Add support for piping COPY to/from an external program.

This includes backend "COPY TO/FROM PROGRAM '...'" syntax, and corresponding
psql \copy syntax. Like with reading/writing files, the backend version is
superuser-only, and in the psql version, the program is run in the client.

In the passing, the psql \copy STDIN/STDOUT syntax is subtly changed: if you
the stdin/stdout is quoted, it's now interpreted as a filename. For example,
"\copy foo from 'stdin'" now reads from a file called 'stdin', not from
standard input. Before this, there was no way to specify a filename called
stdin, stdout, pstdin or pstdout.

This creates a new function in pgport, wait_result_to_str(), which can
be used to convert the exit status of a process, as returned by wait(3),
to a human-readable string.

Etsuro Fujita, reviewed by Amit Kapila.

src/include/commands/copy.h

@@ -26,7 +26,7 @@ extern Oid DoCopy(const CopyStmt *stmt, const char *queryString,
 
 extern void ProcessCopyOptions(CopyState cstate, bool is_from, List *options);
 extern CopyState BeginCopyFrom(Relation rel, const char *filename,
-			  List *attnamelist, List *options);
+			  bool is_program, List *attnamelist, List *options);
 extern void EndCopyFrom(CopyState cstate);
 extern bool NextCopyFrom(CopyState cstate, ExprContext *econtext,
 			 Datum *values, bool *nulls, Oid *tupleOid);

src/include/nodes/parsenodes.h

@@ -1407,6 +1407,7 @@ typedef struct CopyStmt
 	List	   *attlist;		/* List of column names (as Strings), or NIL
 								 * for all columns */
 	bool		is_from;		/* TO or FROM */
+	bool		is_program;		/* is 'filename' a program to popen? */
 	char	   *filename;		/* filename, or NULL for STDIN/STDOUT */
 	List	   *options;		/* List of DefElem nodes */
 } CopyStmt;

src/include/parser/kwlist.h

@@ -292,6 +292,7 @@ PG_KEYWORD("prior", PRIOR, UNRESERVED_KEYWORD)
 PG_KEYWORD("privileges", PRIVILEGES, UNRESERVED_KEYWORD)
 PG_KEYWORD("procedural", PROCEDURAL, UNRESERVED_KEYWORD)
 PG_KEYWORD("procedure", PROCEDURE, UNRESERVED_KEYWORD)
+PG_KEYWORD("program", PROGRAM, UNRESERVED_KEYWORD)
 PG_KEYWORD("quote", QUOTE, UNRESERVED_KEYWORD)
 PG_KEYWORD("range", RANGE, UNRESERVED_KEYWORD)
 PG_KEYWORD("read", READ, UNRESERVED_KEYWORD)

src/include/port.h

@@ -465,4 +465,7 @@ extern int	pg_mkdir_p(char *path, int omode);
 /* port/quotes.c */
 extern char *escape_single_quotes_ascii(const char *src);
 
+/* port/wait_error.c */
+extern char *wait_result_to_str(int exit_status);
+
 #endif   /* PG_PORT_H */

src/include/storage/fd.h

@@ -80,6 +80,10 @@ extern char *FilePathName(File file);
 extern FILE *AllocateFile(const char *name, const char *mode);
 extern int	FreeFile(FILE *file);
 
+/* Operations that allow use of pipe streams (popen/pclose) */
+extern FILE *OpenPipeStream(const char *command, const char *mode);
+extern int	ClosePipeStream(FILE *file);
+
 /* Operations to allow use of the <dirent.h> library routines */
 extern DIR *AllocateDir(const char *dirname);
 extern struct dirent *ReadDir(DIR *dir, const char *dirname);