Do not force internal collectors to call rrdset_next. (#13926)

* Remove calls to rrdset_next().

* Rm checks plugin

* Update documentantion

* Call rrdset_next from within rrdset_done

This wraps up the removal of rrdset_next from internal collectors, which
removes a lot of unecessary code and the need for if/else clauses in
every place.

The pluginsd parser is the only component that calls rrdset_next*()
functions because it's not strictly speaking a collector but more of a
collector manager/proxy.

With the current changes it's possible to simplify the API we expose
from RRD significantly, but this will be follow-up work in the future.

* Remove stale reference to checks.plugin

* Fix RRD unit test

rrdset_next is not meant to be called from these tests.

* Fix db engine unit test.

* Schedule rrdset_next when we have completed at least one collection.

* Mark chart creation clauses as unlikely.

* Add missing brace to fix FreeBSD plugin.

68 files changed, 278 insertions, 1805 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 8c75cfd39c..c12e9c81b9 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -525,10 +525,6 @@ target_include_directories(libnetdata BEFORE PUBLIC ${GENERATED_CONFIG_H_DIR})
 set(APPS_PLUGIN_FILES
         collectors/apps.plugin/apps_plugin.c)
 
-set(CHECKS_PLUGIN_FILES
-        collectors/checks.plugin/plugin_checks.c
-        )
-
 set(FREEBSD_PLUGIN_FILES
         collectors/freebsd.plugin/plugin_freebsd.c
         collectors/freebsd.plugin/plugin_freebsd.h
@@ -1059,7 +1055,6 @@ set(NETDATA_FILES
         ${DAEMON_FILES}
         ${API_PLUGIN_FILES}
         ${EXPORTING_ENGINE_FILES}
-        ${CHECKS_PLUGIN_FILES}
         ${HEALTH_PLUGIN_FILES}
         ${IDLEJITTER_PLUGIN_FILES}
         ${ML_FILES}
diff --git a/Makefile.am b/Makefile.am
index 95a2e28f46..5e16052374 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -203,10 +203,6 @@ APPS_PLUGIN_FILES = \
     $(LIBNETDATA_FILES) \
     $(NULL)
 
-CHECKS_PLUGIN_FILES = \
-    collectors/checks.plugin/plugin_checks.c \
-    $(NULL)
-
 FREEBSD_PLUGIN_FILES = \
     collectors/freebsd.plugin/plugin_freebsd.c \
     collectors/freebsd.plugin/plugin_freebsd.h \
@@ -918,7 +914,6 @@ NETDATA_FILES = \
     $(LIBNETDATA_FILES) \
     $(API_PLUGIN_FILES) \
     $(EXPORTING_ENGINE_FILES) \
-    $(CHECKS_PLUGIN_FILES) \
     $(HEALTH_PLUGIN_FILES) \
     $(ML_FILES) \
     $(ML_TESTS_FILES) \
diff --git a/aclk/aclk_stats.c b/aclk/aclk_stats.c
index a6d2e702a6..215313ff9a 100644
--- a/aclk/aclk_stats.c
+++ b/aclk/aclk_stats.c
@@ -39,8 +39,7 @@ static void aclk_stats_collect(struct aclk_metrics_per_sample *per_sample, struc
             "connected", "netdata", "stats", 200000, localhost->rrd_update_every, RRDSET_TYPE_LINE);
 
         rd_online_status = rrddim_add(st_aclkstats, "online", NULL, 1, 1, RRD_ALGORITHM_ABSOLUTE);
-    } else
-        rrdset_next(st_aclkstats);
+    }
 
     rrddim_set_by_pointer(st_aclkstats, rd_online_status, per_sample->offline_during_sample ? 0 : permanent->online);
 
@@ -60,8 +59,7 @@ static void aclk_stats_query_queue(struct aclk_metrics_per_sample *per_sample)
 
         rd_queued = rrddim_add(st_query_thread, "added", NULL, 1, localhost->rrd_update_every, RRD_ALGORITHM_ABSOLUTE);
         rd_dispatched = rrddim_add(st_query_thread, "dispatched", NULL, -1, localhost->rrd_update_every, RRD_ALGORITHM_ABSOLUTE);
-    } else
-        rrdset_next(st_query_thread);
+    }
 
     rrddim_set_by_pointer(st_query_thread, rd_queued, per_sample->queries_queued);
     rrddim_set_by_pointer(st_query_thread, rd_dispatched, per_sample->queries_dispatched);
@@ -83,8 +81,8 @@ static void aclk_stats_latency(struct aclk_metrics_per_sample *per_sample)
 
         rd_avg = rrddim_add(st, "avg", NULL, 1, 1, RRD_ALGORITHM_ABSOLUTE);
         rd_max = rrddim_add(st, "max", NULL, 1, 1, RRD_ALGORITHM_ABSOLUTE);
-    } else
-        rrdset_next(st);
+    }
+
     if(per_sample->latency_count)
         rrddim_set_by_pointer(st, rd_avg, roundf((float)per_sample->latency_total / per_sample->latency_count));
     else
@@ -109,8 +107,7 @@ static void aclk_stats_cloud_req(struct aclk_metrics_per_sample *per_sample)
 
