Allow "in place" tablespaces.

This is a backpatch to branches 10-14 of the following commits:

7170f2159f Allow "in place" tablespaces.
c6f2f01611 Fix pg_basebackup with in-place tablespaces.
f6f0db4d62 Fix pg_tablespace_location() with in-place tablespaces
7a7cd84893 doc: Remove mention to in-place tablespaces for pg_tablespace_location()
5344723755 Remove unnecessary Windows-specific basebackup code.

In-place tablespaces were introduced as a testing helper mechanism, but
they are going to be used for a bugfix in WAL replay to be backpatched
to all stable branches.

I (Álvaro) had to adjust some code to account for lack of
get_dirent_type() in branches prior to 14.

Author: Thomas Munro <thomas.munro@gmail.com>
Author: Michaël Paquier <michael@paquier.xyz>
Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/20220722081858.omhn2in5zt3g4nek@alvherre.pgsql

src/backend/access/transam/xlog.c

@@ -10674,6 +10674,9 @@ do_pg_start_backup(const char *backupidstr, bool fast, TimeLineID *starttli_p,
 			int			rllen;
 			StringInfoData buflinkpath;
 			char	   *s = linkpath;
+#ifndef WIN32
+			struct stat st;
+#endif
 
 			/* Skip special stuff */
 			if (strcmp(de->d_name, ".") == 0 || strcmp(de->d_name, "..") == 0)
@@ -10681,6 +10684,23 @@ do_pg_start_backup(const char *backupidstr, bool fast, TimeLineID *starttli_p,
 
 			snprintf(fullpath, sizeof(fullpath), "pg_tblspc/%s", de->d_name);
 
+			/*
+			 * Skip anything that isn't a symlink/junction.  For testing only,
+			 * we sometimes use allow_in_place_tablespaces to create
+			 * directories directly under pg_tblspc, which would fail below.
+			 */
+#ifndef WIN32
+			if (lstat(fullpath, &st) < 0)
+				ereport(LOG,
+						(errcode_for_file_access(),
+						 errmsg("could not stat file \"%s\": %m",
+								fullpath)));
+			else if (!S_ISLNK(st.st_mode))
+#else			/* WIN32 */
+			if (!pgwin32_is_junction(fullpath))
+#endif
+				continue;
+
 #if defined(HAVE_READLINK) || defined(WIN32)
 			rllen = readlink(fullpath, linkpath, sizeof(linkpath));
 			if (rllen < 0)

src/backend/commands/tablespace.c

@@ -88,6 +88,7 @@
 /* GUC variables */
 char	   *default_tablespace = NULL;
 char	   *temp_tablespaces = NULL;
+bool		allow_in_place_tablespaces = false;
 
 
 static void create_tablespace_directories(const char *location,
@@ -242,6 +243,7 @@ CreateTableSpace(CreateTableSpaceStmt *stmt)
 	char	   *location;
 	Oid			ownerId;
 	Datum		newOptions;
+	bool		in_place;
 
 	/* Must be super user */
 	if (!superuser())
@@ -267,12 +269,15 @@ CreateTableSpace(CreateTableSpaceStmt *stmt)
 				(errcode(ERRCODE_INVALID_NAME),
 				 errmsg("tablespace location cannot contain single quotes")));
 
+	in_place = allow_in_place_tablespaces && strlen(location) == 0;
+
 	/*
 	 * Allowing relative paths seems risky
 	 *
-	 * this also helps us ensure that location is not empty or whitespace
+	 * This also helps us ensure that location is not empty or whitespace,
+	 * unless specifying a developer-only in-place tablespace.
 	 */
-	if (!is_absolute_path(location))
+	if (!in_place && !is_absolute_path(location))
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
 				 errmsg("tablespace location must be an absolute path")));
@@ -593,16 +598,36 @@ create_tablespace_directories(const char *location, const Oid tablespaceoid)
 	char	   *linkloc;
 	char	   *location_with_version_dir;
 	struct stat st;
+	bool		in_place;
 
 	linkloc = psprintf("pg_tblspc/%u", tablespaceoid);
-	location_with_version_dir = psprintf("%s/%s", location,
+
+	/*
+	 * If we're asked to make an 'in place' tablespace, create the directory
+	 * directly where the symlink would normally go.  This is a developer-only
+	 * option for now, to facilitate regression testing.
+	 */
+	in_place = strlen(location) == 0;
+
+	if (in_place)
+	{
+		if (MakePGDirectory(linkloc) < 0 && errno != EEXIST)
+			ereport(ERROR,
+					(errcode_for_file_access(),
+					 errmsg("could not create directory \"%s\": %m",
+							linkloc)));
+	}
+
+	location_with_version_dir = psprintf("%s/%s", in_place ? linkloc : location,
 										 TABLESPACE_VERSION_DIRECTORY);
 
