Add contrib/pg_logicalinspect.

This module provides SQL functions that allow to inspect logical
decoding components.

It currently allows to inspect the contents of serialized logical
snapshots of a running database cluster, which is useful for debugging
or educational purposes.

Author: Bertrand Drouvot
Reviewed-by: Amit Kapila, Shveta Malik, Peter Smith, Peter Eisentraut
Reviewed-by: David G. Johnston
Discussion: https://postgr.es/m/ZscuZ92uGh3wm4tW%40ip-10-97-1-34.eu-west-3.compute.internal

src/include/replication/snapbuild.h

@@ -15,6 +15,10 @@
 #include "access/xlogdefs.h"
 #include "utils/snapmgr.h"
 
+/*
+ * Please keep get_snapbuild_state_desc() (located in the pg_logicalinspect
+ * module) updated if a change needs to be made to SnapBuildState.
+ */
 typedef enum
 {
 	/*

src/include/replication/snapbuild_internal.h

@@ -193,4 +193,7 @@ typedef struct SnapBuildOnDisk
 	/* variable amount of TransactionIds follows */
 } SnapBuildOnDisk;
 
+extern bool SnapBuildRestoreSnapshot(SnapBuildOnDisk *ondisk, const char *path,
+									 MemoryContext context, bool missing_ok);
+
 #endif							/* SNAPBUILD_INTERNAL_H */