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

1 files changed, 128 insertions, 5 deletions

diff --git a/src/bin/pg_rewind/file_ops.c b/src/bin/pg_rewind/file_ops.c
index ec37d0b2e0d..065368a2208 100644
--- a/src/bin/pg_rewind/file_ops.c
+++ b/src/bin/pg_rewind/file_ops.c
@@ -15,6 +15,7 @@
 #include "postgres_fe.h"
 
 #include <sys/stat.h>
+#include <dirent.h>
 #include <fcntl.h>
 #include <unistd.h>
 
@@ -35,6 +36,9 @@ static void remove_target_dir(const char *path);
 static void create_target_symlink(const char *path, const char *link);
 static void remove_target_symlink(const char *path);
 
+static void recurse_dir(const char *datadir, const char *parentpath,
+						process_file_callback_t callback);
+
 /*
  * Open a target file for writing. If 'trunc' is true and the file already
  * exists, it will be truncated.
@@ -83,7 +87,7 @@ close_target_file(void)
 void
 write_target_range(char *buf, off_t begin, size_t size)
 {
-	int			writeleft;
+	size_t		writeleft;
 	char	   *p;
 
 	/* update progress report */
@@ -101,7 +105,7 @@ write_target_range(char *buf, off_t begin, size_t size)
 	p = buf;
 	while (writeleft > 0)
 	{
-		int			writelen;
+		ssize_t		writelen;
 
 		errno = 0;
 		writelen = write(dstfd, p, writeleft);
@@ -305,9 +309,6 @@ sync_target_dir(void)
  * buffer is actually *filesize + 1. That's handy when reading a text file.
  * This function can be used to read binary files as well, you can just
  * ignore the zero-terminator in that case.
- *
- * This function is used to implement the fetchFile function in the "fetch"
- * interface (see fetch.c), but is also called directly.
  */
 char *
 slurpFile(const char *datadir, const char *path, size_t *filesize)
@@ -352,3 +353,125 @@ slurpFile(const char *datadir, const char *path, size_t *filesize)
 		*filesize = len;
 	return buffer;
 }
+
+/*
+ * Traverse through all files in a data directory, calling 'callback'
+ * for each file.
+ */
+void
+traverse_datadir(const char *datadir, process_file_callback_t callback)
+{
+	recurse_dir(datadir, NULL, callback);
+}
+
+/*
+ * recursive part of traverse_datadir
+ *
+ * parentpath is the current subdirectory's path relative to datadir,
+ * or NULL at the top level.
+ */
+static void
+recurse_dir(const char *datadir, const char *parentpath,
+			process_file_callback_t callback)
+{
+	DIR		   *xldir;
+	struct dirent *xlde;
+	char		fullparentpath[MAXPGPATH];
+
+	if (parentpath)
+		snprintf(fullparentpath, MAXPGPATH, "%s/%s", datadir, parentpath);
+	else
+		snprintf(fullparentpath, MAXPGPATH, "%s", datadir);
+
+	xldir = opendir(fullparentpath);
+	if (xldir == NULL)
+		pg_fatal("could not open directory \"%s\": %m",
+				 fullparentpath);
+
+	while (errno = 0, (xlde = readdir(xldir)) != NULL)
+	{
+		struct stat fst;
+		char		fullpath[MAXPGPATH * 2];
+		char		path[MAXPGPATH * 2];
+
+		if (strcmp(xlde->d_name, ".") == 0 ||
+			strcmp(xlde->d_name, "..") == 0)
+			continue;
+
+		snprintf(fullpath, sizeof(fullpath), "%s/%s", fullparentpath, xlde->d_name);
+
+		if (lstat(fullpath, &fst) < 0)
+		{
+			if (errno == ENOENT)
+			{
+				/*
+				 * File doesn't exist anymore. This is ok, if the new primary
+				 * is running and the file was just removed. If it was a data
+				 * file, there should be a WAL record of the removal. If it
+				 * was something else, it couldn't have been anyway.
+				 *
+				 * TODO: But complain if we're processing the target dir!
+				 */
+			}
+			else
+				pg_fatal("could not stat file \"%s\": %m",
+						 fullpath);
+		}
+
+		if (parentpath)
+			snprintf(path, sizeof(path), "%s/%s", parentpath, xlde->d_name);
+		else
+			snprintf(path, sizeof(path), "%s", xlde->d_name);
+
+		if (S_ISREG(fst.st_mode))
+			callback(path, FILE_TYPE_REGULAR, fst.st_size, NULL);
+		else if (S_ISDIR(fst.st_mode))
+		{
+			callback(path, FILE_TYPE_DIRECTORY, 0, NULL);
+			/* recurse to handle subdirectories */
+			recurse_dir(datadir, path, callback);
+		}
+#ifndef WIN32
+		else if (S_ISLNK(fst.st_mode))
+#else
+		else if (pgwin32_is_junction(fullpath))
+#endif
+		{
+#if defined(HAVE_READLINK) || defined(WIN32)
+			char		link_target[MAXPGPATH];
+			int			len;
+
+			len = readlink(fullpath, link_target, sizeof(link_target));
+			if (len < 0)
+				pg_fatal("could not read symbolic link \"%s\": %m",
+						 fullpath);
+			if (len >= sizeof(link_target))
+				pg_fatal("symbolic link \"%s\" target is too long",
+						 fullpath);
+			link_target[len] = '\0';
+
+			callback(path, FILE_TYPE_SYMLINK, 0, link_target);
+
+			/*
+			 * If it's a symlink within pg_tblspc, we need to recurse into it,
+			 * to process all the tablespaces.  We also follow a symlink if
+			 * it's for pg_wal.  Symlinks elsewhere are ignored.
+			 */
+			if ((parentpath && strcmp(parentpath, "pg_tblspc") == 0) ||
+				strcmp(path, "pg_wal") == 0)
+				recurse_dir(datadir, path, callback);
+#else
+			pg_fatal("\"%s\" is a symbolic link, but symbolic links are not supported on this platform",
+					 fullpath);
+#endif							/* HAVE_READLINK */
+		}
+	}
+
+	if (errno)
+		pg_fatal("could not read directory \"%s\": %m",
+				 fullparentpath);
+
+	if (closedir(xldir))
+		pg_fatal("could not close directory \"%s\": %m",
+				 fullparentpath);
+}