         rd_rq_rcvd = rrddim_add(st, "received", NULL, 1, localhost->rrd_update_every, RRD_ALGORITHM_ABSOLUTE);
         rd_rq_err = rrddim_add(st, "malformed", NULL, 1, localhost->rrd_update_every, RRD_ALGORITHM_ABSOLUTE);
-    } else
-        rrdset_next(st);
+    }
 
     rrddim_set_by_pointer(st, rd_rq_rcvd, per_sample->cloud_req_recvd - per_sample->cloud_req_err);
     rrddim_set_by_pointer(st, rd_rq_err, per_sample->cloud_req_err);
@@ -131,8 +128,7 @@ static void aclk_stats_cloud_req_type(struct aclk_metrics_per_sample *per_sample
         for (int i = 0; i < ACLK_QUERY_TYPE_COUNT; i++)
             dims[i] = rrddim_add(st, aclk_query_get_name(i, 1), NULL, 1, localhost->rrd_update_every, RRD_ALGORITHM_ABSOLUTE);
 
-    } else
-        rrdset_next(st);
+    }
 
     for (int i = 0; i < ACLK_QUERY_TYPE_COUNT; i++)
         rrddim_set_by_pointer(st, dims[i], per_sample->queries_per_type[i]);
@@ -171,8 +167,7 @@ static void aclk_stats_cloud_req_http_type(struct aclk_metrics_per_sample *per_s
 
         for (int i = 0; i < ACLK_STATS_CLOUD_HTTP_REQ_TYPE_CNT; i++)
             rd_rq_types[i] = rrddim_add(st, cloud_req_http_type_names[i], NULL, 1, localhost->rrd_update_every, RRD_ALGORITHM_ABSOLUTE);
-    } else
-        rrdset_next(st);
+    }
 
     for (int i = 0; i < ACLK_STATS_CLOUD_HTTP_REQ_TYPE_CNT; i++)
         rrddim_set_by_pointer(st, rd_rq_types[i], per_sample->cloud_req_http_by_type[i]);
@@ -197,8 +192,7 @@ static void aclk_stats_query_threads(uint32_t *queries_per_thread)
                 error("snprintf encoding error");
             aclk_qt_data[i].dim = rrddim_add(st, dim_name, NULL, 1, localhost->rrd_update_every, RRD_ALGORITHM_ABSOLUTE);
         }
-    } else
-        rrdset_next(st);
+    }
 
     for (int i = 0; i < aclk_stats_cfg.query_thread_count; i++) {
         rrddim_set_by_pointer(st, aclk_qt_data[i].dim, queries_per_thread[i]);
@@ -222,8 +216,7 @@ static void aclk_stats_query_time(struct aclk_metrics_per_sample *per_sample)
         rd_rq_avg = rrddim_add(st, "avg", NULL, 1, localhost->rrd_update_every, RRD_ALGORITHM_ABSOLUTE);
         rd_rq_max = rrddim_add(st, "max", NULL, 1, localhost->rrd_update_every, RRD_ALGORITHM_ABSOLUTE);
         rd_rq_total = rrddim_add(st, "total", NULL, 1, localhost->rrd_update_every, RRD_ALGORITHM_ABSOLUTE);
-    } else
-        rrdset_next(st);
+    }
 
     if(per_sample->cloud_q_process_count)
         rrddim_set_by_pointer(st, rd_rq_avg, roundf((float)per_sample->cloud_q_process_total / per_sample->cloud_q_process_count));
@@ -248,8 +241,7 @@ static void aclk_stats_newproto_rx(uint32_t *rx_msgs_sample)
         for (unsigned int i = 0; i < aclk_stats_cfg.proto_hdl_cnt; i++) {
             aclk_stats_cfg.rx_msg_dims[i] = rrddim_add(st, rx_handler_get_name(i), NULL, 1, localhost->rrd_update_every, RRD_ALGORITHM_ABSOLUTE);
         }
-    } else
-        rrdset_next(st);
+    }
 
     for (unsigned int i = 0; i < aclk_stats_cfg.proto_hdl_cnt; i++)
         rrddim_set_by_pointer(st, aclk_stats_cfg.rx_msg_dims[i], rx_msgs_sample[i]);
@@ -275,8 +267,7 @@ static void aclk_stats_mqtt_wss(struct mqtt_wss_stats *stats)
 
         rd_sent  = rrddim_add(st, "sent", NULL, -1, 1, RRD_ALGORITHM_INCREMENTAL);
         rd_recvd = rrddim_add(st, "received", NULL, 1, 1, RRD_ALGORITHM_INCREMENTAL);
-    } else
-        rrdset_next(st);
+    }
 
     rrddim_set_by_pointer(st, rd_sent, sent);
     rrddim_set_by_pointer(st, rd_recvd, recvd);
diff --git a/collectors/COLLECTORS.md b/collectors/COLLECTORS.md
index 5c41b704fd..7f66076ff0 100644
--- a/collectors/COLLECTORS.md
+++ b/collectors/COLLECTORS.md
@@ -514,6 +514,5 @@ default. To use a third-party collector, visit their GitHub/documentation page a
 
