Various agency improvements. (#8380)

* Ignore satellite collections in shrinkCluster in agency.
* Abort RemoveFollower job if not enough in-sync followers or leader failure.
* Break quick wait loop in supervision if leadership is lost.
* In case of resigned leader, set isReady=false in clusterInventory.
* Fix catch tests.

arangod/Agency/RemoveFollower.cpp

@@ -235,6 +235,7 @@ bool RemoveFollower::start(bool&) {
         << " does not have a leader that has confirmed leadership, waiting, "
            "jobId="
         << _jobId;
+    finish("", "", false, "job no longer sensible, leader has gone bad");
     return false;
   }
 
@@ -245,7 +246,8 @@ bool RemoveFollower::start(bool&) {
         << " does not have enough in sync followers to remove one, waiting, "
            "jobId="
         << _jobId;
-    return false;
+    finish("", "", false, "job no longer sensible, do not have few enough replicas");
+    return true;
   }
 
   // We now know actualReplFactor >= inSyncCount + noGoodCount and

arangod/Agency/Supervision.cpp

@@ -862,8 +862,9 @@ void Supervision::run() {
           }
 
           auto result = _agent->waitFor(leaderIndex);
-          if (result == Agent::raft_commit_t::UNKNOWN ||
-              result == Agent::raft_commit_t::TIMEOUT) { // Oh snap
+          if (result == Agent::raft_commit_t::TIMEOUT) { // Oh snap
+            // Note that we can get UNKNOWN if we have lost leadership or
+            // if we are shutting down. In both cases we just leave the loop.
             LOG_TOPIC(WARN, Logger::SUPERVISION) << "Waiting for commits to be done ... ";
             continue; 
           } else {                                       // Good we can continue
@@ -1457,11 +1458,8 @@ void Supervision::shrinkCluster() {
           if (replFact > maxReplFact) {
             maxReplFact = replFact;
           }
-        } else {
-          LOG_TOPIC(WARN, Logger::SUPERVISION)
-              << "Cannot retrieve replication factor for collection " << collptr.first;
-          return;
         }
+        // Note that this could be a satellite collection, in any case, ignore:
       }
     }
 

arangod/RestHandler/RestReplicationHandler.cpp

@@ -699,7 +699,8 @@ void RestReplicationHandler::handleCommandClusterInventory() {
     for (auto const& p : *shardMap) {
       auto currentServerList = cic->servers(p.first /* shardId */);
       if (currentServerList.size() == 0 || p.second.size() == 0 ||
-          currentServerList[0] != p.second[0]) {
+          currentServerList[0] != p.second[0] ||
+          (!p.second[0].empty() && p.second[0][0] == '_')) {
         isReady = false;
       }
       if (!ClusterHelpers::compareServerLists(p.second, currentServerList)) {

tests/Agency/RemoveFollowerTest.cpp

@@ -399,7 +399,7 @@ TEST_CASE("RemoveFollower", "[agency][supervision]") {
           REQUIRE(typeName(q->slice()) == "array");
           REQUIRE(q->slice().length() == 1);
           REQUIRE(typeName(q->slice()[0]) == "array");
-          REQUIRE(q->slice()[0].length() == 1); // we always simply override! no preconditions...
+          REQUIRE(q->slice()[0].length() == 2); // precondition
           REQUIRE(typeName(q->slice()[0][0]) == "object");
 
           auto writes = q->slice()[0][0];
@@ -407,8 +407,12 @@ TEST_CASE("RemoveFollower", "[agency][supervision]") {
           REQUIRE(typeName(writes.get("/arango/Target/ToDo/1").get("op")) == "string");
           CHECK(writes.get("/arango/Target/ToDo/1").get("op").copyString() == "delete");
           CHECK(writes.get("/arango/Target/Finished/1").get("collection").copyString() == COLLECTION);
-          CHECK(writes.get("/arango/Target/Pending/1").get("op").copyString() == "delete");
           CHECK(typeName(writes.get("/arango/Target/Failed/1")) == "none");
+
+          auto precond = q->slice()[0][1];
+          REQUIRE(typeName(precond) == "object");
+          REQUIRE(typeName(precond.get("/arango/Supervision/Health/follower1/Status")) == "object");
+
           return fakeWriteResult;
         }
       );

tests/Agency/RemoveFollowerTest.json

@@ -11,7 +11,7 @@ R"=(
           },
           "collection2": {
             "s2": {
-              "servers": ["leader"]
+              "servers": ["leader", "follower1"]
             }
           },
           "collection3": {