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

1 files changed, 29 insertions, 0 deletions

diff --git a/src/backend/utils/adt/misc.c b/src/backend/utils/adt/misc.c
index 4568749d230..89690be2ed1 100644
--- a/src/backend/utils/adt/misc.c
+++ b/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,