Major - P3 In 3.0.3, explained updates against the 'config' and 'admin' databases return explain plans.
In newer 3.1.x versions, the explained updates cause invariant failures because there's no shard information for the 'config' and 'admin' databases.
Invariant failure shard src/mongo/s/client/shard.cpp 120
#0 0x00007fb841f6920b in raise (sig=5) at ../nptl/sysdeps/unix/sysv/linux/pt-raise.c:37
#1 0x0000000000fd06b4 in mongo::breakpoint () at src/mongo/util/debugger.cpp:57
#2 0x0000000000fc2e49 in mongo::invariantFailed (expr=0x11ef5ad "shard", file=0x11ef582 "src/mongo/s/client/shard.cpp", line=120) at src/mongo/util/assert_util.cpp:153
#3 0x0000000000f08251 in mongo::Shard::reset (this=0x7fb8404b90b0, ident=...) at src/mongo/s/client/shard.cpp:120
#4 0x0000000000e71908 in mongo::Shard::make (ident=...) at src/mongo/s/client/shard.h:82
#5 0x0000000000f7cda9 in mongo::Strategy::commandOpWrite (this=0x1e18210, dbName=..., command=..., targetingBatchItem=..., results=0x7fb8404b9370) at src/mongo/s/strategy.cpp:481
#6 0x0000000000f47a5f in mongo::(anonymous namespace)::ClusterWriteCmd::explain (this=0x1740720 <mongo::(anonymous namespace)::clusterUpdateCmd>, txn=0x0, dbname=..., cmdObj=..., verbosity=mongo::ExplainCommon::EXEC_ALL_PLANS,
out=0x7fb8404b9930) at src/mongo/s/commands/cluster_write_cmd.cpp:129
#7 0x0000000000f20115 in mongo::ClusterExplainCmd::run (this=0x173ef20 <mongo::cmdExplainCluster>, txn=0x0, dbName=..., cmdObj=..., options=0, errmsg=..., result=...) at src/mongo/s/commands/cluster_explain_cmd.cpp:120
#8 0x0000000000f731fe in mongo::Command::execCommandClientBasic (txn=0x0, c=0x173ef20 <mongo::cmdExplainCluster>, client=..., queryOptions=0, ns=0x7fb8140717f4 "admin.$cmd", cmdObj=..., result=...) at src/mongo/s/s_only.cpp:128
#9 0x0000000000f73678 in mongo::Command::runAgainstRegistered (ns=0x7fb8140717f4 "admin.$cmd", jsobj=..., anObjBuilder=..., queryOptions=0) at src/mongo/s/s_only.cpp:172
#10 0x0000000000f7b908 in mongo::Strategy::clientCommandOp (this=0x1e18210, r=...) at src/mongo/s/strategy.cpp:308
#11 0x0000000000f72304 in mongo::Request::process (this=0x7fb8404b9bd0, attempt=0) at src/mongo/s/request.cpp:121
#12 0x0000000000b3eab0 in mongo::ShardedMessageHandler::process (this=0x7fffbabafaa0, m=..., p=0x1e6dcc0) at src/mongo/s/server.cpp:141
#13 0x0000000000fe3acd in mongo::PortMessageServer::handleIncomingMsg (arg=0x1e6dcc0) at src/mongo/util/net/message_server_port.cpp:227
#14 0x00007fb841f61182 in start_thread (arg=0x7fb8404ba700) at pthread_create.c:312
#15 0x00007fb841c8e47d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:111