULL).
  * ----------------
  */
 
@@ -3002,15 +3002,43 @@ PQfn(PGconn *conn,
 	 const PQArgBlock *args,
 	 int nargs)
 {
-	return PQnfn(conn, fnid, result_buf, -1, result_len,
-				 result_is_int, args, nargs);
+	*result_len = 0;
+
+	if (!conn)
+		return NULL;
+
+	/*
+	 * Since this is the beginning of a query cycle, reset the error state.
+	 * However, in pipeline mode with something already queued, the error
+	 * buffer belongs to that command and we shouldn't clear it.
+	 */
+	if (conn->cmd_queue_head == NULL)
+		pqClearConnErrorState(conn);
+
+	if (conn->pipelineStatus != PQ_PIPELINE_OFF)
+	{
+		libpq_append_conn_error(conn, "%s not allowed in pipeline mode", "PQfn");
+		return NULL;
+	}
+
+	if (conn->sock == PGINVALID_SOCKET || conn->asyncStatus != PGASYNC_IDLE ||
+		pgHavePendingResult(conn))
+	{
+		libpq_append_conn_error(conn, "connection in wrong state");
+		return NULL;
+	}
+
+	libpq_append_conn_error(conn, "PQfn() is no longer supported; use a prepared statement or PQexecParams() with binary results instead");
+	pqSaveErrorResult(conn);
+	return pqPrepareAsyncResult(conn);
 }
 
 /*
  * PQnfn
  *		Private version of PQfn() with verification that returned data fits in
  *		result_buf when result_is_int == 0.  Setting buf_size to -1 disables
- *		this verification.
+ *		this verification.  This is currently only used by the frontend LO
+ *		interface and will hopefully be removed down the road.
  */
 PGresult *
 PQnfn(PGconn *conn, int fnid, int *result_buf, int buf_size, int *result_len,
diff --git a/src/interfaces/libpq/fe-lobj.c b/src/interfaces/libpq/fe-lobj.c
index 12a32fcbaf3..1660c969f58 100644
--- a/src/interfaces/libpq/fe-lobj.c
+++ b/src/interfaces/libpq/fe-lobj.c
@@ -72,7 +72,7 @@ lo_open(PGconn *conn, Oid lobjId, int mode)
 	argv[1].len = 4;
 	argv[1].u.integer = mode;
 
-	res = PQfn(conn, conn->lobjfuncs->fn_lo_open, &fd, &result_len, 1, argv, 2);
+	res = PQnfn(conn, conn->lobjfuncs->fn_lo_open, &fd, -1, &result_len, 1, argv, 2);
 	if (PQresultStatus(res) == PGRES_COMMAND_OK)
 	{
 		PQclear(res);
@@ -106,8 +106,8 @@ lo_close(PGconn *conn, int fd)
 	argv[0].isint = 1;
 	argv[0].len = 4;
 	argv[0].u.integer = fd;
-	res = PQfn(conn, conn->lobjfuncs->fn_lo_close,
-			   &retval, &result_len, 1, argv, 1);
+	res = PQnfn(conn, conn->lobjfuncs->fn_lo_close,
+				&retval, -1, &result_len, 1, argv, 1);
 	if (PQresultStatus(res) == PGRES_COMMAND_OK)
 	{
 		PQclear(res);
@@ -169,8 +169,8 @@ lo_truncate(PGconn *conn, int fd, size_t len)
 	argv[1].len = 4;
 	argv[1].u.integer = (int) len;
 
-	res = PQfn(conn, conn->lobjfuncs->fn_lo_truncate,
-			   &retval, &result_len, 1, argv, 2);
+	res = PQnfn(conn, conn->lobjfuncs->fn_lo_truncate,
+				&retval, -1, &result_len, 1, argv, 2);
 
 	if (PQresultStatus(res) == PGRES_COMMAND_OK)
 	{
@@ -218,8 +218,8 @@ lo_truncate64(PGconn *conn, int fd, int64_t len)
 	argv[1].len = 8;
 	argv[1].u.ptr = (int *) &len;
 