 	/*
 	 * Attempt to coerce target directory to safe permissions.  If this fails,
-	 * it doesn't exist or has the wrong owner.
+	 * it doesn't exist or has the wrong owner.  Not needed for in-place mode,
+	 * because in that case we created the directory with the desired
+	 * permissions.
 	 */
-	if (chmod(location, pg_dir_create_mode) != 0)
+	if (!in_place && chmod(location, pg_dir_create_mode) != 0)
 	{
 		if (errno == ENOENT)
 			ereport(ERROR,
@@ -651,13 +676,13 @@ create_tablespace_directories(const char *location, const Oid tablespaceoid)
 	/*
 	 * In recovery, remove old symlink, in case it points to the wrong place.
 	 */
-	if (InRecovery)
+	if (!in_place && InRecovery)
 		remove_tablespace_symlink(linkloc);
 
 	/*
 	 * Create the symlink under PGDATA
 	 */
-	if (symlink(location, linkloc) < 0)
+	if (!in_place && symlink(location, linkloc) < 0)
 		ereport(ERROR,
 				(errcode_for_file_access(),
 				 errmsg("could not create symbolic link \"%s\": %m",

src/backend/utils/adt/misc.c

@@ -15,6 +15,7 @@
 #include "postgres.h"
 
 #include <sys/file.h>
+#include <sys/stat.h>
 #include <dirent.h>
 #include <fcntl.h>
 #include <math.h>
@@ -307,6 +308,9 @@ pg_tablespace_location(PG_FUNCTION_ARGS)
 	char		sourcepath[MAXPGPATH];
 	char		targetpath[MAXPGPATH];
 	int			rllen;
+#ifndef WIN32
+	struct stat st;
+#endif
 
 	/*
 	 * It's useful to apply this function to pg_class.reltablespace, wherein
@@ -331,6 +335,31 @@ pg_tablespace_location(PG_FUNCTION_ARGS)
 	 */
 	snprintf(sourcepath, sizeof(sourcepath), "pg_tblspc/%u", tablespaceOid);
 
+	/*
+	 * Before reading the link, check if the source path is a link or a
+	 * junction point.  Note that a directory is possible for a tablespace
+	 * created with allow_in_place_tablespaces enabled.  If a directory is
+	 * found, a relative path to the data directory is returned.
+	 */
+#ifdef WIN32
+	if (!pgwin32_is_junction(sourcepath))
+		PG_RETURN_TEXT_P(cstring_to_text(sourcepath));
+#else
+	if (lstat(sourcepath, &st) < 0)
+	{
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not stat file \"%s\": %m",
+						sourcepath)));
+	}
+
+	if (!S_ISLNK(st.st_mode))
+		PG_RETURN_TEXT_P(cstring_to_text(sourcepath));
+#endif
+
+	/*
+	 * In presence of a link or a junction point, return the path pointing to.
+	 */
 	rllen = readlink(sourcepath, targetpath, sizeof(targetpath));
 	if (rllen < 0)
 		ereport(ERROR,

src/backend/utils/misc/guc.c

@@ -38,6 +38,7 @@
 #include "catalog/pg_authid.h"
 #include "commands/async.h"
 #include "commands/prepare.h"
+#include "commands/tablespace.h"
 #include "commands/user.h"
 #include "commands/vacuum.h"
 #include "commands/variable.h"
@@ -1792,6 +1793,17 @@ static struct config_bool ConfigureNamesBool[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"allow_in_place_tablespaces", PGC_SUSET, DEVELOPER_OPTIONS,
+			gettext_noop("Allows tablespaces directly inside pg_tblspc, for testing."),
+			NULL,
+			GUC_NOT_IN_SAMPLE
+		},
+		&allow_in_place_tablespaces,
+		false,
+		NULL, NULL, NULL
+	},
+
 	{
 		{"lo_compat_privileges", PGC_SUSET, COMPAT_OPTIONS_PREVIOUS,
 			gettext_noop("Enables backward compatibility mode for privilege checks on large objects."),

src/include/commands/tablespace.h

@@ -19,6 +19,8 @@
 #include "lib/stringinfo.h"
 #include "nodes/parsenodes.h"
 
+extern bool allow_in_place_tablespaces;
+
 /* XLOG stuff */
 #define XLOG_TBLSPC_CREATE		0x00
 #define XLOG_TBLSPC_DROP		0x10