Allow "in place" tablespaces.

Provide a developer-only GUC allow_in_place_tablespaces, disabled by
default.  When enabled, tablespaces can be created with an empty
LOCATION string, meaning that they should be created as a directory
directly beneath pg_tblspc.  This can be used for new testing scenarios,
in a follow-up patch.  Not intended for end-user usage, since it might
confuse backup tools that expect symlinks.

Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/CA%2BhUKGKpRWQ9SxdxxDmTBCJoR0YnFpMBe7kyzY8SUQk%2BHeskxg%40mail.gmail.com

doc/src/sgml/config.sgml

@@ -10433,6 +10433,25 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
     </para>
 
     <variablelist>
+     <varlistentry id="guc-allow-in-place-tablespaces" xreflabel="allow_in_place_tablespaces">
+      <term><varname>allow_in_place_tablespaces</varname> (<type>boolean</type>)
+      <indexterm>
+        <primary><varname>allow_in_place_tablespaces</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Allows tablespaces to be created as directories inside
+        <filename>pg_tblspc</filename>, when an empty location string
+        is provided to the <command>CREATE TABLESPACE</command> command.  This
+        is intended to allow testing replication scenarios where primary and
+        standby servers are running on the same machine.  Such directories
+        are likely to confuse backup tools that expect to find only symbolic
+        links in that location.  Only superusers can change this setting.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-allow-system-table-mods" xreflabel="allow_system_table_mods">
       <term><varname>allow_system_table_mods</varname> (<type>boolean</type>)
       <indexterm>

src/backend/commands/tablespace.c

@@ -87,6 +87,7 @@
 /* GUC variables */
 char	   *default_tablespace = NULL;
 char	   *temp_tablespaces = NULL;
+bool		allow_in_place_tablespaces = false;
 
 
 static void create_tablespace_directories(const char *location,
@@ -241,6 +242,7 @@ CreateTableSpace(CreateTableSpaceStmt *stmt)
 	char	   *location;
 	Oid			ownerId;
 	Datum		newOptions;
+	bool		in_place;
 
 	/* Must be superuser */
 	if (!superuser())
@@ -266,12 +268,15 @@ 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")));
@@ -590,16 +595,36 @@ 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,
@@ -648,13 +673,13 @@ 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/misc/guc.c

@@ -46,6 +46,7 @@
 #include "catalog/storage.h"
 #include "commands/async.h"
 #include "commands/prepare.h"
+#include "commands/tablespace.h"
 #include "commands/trigger.h"
 #include "commands/user.h"
 #include "commands/vacuum.h"
@@ -1961,6 +1962,17 @@ 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