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/backend/storage/file/fd.c

@@ -39,13 +39,13 @@
  * for a long time, like relation files. It is the caller's responsibility
  * to close them, there is no automatic mechanism in fd.c for that.
  *
- * AllocateFile, AllocateDir and OpenTransientFile are wrappers around
- * fopen(3), opendir(3), and open(2), respectively. They behave like the
- * corresponding native functions, except that the handle is registered with
- * the current subtransaction, and will be automatically closed at abort.
- * These are intended for short operations like reading a configuration file,
- * and there is a fixed limit on the number of files that can be opened using
- * these functions at any one time.
+ * AllocateFile, AllocateDir, OpenPipeStream and OpenTransientFile are
+ * wrappers around fopen(3), opendir(3), popen(3) and open(2), respectively.
+ * They behave like the corresponding native functions, except that the handle
+ * is registered with the current subtransaction, and will be automatically
+ * closed at abort. These are intended for short operations like reading a
+ * configuration file, and there is a fixed limit on the number of files that
+ * can be opened using these functions at any one time.
  *
  * Finally, BasicOpenFile is just a thin wrapper around open() that can
  * release file descriptors in use by the virtual file descriptors if
@@ -202,6 +202,7 @@ static uint64 temporary_files_size = 0;
 typedef enum
 {
 	AllocateDescFile,
+	AllocateDescPipe,
 	AllocateDescDir,
 	AllocateDescRawFD
 } AllocateDescKind;
@@ -1585,6 +1586,61 @@ OpenTransientFile(FileName fileName, int fileFlags, int fileMode)
 	return -1;					/* failure */
 }
 
+/*
+ * Routines that want to initiate a pipe stream should use OpenPipeStream
+ * rather than plain popen().  This lets fd.c deal with freeing FDs if
+ * necessary.  When done, call ClosePipeStream rather than pclose.
+ */
+FILE *
+OpenPipeStream(const char *command, const char *mode)
+{
+	FILE	   *file;
+
+	DO_DB(elog(LOG, "OpenPipeStream: Allocated %d (%s)",
+			   numAllocatedDescs, command));
+
+	/*
+	 * The test against MAX_ALLOCATED_DESCS prevents us from overflowing
+	 * allocatedFiles[]; the test against max_safe_fds prevents AllocateFile
+	 * from hogging every one of the available FDs, which'd lead to infinite
+	 * looping.
+	 */
+	if (numAllocatedDescs >= MAX_ALLOCATED_DESCS ||
+		numAllocatedDescs >= max_safe_fds - 1)
+		elog(ERROR, "exceeded MAX_ALLOCATED_DESCS while trying to execute command \"%s\"",
+			 command);
+
+TryAgain:
+	fflush(stdout);
+	fflush(stderr);
+	errno = 0;
+	if ((file = popen(command, mode)) != NULL)
+	{
+		AllocateDesc *desc = &allocatedDescs[numAllocatedDescs];
+
+		desc->kind = AllocateDescPipe;
+		desc->desc.file = file;
+		desc->create_subid = GetCurrentSubTransactionId();
+		numAllocatedDescs++;
+		return desc->desc.file;
+	}
+
+	if (errno == EMFILE || errno == ENFILE)
+	{
+		int			save_errno = errno;
+
+		ereport(LOG,
+				(errcode(ERRCODE_INSUFFICIENT_RESOURCES),
+				 errmsg("out of file descriptors: %m; release and retry")));
+		errno = 0;
+		if (ReleaseLruFile())
+			goto TryAgain;
+		errno = save_errno;
+	}
+
+	return NULL;
+}
+
 /*
  * Free an AllocateDesc of any type.
  *
@@ -1601,6 +1657,9 @@ FreeDesc(AllocateDesc *desc)
 		case AllocateDescFile:
 			result = fclose(desc->desc.file);
 			break;
+		case AllocateDescPipe:
+			result = pclose(desc->desc.file);
+			break;
 		case AllocateDescDir:
 			result = closedir(desc->desc.dir);
 			break;
@@ -1814,6 +1873,31 @@ FreeDir(DIR *dir)
 }
 
 
+/*
+ * Close a pipe stream returned by OpenPipeStream.
+ */
+int
+ClosePipeStream(FILE *file)
+{
+	int			i;
+
+	DO_DB(elog(LOG, "ClosePipeStream: Allocated %d", numAllocatedDescs));
+
+	/* Remove file from list of allocated files, if it's present */
+	for (i = numAllocatedDescs; --i >= 0;)
+	{
+		AllocateDesc *desc = &allocatedDescs[i];
+
+		if (desc->kind == AllocateDescPipe && desc->desc.file == file)
+			return FreeDesc(desc);
+	}
+
+	/* Only get here if someone passes us a file not in allocatedDescs */
+	elog(WARNING, "file passed to ClosePipeStream was not obtained from OpenPipeStream");
+
+	return pclose(file);
+}
+
 /*
  * closeAllVfds
  *