Prevent pg_dist_node changes during master_create_empty_shard

src/backend/distributed/master/master_stage_protocol.c

@@ -44,6 +44,7 @@
 #include "distributed/transaction_management.h"
 #include "distributed/worker_manager.h"
 #include "distributed/worker_protocol.h"
+#include "storage/lmgr.h"
 #include "utils/builtins.h"
 #include "utils/fmgroids.h"
 #include "utils/inval.h"
@@ -94,6 +95,12 @@ master_create_empty_shard(PG_FUNCTION_ARGS)
 	EnsureTablePermissions(relationId, ACL_INSERT);
 	CheckDistributedTable(relationId);
 
+	/* don't allow the table to be dropped */
+	LockRelationOid(relationId, AccessShareLock);
+
+	/* don't allow concurrent node list changes that require an exclusive lock */
+	LockRelationOid(DistNodeRelationId(), RowShareLock);
+
 	/*
 	 * We check whether the table is a foreign table or not. If it is, we set
 	 * storage type as foreign also. Only exception is if foreign table is a

src/test/regress/expected/isolation_create_table_vs_add_remove_node.out

@@ -364,7 +364,7 @@ master_remove_node
 
                
 
-starting permutation: s1-add-node-2 s1-begin s2-create-table-2 s1-remove-node-2 s1-commit s2-select
+starting permutation: s1-add-node-2 s2-begin s2-create-table-2 s1-remove-node-2 s2-commit s2-select
 node_name      node_port      
 
 localhost      57637          
@@ -374,7 +374,7 @@ step s1-add-node-2:
 ?column?       
 
 1              
-step s1-begin: 
+step s2-begin: 
 	BEGIN;
 
 step s2-create-table-2: 
@@ -388,11 +388,12 @@ create_distributed_table
                
 step s1-remove-node-2: 
 	SELECT * FROM master_remove_node('localhost', 57638);
-
+ <waiting ...>
-ERROR:  you cannot remove the primary node of a node group which has shard placements
+step s2-commit: 
-step s1-commit: 
 	COMMIT;
 
+step s1-remove-node-2: <... completed>
+error in steps s2-commit s1-remove-node-2: ERROR:  you cannot remove the primary node of a node group which has shard placements
 step s2-select: 
 	SELECT * FROM dist_table;
 
@@ -402,3 +403,91 @@ master_remove_node
 
                
                
+
+starting permutation: s1-add-node-2 s1-begin s1-remove-node-2 s2-create-append-table s1-commit s2-select
+node_name      node_port      
+
+localhost      57637          
+step s1-add-node-2: 
+	SELECT 1 FROM master_add_node('localhost', 57638);
+
+?column?       
+
+1              
+step s1-begin: 
+	BEGIN;
+
+step s1-remove-node-2: 
+	SELECT * FROM master_remove_node('localhost', 57638);
+
+master_remove_node
+
+               
+step s2-create-append-table: 
+	SET citus.shard_replication_factor TO 1;
+	CREATE TABLE dist_table (x int, y int);
+	SELECT create_distributed_table('dist_table', 'x', 'append');
+	SELECT 1 FROM master_create_empty_shard('dist_table');
+ <waiting ...>
+step s1-commit: 
+	COMMIT;
+
+step s2-create-append-table: <... completed>
+create_distributed_table
+
+               
+?column?       
+
+1              
+step s2-select: 
+	SELECT * FROM dist_table;
+
+x              y              
+
+master_remove_node
+
+               
+
+starting permutation: s1-add-node-2 s2-begin s2-create-append-table s1-remove-node-2 s2-commit s2-select
+node_name      node_port      
+
+localhost      57637          
+step s1-add-node-2: 
+	SELECT 1 FROM master_add_node('localhost', 57638);
+
+?column?       
+
+1              
+step s2-begin: 
+	BEGIN;
+
+step s2-create-append-table: 
+	SET citus.shard_replication_factor TO 1;
+	CREATE TABLE dist_table (x int, y int);
+	SELECT create_distributed_table('dist_table', 'x', 'append');
+	SELECT 1 FROM master_create_empty_shard('dist_table');
+
+create_distributed_table
+
+               
+?column?       
+
+1              
+step s1-remove-node-2: 
+	SELECT * FROM master_remove_node('localhost', 57638);
+ <waiting ...>
+step s2-commit: 
+	COMMIT;
+
+step s1-remove-node-2: <... completed>
+master_remove_node
+
+               
+step s2-select: 
+	SELECT * FROM dist_table;
+
+x              y              
+
+master_remove_node
+
+               

src/test/regress/specs/isolation_create_table_vs_add_remove_node.spec

@@ -83,6 +83,14 @@ step "s2-create-table-2"
 	SELECT create_distributed_table('dist_table', 'x');
 }
 
+step "s2-create-append-table"
+{
+	SET citus.shard_replication_factor TO 1;
+	CREATE TABLE dist_table (x int, y int);
+	SELECT create_distributed_table('dist_table', 'x', 'append');
+	SELECT 1 FROM master_create_empty_shard('dist_table');
+}
+
 step "s2-select"
 {
 	SELECT * FROM dist_table;
@@ -105,4 +113,8 @@ permutation "s1-add-node-2" "s2-begin" "s2-create-table-1" "s1-remove-node-2" "s
 
 # session 1 removes a node, session 2 creates a distributed table with replication factor 2, should throw a sane error
 permutation "s1-add-node-2" "s1-begin" "s1-remove-node-2" "s2-create-table-2" "s1-commit" "s2-select"
-permutation "s1-add-node-2" "s1-begin" "s2-create-table-2" "s1-remove-node-2" "s1-commit" "s2-select"
+permutation "s1-add-node-2" "s2-begin" "s2-create-table-2" "s1-remove-node-2" "s2-commit" "s2-select"
+
+# session 1 removes a node, session 2 creates a shard in an append-distributed table
+permutation "s1-add-node-2" "s1-begin" "s1-remove-node-2" "s2-create-append-table" "s1-commit" "s2-select"
+permutation "s1-add-node-2" "s2-begin" "s2-create-append-table" "s1-remove-node-2" "s2-commit" "s2-select"