Split copy.c into four files.

Copy.c has grown really large. Split it into more manageable parts:

- copy.c now contains only a few functions that are common to COPY FROM
  and COPY TO.

- copyto.c contains code for COPY TO.

- copyfrom.c contains code for initializing COPY FROM, and inserting the
  tuples to the correct table.

- copyfromparse.c contains code for reading from the client/file/program,
  and parsing the input text/CSV/binary format into tuples.

All of these parts are fairly complicated, and fairly independent of each
other. There is a patch being discussed to implement parallel COPY FROM,
which will add a lot of new code to the COPY FROM path, and another patch
which would allow INSERTs to use the same multi-insert machinery as COPY
FROM, both of which will require refactoring that code. With those two
patches, there's going to be a lot of code churn in copy.c anyway, so now
seems like a good time to do this refactoring.

The CopyStateData struct is also split. All the formatting options, like
FORMAT, QUOTE, ESCAPE, are put in a new CopyFormatOption struct, which
is used by both COPY FROM and TO. Other state data are kept in separate
CopyFromStateData and CopyToStateData structs.

Reviewed-by: Soumyadeep Chakraborty, Erik Rijkers, Vignesh C, Andres Freund
Discussion: https://www.postgresql.org/message-id/8e15b560-f387-7acc-ac90-763986617bfb%40iki.fi

Author: hlinnaka

contrib/file_fdw/file_fdw.c

@@ -105,7 +105,7 @@ typedef struct FileFdwExecutionState
 	bool		is_program;		/* true if filename represents an OS command */
 	List	   *options;		/* merged COPY options, excluding filename and
 								 * is_program */
-	CopyState	cstate;			/* COPY execution state */
+	CopyFromState cstate;		/* COPY execution state */
 } FileFdwExecutionState;
 
 /*
@@ -655,7 +655,7 @@ fileBeginForeignScan(ForeignScanState *node, int eflags)
 	char	   *filename;
 	bool		is_program;
 	List	   *options;
-	CopyState	cstate;
+	CopyFromState cstate;
 	FileFdwExecutionState *festate;
 
 	/*
@@ -677,6 +677,7 @@ fileBeginForeignScan(ForeignScanState *node, int eflags)
 	 */
 	cstate = BeginCopyFrom(NULL,
 						   node->ss.ss_currentRelation,
+						   NULL,
 						   filename,
 						   is_program,
 						   NULL,
@@ -752,6 +753,7 @@ fileReScanForeignScan(ForeignScanState *node)
 
 	festate->cstate = BeginCopyFrom(NULL,
 									node->ss.ss_currentRelation,
+									NULL,
 									festate->filename,
 									festate->is_program,
 									NULL,
@@ -1107,7 +1109,7 @@ file_acquire_sample_rows(Relation onerel, int elevel,
 	char	   *filename;
 	bool		is_program;
 	List	   *options;
-	CopyState	cstate;
+	CopyFromState cstate;
 	ErrorContextCallback errcallback;
 	MemoryContext oldcontext = CurrentMemoryContext;
 	MemoryContext tupcontext;
@@ -1125,7 +1127,7 @@ file_acquire_sample_rows(Relation onerel, int elevel,
 	/*
 	 * Create CopyState from FDW options.
 	 */
-	cstate = BeginCopyFrom(NULL, onerel, filename, is_program, NULL, NIL,
+	cstate = BeginCopyFrom(NULL, onerel, NULL, filename, is_program, NULL, NIL,
 						   options);
 
 	/*

src/backend/commands/Makefile

@@ -24,6 +24,9 @@ OBJS = \
 	constraint.o \
 	conversioncmds.o \
 	copy.o \
+	copyfrom.o \
+	copyfromparse.o \
+	copyto.o \
 	createas.o \
 	dbcommands.o \
 	define.o \