Add support for COPY TO callback functions

This is useful as a way for extensions to process COPY TO rows in the
way they see fit (say auditing, analytics, backend, etc.) without the
need to invoke an external process running as the OS user running the
backend through PROGRAM that requires superuser rights.  COPY FROM
already provides a similar callback for logical replication.  For COPY
TO, the callback is triggered when we are ready to send a row in
CopySendEndOfRow(), which is the same code path as when sending a row
to a frontend or a pipe/file.

A small test module, test_copy_callbacks, is added to provide some
coverage for this facility.

Author: Bilva Sanaba, Nathan Bossart
Discussion: https://postgr.es/m/253C21D1-FCEB-41D9-A2AF-E6517015B7D7@amazon.com

src/backend/commands/copy.c

@@ -310,7 +310,7 @@ DoCopy(ParseState *pstate, const CopyStmt *stmt,
 
 		cstate = BeginCopyTo(pstate, rel, query, relid,
 							 stmt->filename, stmt->is_program,
-							 stmt->attlist, stmt->options);
+							 NULL, stmt->attlist, stmt->options);
 		*processed = DoCopyTo(cstate);	/* copy from database to file */
 		EndCopyTo(cstate);
 	}

src/backend/commands/copyto.c

@@ -51,6 +51,7 @@ typedef enum CopyDest
 {
 	COPY_FILE,					/* to file (or a piped program) */
 	COPY_FRONTEND,				/* to frontend */
+	COPY_CALLBACK				/* to callback function */
 } CopyDest;
 
 /*
@@ -85,6 +86,7 @@ typedef struct CopyToStateData
 	List	   *attnumlist;		/* integer list of attnums to copy */
 	char	   *filename;		/* filename, or NULL for STDOUT */
 	bool		is_program;		/* is 'filename' a program to popen? */
+	copy_data_dest_cb data_dest_cb; /* function for writing data */
 
 	CopyFormatOptions opts;
 	Node	   *whereClause;	/* WHERE condition (or NULL) */
@@ -247,6 +249,9 @@ CopySendEndOfRow(CopyToState cstate)
 			/* Dump the accumulated row as one CopyData message */
 			(void) pq_putmessage('d', fe_msgbuf->data, fe_msgbuf->len);
 			break;
+		case COPY_CALLBACK:
+			cstate->data_dest_cb(fe_msgbuf->data, fe_msgbuf->len);
+			break;
 	}
 
 	/* Update the progress */
@@ -336,6 +341,17 @@ EndCopy(CopyToState cstate)
 
 /*
  * Setup CopyToState to read tuples from a table or a query for COPY TO.
+ *
+ * 'rel': Relation to be copied
+ * 'raw_query': Query whose results are to be copied
+ * 'queryRelId': OID of base relation to convert to a query (for RLS)
+ * 'filename': Name of server-local file to write, NULL for STDOUT
+ * 'is_program': true if 'filename' is program to execute
+ * 'data_dest_cb': Callback that processes the output data
+ * 'attnamelist': List of char *, columns to include. NIL selects all cols.
+ * 'options': List of DefElem. See copy_opt_item in gram.y for selections.
+ *
+ * Returns a CopyToState, to be passed to DoCopyTo() and related functions.
  */
 CopyToState
 BeginCopyTo(ParseState *pstate,
@@ -344,11 +360,12 @@ BeginCopyTo(ParseState *pstate,
 			Oid queryRelId,
 			const char *filename,
 			bool is_program,
+			copy_data_dest_cb data_dest_cb,
 			List *attnamelist,
 			List *options)
 {
 	CopyToState cstate;
-	bool		pipe = (filename == NULL);
+	bool		pipe = (filename == NULL && data_dest_cb == NULL);
 	TupleDesc	tupDesc;
 	int			num_phys_attrs;
 	MemoryContext oldcontext;
@@ -656,7 +673,13 @@ BeginCopyTo(ParseState *pstate,
 
 	cstate->copy_dest = COPY_FILE;	/* default */
 
-	if (pipe)
+	if (data_dest_cb)
+	{
+		progress_vals[1] = PROGRESS_COPY_TYPE_CALLBACK;
+		cstate->copy_dest = COPY_CALLBACK;
+		cstate->data_dest_cb = data_dest_cb;
+	}
+	else if (pipe)
 	{
 		progress_vals[1] = PROGRESS_COPY_TYPE_PIPE;
 