-	res = PQfn(conn, conn->lobjfuncs->fn_lo_truncate64,
-			   &retval, &result_len, 1, argv, 2);
+	res = PQnfn(conn, conn->lobjfuncs->fn_lo_truncate64,
+				&retval, -1, &result_len, 1, argv, 2);
 
 	if (PQresultStatus(res) == PGRES_COMMAND_OK)
 	{
@@ -322,8 +322,8 @@ lo_write(PGconn *conn, int fd, const char *buf, size_t len)
 	argv[1].len = (int) len;
 	argv[1].u.ptr = (int *) unconstify(char *, buf);
 
-	res = PQfn(conn, conn->lobjfuncs->fn_lo_write,
-			   &retval, &result_len, 1, argv, 2);
+	res = PQnfn(conn, conn->lobjfuncs->fn_lo_write,
+				&retval, -1, &result_len, 1, argv, 2);
 	if (PQresultStatus(res) == PGRES_COMMAND_OK)
 	{
 		PQclear(res);
@@ -363,8 +363,8 @@ lo_lseek(PGconn *conn, int fd, int offset, int whence)
 	argv[2].len = 4;
 	argv[2].u.integer = whence;
 
-	res = PQfn(conn, conn->lobjfuncs->fn_lo_lseek,
-			   &retval, &result_len, 1, argv, 3);
+	res = PQnfn(conn, conn->lobjfuncs->fn_lo_lseek,
+				&retval, -1, &result_len, 1, argv, 3);
 	if (PQresultStatus(res) == PGRES_COMMAND_OK)
 	{
 		PQclear(res);
@@ -448,8 +448,8 @@ lo_creat(PGconn *conn, int mode)
 	argv[0].isint = 1;
 	argv[0].len = 4;
 	argv[0].u.integer = mode;
-	res = PQfn(conn, conn->lobjfuncs->fn_lo_creat,
-			   &retval, &result_len, 1, argv, 1);
+	res = PQnfn(conn, conn->lobjfuncs->fn_lo_creat,
+				&retval, -1, &result_len, 1, argv, 1);
 	if (PQresultStatus(res) == PGRES_COMMAND_OK)
 	{
 		PQclear(res);
@@ -492,8 +492,8 @@ lo_create(PGconn *conn, Oid lobjId)
 	argv[0].isint = 1;
 	argv[0].len = 4;
 	argv[0].u.integer = lobjId;
-	res = PQfn(conn, conn->lobjfuncs->fn_lo_create,
-			   &retval, &result_len, 1, argv, 1);
+	res = PQnfn(conn, conn->lobjfuncs->fn_lo_create,
+				&retval, -1, &result_len, 1, argv, 1);
 	if (PQresultStatus(res) == PGRES_COMMAND_OK)
 	{
 		PQclear(res);
@@ -526,8 +526,8 @@ lo_tell(PGconn *conn, int fd)
 	argv[0].len = 4;
 	argv[0].u.integer = fd;
 
-	res = PQfn(conn, conn->lobjfuncs->fn_lo_tell,
-			   &retval, &result_len, 1, argv, 1);
+	res = PQnfn(conn, conn->lobjfuncs->fn_lo_tell,
+				&retval, -1, &result_len, 1, argv, 1);
 	if (PQresultStatus(res) == PGRES_COMMAND_OK)
 	{
 		PQclear(res);
@@ -600,8 +600,8 @@ lo_unlink(PGconn *conn, Oid lobjId)
 	argv[0].len = 4;
 	argv[0].u.integer = lobjId;
 
-	res = PQfn(conn, conn->lobjfuncs->fn_lo_unlink,
-			   &retval, &result_len, 1, argv, 1);
+	res = PQnfn(conn, conn->lobjfuncs->fn_lo_unlink,
+				&retval, -1, &result_len, 1, argv, 1);
 	if (PQresultStatus(res) == PGRES_COMMAND_OK)
 	{
 		PQclear(res);
-- 
2.50.1 (Apple Git-155)

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