Add wait events for COPY file/program operations

Add two new IO wait events for COPY operations:
- COPY_DATA_READ: When COPY FROM blocks reading from file or program
- COPY_DATA_WRITE: When COPY TO blocks writing to file or program

These events cover:
- COPY FROM/TO file (including slow storage I/O)
- COPY FROM/TO PROGRAM (pipe buffer congestion in ETL pipelines)

Note: COPY FROM/TO STDIN/STDOUT already have wait event coverage via
existing Client/ClientRead and Client/ClientWrite events emitted at
the protocol layer, so no additional instrumentation is needed there.

This enables DBAs to:
- Diagnose slow pg_dump/pg_restore operations
- Identify storage I/O bottlenecks during bulk loads
- Analyze pipe buffer congestion in ETL pipelines
- Perform ASH analysis of COPY operations

Author: NikolayS

src/backend/commands/copyfromparse.c

@@ -249,7 +249,9 @@ CopyGetData(CopyFromState cstate, void *databuf, int minread, int maxread)
 	switch (cstate->copy_src)
 	{
 		case COPY_FILE:
+			pgstat_report_wait_start(WAIT_EVENT_COPY_DATA_READ);
 			bytesread = fread(databuf, 1, maxread, cstate->copy_file);
+			pgstat_report_wait_end();
 			if (ferror(cstate->copy_file))
 				ereport(ERROR,
 						(errcode_for_file_access(),

src/backend/commands/copyto.c

@@ -454,10 +454,12 @@ CopySendEndOfRow(CopyToState cstate)
 	switch (cstate->copy_dest)
 	{
 		case COPY_FILE:
+			pgstat_report_wait_start(WAIT_EVENT_COPY_DATA_WRITE);
 			if (fwrite(fe_msgbuf->data, fe_msgbuf->len, 1,
 					   cstate->copy_file) != 1 ||
 				ferror(cstate->copy_file))
 			{
+				pgstat_report_wait_end();
 				if (cstate->is_program)
 				{
 					if (errno == EPIPE)
@@ -486,6 +488,7 @@ CopySendEndOfRow(CopyToState cstate)
 							(errcode_for_file_access(),
 							 errmsg("could not write to COPY file: %m")));
 			}
+			pgstat_report_wait_end();
 			break;
 		case COPY_FRONTEND:
 			/* Dump the accumulated row as one CopyData message */

src/backend/utils/activity/wait_event_names.txt

@@ -210,6 +210,8 @@ CONTROL_FILE_SYNC	"Waiting for the <filename>pg_control</filename> file to reach
 CONTROL_FILE_SYNC_UPDATE	"Waiting for an update to the <filename>pg_control</filename> file to reach durable storage."
 CONTROL_FILE_WRITE	"Waiting for a write to the <filename>pg_control</filename> file."
 CONTROL_FILE_WRITE_UPDATE	"Waiting for a write to update the <filename>pg_control</filename> file."
+COPY_DATA_READ	"Waiting to read data from file or program during COPY FROM."
+COPY_DATA_WRITE	"Waiting to write data to file or program during COPY TO."
 COPY_FILE_COPY	"Waiting for a file copy operation."
 COPY_FILE_READ	"Waiting for a read during a file copy operation."
 COPY_FILE_WRITE	"Waiting for a write during a file copy operation."