@@ -765,11 +788,13 @@ EndCopyTo(CopyToState cstate)
 
 /*
  * Copy from relation or query TO file.
+ *
+ * Returns the number of rows processed.
  */
 uint64
 DoCopyTo(CopyToState cstate)
 {
-	bool		pipe = (cstate->filename == NULL);
+	bool		pipe = (cstate->filename == NULL && cstate->data_dest_cb == NULL);
 	bool		fe_copy = (pipe && whereToSendOutput == DestRemote);
 	TupleDesc	tupDesc;
 	int			num_phys_attrs;

src/include/commands/copy.h

@@ -66,6 +66,7 @@ typedef struct CopyFromStateData *CopyFromState;
 typedef struct CopyToStateData *CopyToState;
 
 typedef int (*copy_data_source_cb) (void *outbuf, int minread, int maxread);
+typedef void (*copy_data_dest_cb) (void *data, int len);
 
 extern void DoCopy(ParseState *pstate, const CopyStmt *stmt,
 				   int stmt_location, int stmt_len,
@@ -91,7 +92,7 @@ extern DestReceiver *CreateCopyDestReceiver(void);
  */
 extern CopyToState BeginCopyTo(ParseState *pstate, Relation rel, RawStmt *raw_query,
 							   Oid queryRelId, const char *filename, bool is_program,
-							   List *attnamelist, List *options);
+							   copy_data_dest_cb data_dest_cb, List *attnamelist, List *options);
 extern void EndCopyTo(CopyToState cstate);
 extern uint64 DoCopyTo(CopyToState cstate);
 extern List *CopyGetAttnums(TupleDesc tupDesc, Relation rel,

src/test/modules/Makefile

@@ -15,6 +15,7 @@ SUBDIRS = \
 		  snapshot_too_old \
 		  spgist_name_ops \
 		  test_bloomfilter \
+		  test_copy_callbacks \
 		  test_ddl_deparse \
 		  test_extensions \
 		  test_ginpostinglist \

src/test/modules/meson.build

@@ -9,6 +9,7 @@ subdir('snapshot_too_old')
 subdir('spgist_name_ops')
 subdir('ssl_passphrase_callback')
 subdir('test_bloomfilter')
+subdir('test_copy_callbacks')
 subdir('test_ddl_deparse')
 subdir('test_extensions')
 subdir('test_ginpostinglist')

src/test/modules/test_copy_callbacks/.gitignore

@@ -0,0 +1,4 @@
+# Generated subdirectories
+/log/
+/results/
+/tmp_check/

src/test/modules/test_copy_callbacks/Makefile

@@ -0,0 +1,23 @@
+# src/test/modules/test_copy_callbacks/Makefile
+
+MODULE_big = test_copy_callbacks
+OBJS = \
+	$(WIN32RES) \
+	test_copy_callbacks.o
+PGFILEDESC = "test_copy_callbacks - test COPY callbacks"
+
+EXTENSION = test_copy_callbacks
+DATA = test_copy_callbacks--1.0.sql
+
+REGRESS = test_copy_callbacks
+
+ifdef USE_PGXS
+PG_CONFIG = pg_config
+PGXS := $(shell $(PG_CONFIG) --pgxs)
+include $(PGXS)
+else
+subdir = src/test/modules/test_copy_callbacks
+top_builddir = ../../../..
+include $(top_builddir)/src/Makefile.global
+include $(top_srcdir)/contrib/contrib-global.mk
+endif

src/test/modules/test_copy_callbacks/expected/test_copy_callbacks.out

@@ -0,0 +1,13 @@
+CREATE EXTENSION test_copy_callbacks;
+CREATE TABLE public.test (a INT, b INT, c INT);
+INSERT INTO public.test VALUES (1, 2, 3), (12, 34, 56), (123, 456, 789);
+SELECT test_copy_to_callback('public.test'::pg_catalog.regclass);
+NOTICE:  COPY TO callback called with data "1	2	3" and length 5
+NOTICE:  COPY TO callback called with data "12	34	56" and length 8
+NOTICE:  COPY TO callback called with data "123	456	789" and length 11
+NOTICE:  COPY TO callback has processed 3 rows
+ test_copy_to_callback 
+-----------------------
+ 
+(1 row)
+

