add_check_hook.patch

diff --git a/src/backend/access/transam/xloglevelworker.c b/src/backend/access/transam/xloglevelworker.c
index 286ee411f6..cca0587012 100644
--- a/src/backend/access/transam/xloglevelworker.c
+++ b/src/backend/access/transam/xloglevelworker.c
@@ -28,6 +28,7 @@
 #include "postmaster/pgarch.h"
 #include "replication/logicalxlog.h"
 #include "replication/slot.h"
+#include "replication/walsender.h"
 #include "storage/ipc.h"
 #include "storage/lmgr.h"
 #include "storage/procsignal.h"
@@ -612,3 +613,22 @@ show_wal_level(void)
 	else
 		return "logical";
 }
+
+bool
+check_wal_level(int *newval, void **extra, GucSource source)
+{
+	int newWalLevel = *newval;
+
+	if (newWalLevel >= wal_level)
+		return true;
+
+	if (newWalLevel == WAL_LEVEL_MINIMAL && DoesWalSndExists())
+	{
+		GUC_check_errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE);
+		GUC_check_errmsg("wal_level cannot be set to \"minimal\" while walsender exists");
+		return false;
+	}
+
+	return true;
+
+}
diff --git a/src/backend/replication/walsender.c b/src/backend/replication/walsender.c
index 9d0b33cf83..5a8e0b001f 100644
--- a/src/backend/replication/walsender.c
+++ b/src/backend/replication/walsender.c
@@ -3529,6 +3529,28 @@ WalSndRqstFileReload(void)
 	}
 }
 
+bool
+DoesWalSndExists(void)
+{
+	bool	result = false;
+
+	for (int i = 0; i < max_wal_senders; i++)
+	{
+		WalSnd	   *walsnd = &WalSndCtl->walsnds[i];
+
+		SpinLockAcquire(&walsnd->mutex);
+		if (walsnd->pid == 0)
+		{
+			SpinLockRelease(&walsnd->mutex);
+			result = true;
+			break;
+		}
+		SpinLockRelease(&walsnd->mutex);
+	}
+
+	return result;
+}
+
 /*
  * Handle PROCSIG_WALSND_INIT_STOPPING signal.
  */
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index b181bb7b6a..6fc0440535 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -5069,7 +5069,7 @@ struct config_enum ConfigureNamesEnum[] =
 		},
 		&wal_level,
 		WAL_LEVEL_REPLICA, wal_level_options,
-		NULL, NULL, show_wal_level
+		check_wal_level, NULL, show_wal_level
 	},
 
 	{
diff --git a/src/include/replication/walsender.h b/src/include/replication/walsender.h
index d6d426915e..b2ed9b430a 100644
--- a/src/include/replication/walsender.h
+++ b/src/include/replication/walsender.h
@@ -49,6 +49,7 @@ extern void WalSndInitStopping(void);
 extern void WalSndWaitStopping(void);
 extern void HandleWalSndInitStopping(void);
 extern void WalSndRqstFileReload(void);
+extern bool DoesWalSndExists(void);
 
 /*
  * Remember that we want to wakeup walsenders later
diff --git a/src/include/utils/guc_hooks.h b/src/include/utils/guc_hooks.h
index 22180b2016..b5bb692068 100644
--- a/src/include/utils/guc_hooks.h
+++ b/src/include/utils/guc_hooks.h
@@ -169,6 +169,7 @@ extern bool check_wal_buffers(int *newval, void **extra, GucSource source);
 extern bool check_wal_consistency_checking(char **newval, void **extra,
 										   GucSource source);
 extern void assign_wal_consistency_checking(const char *newval, void *extra);
+extern bool check_wal_level(int *newval, void **extra, GucSource source);
 extern const char *show_wal_level(void);
 extern bool check_wal_segment_size(int *newval, void **extra, GucSource source);
 extern void assign_wal_sync_method(int new_wal_sync_method, void *extra);