Fix pg_tablespace_location() with in-place tablespaces

Using this system function with an in-place tablespace (created when
allow_in_place_tablespaces is enabled by specifying an empty string as
location) caused a failure when using readlink(), as the tablespace is,
in this case, not a symbolic link in pg_tblspc/ but a directory.

Rather than getting a failure, the commit changes
pg_tablespace_location() so as a relative path to the data directory is
returned for in-place tablespaces, to make a difference between
tablespaces created when allow_in_place_tablespaces is enabled or not.
Getting a path rather than an empty string that would match the CREATE
TABLESPACE command in this case is more useful for tests that would like
to rely on this function.

While on it, a regression test is added for this case.  This is simple
to add in the main regression test suite thanks to regexp_replace() to
mask the part of the tablespace location dependent on its OID.

Author: Michael Paquier
Reviewed-by: Kyotaro Horiguchi, Thomas Munro
Discussion: https://postgr.es/m/YiG1RleON1WBcLnX@paquier.xyz

doc/src/sgml/func.sgml

@@ -23924,7 +23924,13 @@ SELECT currval(pg_get_serial_sequence('sometable', 'id'));
        </para>
        <para>
         Returns the file system path that this tablespace is located in.
-       </para></entry>
+       </para>
+       <para>
+        A relative path to the data directory is returned for tablespaces
+        created when <xref linkend="guc-allow-in-place-tablespaces"/> is
+        enabled.
+       </para>
+       </entry>
       </row>
 
       <row>

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>
@@ -282,6 +283,9 @@ 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
@@ -306,6 +310,31 @@ 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/test/regress/expected/tablespace.out

@@ -24,6 +24,15 @@ SELECT spcoptions FROM pg_tablespace WHERE spcname = 'regress_tblspacewith';
 DROP TABLESPACE regress_tblspacewith;
 -- create a tablespace we can use
 CREATE TABLESPACE regress_tblspace LOCATION '';
+-- This returns a relative path as of an effect of allow_in_place_tablespaces,
+-- masking the tablespace OID used in the path name.
+SELECT regexp_replace(pg_tablespace_location(oid), '(pg_tblspc)/(\d+)', '\1/NNN')
+  FROM pg_tablespace  WHERE spcname = 'regress_tblspace';
+ regexp_replace 
+----------------
+ pg_tblspc/NNN
+(1 row)
+
 -- try setting and resetting some properties for the new tablespace
 ALTER TABLESPACE regress_tblspace SET (random_page_cost = 1.0, seq_page_cost = 1.1);
 ALTER TABLESPACE regress_tblspace SET (some_nonexistent_parameter = true);  -- fail

src/test/regress/sql/tablespace.sql

@@ -22,6 +22,10 @@ DROP TABLESPACE regress_tblspacewith;
 
 -- create a tablespace we can use
 CREATE TABLESPACE regress_tblspace LOCATION '';
+-- This returns a relative path as of an effect of allow_in_place_tablespaces,
+-- masking the tablespace OID used in the path name.
+SELECT regexp_replace(pg_tablespace_location(oid), '(pg_tblspc)/(\d+)', '\1/NNN')
+  FROM pg_tablespace  WHERE spcname = 'regress_tblspace';
 
 -- try setting and resetting some properties for the new tablespace
 ALTER TABLESPACE regress_tblspace SET (random_page_cost = 1.0, seq_page_cost = 1.1);