ircd::db: Set max_open_files=-1 when required for secondary mode.

ircd:Ⓜ️:dbs: Condition option from being set in slave/secondary mode.

ircd::db: Skip snapshot creation in slave/secondary mode (required).

ircd::db: Slave implies read-only.

ircd/db.cc

@@ -1208,7 +1208,7 @@ try
 }
 ,read_only
 {
-	ircd::read_only
+	slave || ircd::read_only
 }
 ,env
 {
@@ -1281,7 +1281,9 @@ try
 
 	// limit maxfdto prevent too many small files degrading read perf; too low is
 	// bad for write perf.
-	opts->max_open_files = fs::support::rlimit_nofile();
+	opts->max_open_files = !slave?
+		fs::support::rlimit_nofile():
+		-1;
 
 	// TODO: Check if these values can be increased; RocksDB may keep
 	// thread_local state preventing values > 1.
@@ -2410,7 +2412,10 @@ ircd::db::sequence(const rocksdb::Snapshot *const &rs)
 ircd::db::database::snapshot::snapshot(database &d)
 :s
 {
-	d.d->GetSnapshot(),
+	!d.slave?
+		d.d->GetSnapshot():
+		nullptr,
+
 	[dp(weak_from(d))](const rocksdb::Snapshot *const s)
 	{
 		if(!s)

matrix/dbs.cc

@@ -57,7 +57,7 @@ ircd::m::dbs::sst_write_buffer_size
 	{
 		static const string_view key{"writable_file_max_buffer_size"};
 		const size_t &value{sst_write_buffer_size};
-		if(events)
+		if(events && !events->slave)
 			db::setopt(*events, key, lex_cast(value));
 	}
 };

matrix/vm.cc

@@ -106,7 +106,7 @@ noexcept
 			db::sequence(*m::dbs::events) : 0UL,
 	};
 
-	assert(retired == sequence::retired);
+	assert(retired == sequence::retired || ircd::read_only);
 }
 
 //