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/[email protected]

1 files changed, 28 insertions, 3 deletions

diff --git a/src/backend/commands/copyto.c b/src/backend/commands/copyto.c
index fca29a9a105..2527e660598 100644
--- a/src/backend/commands/copyto.c
+++ b/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;