pg_upgrade: fail early if a tablespace dir already exists for new cluster version

[Justin Pryzby]

This should be caught during --check, or earlier in the upgrade, rather than
only after dumping the schema.

Typically, the tablespace dir would be left behind by a previous failed upgrade
attempt, causing a cascade of failured upgrades.  I guess I may not be the only
one with a 3 year old wrapper which has a hack to check for this.

|rm -fr pgsql12.dat pgsql13.dat
|/usr/pgsql-12/bin/initdb -D pgsql12.dat --no-sync
|/usr/pgsql-13/bin/initdb -D pgsql13.dat --no-sync
|/usr/pgsql-12/bin/postgres -D pgsql12.dat -c port=5678 -k /tmp
|mkdir tblspc tblspc/PG_13_202007203
|psql -h /tmp -p 5678 postgres -c "CREATE TABLESPACE one LOCATION 
'/home/pryzbyj/tblspc'"
|/usr/pgsql-13/bin/pg_upgrade -D pgsql13.dat -d pgsql12.dat -b /usr/pgsql-12/bin
|pg_upgrade_utility.log:
|CREATE TABLESPACE "one" OWNER "pryzbyj" LOCATION '/home/pryzbyj/tblspc';
|psql:pg_upgrade_dump_globals.sql:27: ERROR:  directory 
"/home/pryzbyj/tblspc/PG_13_202007201" already in use as a tablespace

>From 3df104610d73833da60bffcc54756a6ecb2f390f Mon Sep 17 00:00:00 2001
From: Justin Pryzby <pryz...@telsasoft.com>
Date: Thu, 24 Sep 2020 19:49:40 -0500
Subject: [PATCH v1] pg_upgrade --check to avoid tablespace failure mode

---
 src/bin/pg_upgrade/check.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/src/bin/pg_upgrade/check.c b/src/bin/pg_upgrade/check.c
index 2f7aa632c5..b07e63dab2 100644
--- a/src/bin/pg_upgrade/check.c
+++ b/src/bin/pg_upgrade/check.c
@@ -13,6 +13,7 @@
 #include "fe_utils/string_utils.h"
 #include "mb/pg_wchar.h"
 #include "pg_upgrade.h"
+#include "common/relpath.h"
 
 static void check_new_cluster_is_empty(void);
 static void check_databases_are_compatible(void);
@@ -27,6 +28,7 @@ static void check_for_tables_with_oids(ClusterInfo *cluster);
 static void check_for_reg_data_type_usage(ClusterInfo *cluster);
 static void check_for_jsonb_9_4_usage(ClusterInfo *cluster);
 static void check_for_pg_role_prefix(ClusterInfo *cluster);
+static void check_for_existing_tablespace_dirs(ClusterInfo *new_cluster);
 static char *get_canonical_locale_name(int category, const char *locale);
 
 
@@ -187,6 +189,8 @@ check_new_cluster(void)
 	check_is_install_user(&new_cluster);
 
 	check_for_prepared_transactions(&new_cluster);
+
+	check_for_existing_tablespace_dirs(&new_cluster);
 }
 
 
@@ -541,6 +545,34 @@ create_script_for_cluster_analyze(char **analyze_script_file_name)
 	check_ok();
 }
 
+/*
+ * Check that tablespace dirs for new cluster do not already exist.
+ * If they did, they'd cause an error while restoring global objects.
+ * This allows failing earlier rather than only after dumping schema.
+ */
+static void
+check_for_existing_tablespace_dirs(ClusterInfo *new_cluster)
+{
+	char	old_tablespace_dir[MAXPGPATH];
+
+	prep_status("Checking for pre-existing tablespace directories for new cluster version");
+
+// if (GET_MAJOR_VERSION(old_cluster.major_version) <= 804)
+	for (int tblnum = 0; tblnum < os_info.num_old_tablespaces; tblnum++)
+	{
+		struct stat statbuf;
+		snprintf(old_tablespace_dir, MAXPGPATH, "%s%c%s",
+				os_info.old_tablespaces[tblnum],
+				PATH_SEPARATOR, TABLESPACE_VERSION_DIRECTORY);
+
+		canonicalize_path(old_tablespace_dir);
+
+		// XXX: lstat ?
+		if (stat(old_tablespace_dir, &statbuf) == 0)
+			pg_fatal("tablespace directory already exists for new cluster version: \"%s\"\n",
+					 old_tablespace_dir);
+	}
+}
 
 /*
  * create_script_for_old_cluster_deletion()
-- 
2.17.0