src/test/modules/test_copy_callbacks/meson.build

@@ -0,0 +1,34 @@
+# FIXME: prevent install during main install, but not during test :/
+
+test_copy_callbacks_sources = files(
+  'test_copy_callbacks.c',
+)
+
+if host_system == 'windows'
+  test_copy_callbacks_sources += rc_lib_gen.process(win32ver_rc, extra_args: [
+    '--NAME', 'test_copy_callbacks',
+    '--FILEDESC', 'test_copy_callbacks - test COPY callbacks',])
+endif
+
+test_copy_callbacks = shared_module('test_copy_callbacks',
+  test_copy_callbacks_sources,
+  kwargs: pg_mod_args,
+)
+testprep_targets += test_copy_callbacks
+
+install_data(
+  'test_copy_callbacks.control',
+  'test_copy_callbacks--1.0.sql',
+  kwargs: contrib_data_args,
+)
+
+tests += {
+  'name': 'test_copy_callbacks',
+  'sd': meson.current_source_dir(),
+  'bd': meson.current_build_dir(),
+  'regress': {
+    'sql': [
+      'test_copy_callbacks',
+    ],
+  },
+}

src/test/modules/test_copy_callbacks/sql/test_copy_callbacks.sql

@@ -0,0 +1,4 @@
+CREATE EXTENSION test_copy_callbacks;
+CREATE TABLE public.test (a INT, b INT, c INT);
+INSERT INTO public.test VALUES (1, 2, 3), (12, 34, 56), (123, 456, 789);
+SELECT test_copy_to_callback('public.test'::pg_catalog.regclass);

src/test/modules/test_copy_callbacks/test_copy_callbacks--1.0.sql

@@ -0,0 +1,8 @@
+/* src/test/modules/test_copy_callbacks/test_copy_callbacks--1.0.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "CREATE EXTENSION test_copy_callbacks" to load this file. \quit
+
+CREATE FUNCTION test_copy_to_callback(pg_catalog.regclass)
+	RETURNS pg_catalog.void
+	AS 'MODULE_PATHNAME' LANGUAGE C;

src/test/modules/test_copy_callbacks/test_copy_callbacks.c

@@ -0,0 +1,51 @@
+/*--------------------------------------------------------------------------
+ *
+ * test_copy_callbacks.c
+ *		Code for testing COPY callbacks.
+ *
+ * Portions Copyright (c) 1996-2022, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *		src/test/modules/test_copy_callbacks/test_copy_callbacks.c
+ *
+ * -------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "access/table.h"
+#include "commands/copy.h"
+#include "fmgr.h"
+#include "utils/rel.h"
+
+PG_MODULE_MAGIC;
+
+static void
+to_cb(void *data, int len)
+{
+	ereport(NOTICE,
+			(errmsg("COPY TO callback called with data \"%s\" and length %d",
+					(char *) data, len)));
+}
+
+PG_FUNCTION_INFO_V1(test_copy_to_callback);
+Datum
+test_copy_to_callback(PG_FUNCTION_ARGS)
+{
+	Relation	rel = table_open(PG_GETARG_OID(0), AccessShareLock);
+	CopyToState cstate;
+	int64		processed;
+
+	cstate = BeginCopyTo(NULL, rel, NULL, RelationGetRelid(rel), NULL, NULL,
+						 to_cb, NIL, NIL);
+	processed = DoCopyTo(cstate);
+	EndCopyTo(cstate);
+
+	ereport(NOTICE, (errmsg("COPY TO callback has processed %lld rows",
+							(long long) processed)));
+
+	table_close(rel, NoLock);
+
+	PG_RETURN_VOID();
+}

src/test/modules/test_copy_callbacks/test_copy_callbacks.control

@@ -0,0 +1,4 @@
+comment = 'Test code for COPY callbacks'
+default_version = '1.0'
+module_pathname = '$libdir/test_copy_callbacks'
+relocatable = true

src/tools/pgindent/typedefs.list

@@ -3177,6 +3177,7 @@ compare_context
 config_var_value
 contain_aggs_of_level_context
 convert_testexpr_context
+copy_data_dest_cb
 copy_data_source_cb
 core_YYSTYPE
 core_yy_extra_type