pg_rewind: Refactor the abstraction to fetch from local/libpq source.

This makes the abstraction of a "source" server more clear, by introducing
a common abstract class, borrowing the object-oriented programming term,
that represents all the operations that can be done on the source server.
There are two implementations of it, one for fetching via libpq, and
another to fetch from a local directory. This adds some code, but makes it
easier to understand what's going on.

The copy_executeFileMap() and libpq_executeFileMap() functions contained
basically the same logic, just calling different functions to fetch the
source files. Refactor so that the common logic is in one place, in a new
function called perform_rewind().

Reviewed-by: Kyotaro Horiguchi, Soumyadeep Chakraborty
Discussion: https://www.postgresql.org/message-id/0c5b3783-af52-3ee5-f8fa-6e794061f70d%40iki.fi

src/bin/pg_rewind/local_source.c

@@ -0,0 +1,131 @@
+/*-------------------------------------------------------------------------
+ *
+ * local_source.c
+ *	  Functions for using a local data directory as the source.
+ *
+ * Portions Copyright (c) 2013-2020, PostgreSQL Global Development Group
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres_fe.h"
+
+#include <fcntl.h>
+#include <unistd.h>
+
+#include "datapagemap.h"
+#include "file_ops.h"
+#include "filemap.h"
+#include "pg_rewind.h"
+#include "rewind_source.h"
+
+typedef struct
+{
+	rewind_source common;		/* common interface functions */
+
+	const char *datadir;		/* path to the source data directory */
+} local_source;
+
+static void local_traverse_files(rewind_source *source,
+								 process_file_callback_t callback);
+static char *local_fetch_file(rewind_source *source, const char *path,
+							  size_t *filesize);
+static void local_fetch_file_range(rewind_source *source, const char *path,
+								   off_t off, size_t len);
+static void local_finish_fetch(rewind_source *source);
+static void local_destroy(rewind_source *source);
+
+rewind_source *
+init_local_source(const char *datadir)
+{
+	local_source *src;
+
+	src = pg_malloc0(sizeof(local_source));
+
+	src->common.traverse_files = local_traverse_files;
+	src->common.fetch_file = local_fetch_file;
+	src->common.queue_fetch_range = local_fetch_file_range;
+	src->common.finish_fetch = local_finish_fetch;
+	src->common.get_current_wal_insert_lsn = NULL;
+	src->common.destroy = local_destroy;
+
+	src->datadir = datadir;
+
+	return &src->common;
+}
+
+static void
+local_traverse_files(rewind_source *source, process_file_callback_t callback)
+{
+	traverse_datadir(((local_source *) source)->datadir, &process_source_file);
+}
+
+static char *
+local_fetch_file(rewind_source *source, const char *path, size_t *filesize)
+{
+	return slurpFile(((local_source *) source)->datadir, path, filesize);
+}
+
+/*
+ * Copy a file from source to target, starting at 'off', for 'len' bytes.
+ */
+static void
+local_fetch_file_range(rewind_source *source, const char *path, off_t off,
+					   size_t len)
+{
+	const char *datadir = ((local_source *) source)->datadir;
+	PGAlignedBlock buf;
+	char		srcpath[MAXPGPATH];
+	int			srcfd;
+	off_t		begin = off;
+	off_t		end = off + len;
+
+	snprintf(srcpath, sizeof(srcpath), "%s/%s", datadir, path);
+
+	srcfd = open(srcpath, O_RDONLY | PG_BINARY, 0);
+	if (srcfd < 0)
+		pg_fatal("could not open source file \"%s\": %m",
+				 srcpath);
+
+	if (lseek(srcfd, begin, SEEK_SET) == -1)
+		pg_fatal("could not seek in source file: %m");
+
+	open_target_file(path, false);
+
+	while (end - begin > 0)
+	{
+		ssize_t		readlen;
+		size_t		len;
+
+		if (end - begin > sizeof(buf))
+			len = sizeof(buf);
+		else
+			len = end - begin;
+
+		readlen = read(srcfd, buf.data, len);
+
+		if (readlen < 0)
+			pg_fatal("could not read file \"%s\": %m", srcpath);
+		else if (readlen == 0)
+			pg_fatal("unexpected EOF while reading file \"%s\"", srcpath);
+
+		write_target_range(buf.data, begin, readlen);
+		begin += readlen;
+	}
+
+	if (close(srcfd) != 0)
+		pg_fatal("could not close file \"%s\": %m", srcpath);
+}
+
+static void
+local_finish_fetch(rewind_source *source)
+{
+	/*
+	 * Nothing to do, local_fetch_file_range() copies the ranges immediately.
+	 */
+}
+
+static void
+local_destroy(rewind_source *source)
+{
+	pfree(source);
+}