 ## Etc
 
-- [checks.plugin](checks.plugin/README.md): A debugging collector, disabled by default.
 - [charts.d example](charts.d.plugin/example/README.md): An example `charts.d` collector.
 - [python.d example](python.d.plugin/example/README.md): An example `python.d` collector.
diff --git a/collectors/Makefile.am b/collectors/Makefile.am
index a0a972e8f3..9f8bf52807 100644
--- a/collectors/Makefile.am
+++ b/collectors/Makefile.am
@@ -7,7 +7,6 @@ SUBDIRS = \
     apps.plugin \
     cgroups.plugin \
     charts.d.plugin \
-    checks.plugin \
     cups.plugin \
     diskspace.plugin \
     timex.plugin \
diff --git a/collectors/REFERENCE.md b/collectors/REFERENCE.md
index 949858f600..939b189ee7 100644
--- a/collectors/REFERENCE.md
+++ b/collectors/REFERENCE.md
@@ -148,11 +148,6 @@ collect_data() {
         // attach a metric to it
         rd = rrddim_add(st, "id", "name", multiplier, divider, algorithm);
     }
-    else {
-        // this chart is already created
-        // let Netdata know we start a new iteration on it
-        rrdset_next(st);
-    }
 
     // give the collected value(s) to the chart
     rrddim_set_by_pointer(st, rd, collected_value);
diff --git a/collectors/cgroups.plugin/sys_fs_cgroup.c b/collectors/cgroups.plugin/sys_fs_cgroup.c
index 3bee46dcf4..8f7548286c 100644
--- a/collectors/cgroups.plugin/sys_fs_cgroup.c
+++ b/collectors/cgroups.plugin/sys_fs_cgroup.c
@@ -1447,7 +1447,6 @@ static inline void cgroup2_read_pressure(struct pressure *res) {
             return;
         }
 
-    
         res->some.share_time.value10 = strtod(procfile_lineword(ff, 0, 2), NULL);
         res->some.share_time.value60 = strtod(procfile_lineword(ff, 0, 4), NULL);
         res->some.share_time.value300 = strtod(procfile_lineword(ff, 0, 6), NULL);
@@ -2847,57 +2846,45 @@ void update_systemd_services_charts(
 
     // create the charts
 
-    if(likely(do_cpu)) {
-        if(unlikely(!st_cpu)) {
-            char title[CHART_TITLE_MAX + 1];
-            snprintfz(title, CHART_TITLE_MAX, "Systemd Services CPU utilization (100%% = 1 core)");
-
-            st_cpu = rrdset_create_localhost(
-                    "services"
-                    , "cpu"
-                    , NULL
-                    , "cpu"
-                    , "services.cpu"
-                    , title
-                    , "percentage"
-                    , PLUGIN_CGROUPS_NAME
-                    , PLUGIN_CGROUPS_MODULE_SYSTEMD_NAME
-                    , NETDATA_CHART_PRIO_CGROUPS_SYSTEMD
-                    , update_every
-                    , RRDSET_TYPE_STACKED
-            );
-
-        }
-        else
-            rrdset_next(st_cpu);
+    if (unlikely(do_cpu && !st_cpu)) {
+        char title[CHART_TITLE_MAX + 1];
+        snprintfz(title, CHART_TITLE_MAX, "Systemd Services CPU utilization (100%% = 1 core)");
+
+        st_cpu = rrdset_create_localhost(
+                "services"
+                , "cpu"
+                , NULL
+                , "cpu"
+                , "services.cpu"
+                , title
+                , "percentage"
+                , PLUGIN_CGROUPS_NAME
+                , PLUGIN_CGROUPS_MODULE_SYSTEMD_NAME
+                , NETDATA_CHART_PRIO_CGROUPS_SYSTEMD
+                , update_every
+                , RRDSET_TYPE_STACKED
+        );
     }
 
-    if(likely(do_mem_usage)) {
-        if(unlikely(!st_mem_usage)) {
-
-            st_mem_usage = rrdset_create_localhost(
-                    "services"
-                    , "mem_usage"
-                    , NULL
-                    , "mem"
-                    , "services.mem_usage"
-                    , "Systemd Services Used Memory"
-                    , "MiB"
-                    , PLUGIN_CGROUPS_NAME
-                    , PLUGIN_CGROUPS_MODULE_SYSTEMD_NAME
-                    , NETDATA_CHART_PRIO_CGROUPS_SYSTEMD + 10
-                    , update_every
-                    , RRDSET_TYPE_STACKED
-            );
-
-        }
-        else
-            rrdset_next(st_mem_usage);
+    if (unlikely(do_mem_usage && !st_mem_usage)) {
+        st_mem_usage = rrdset_create_localhost(
+                "services"
+                , "mem_usage"
+                , NULL
+                , "mem"
+                , "services.mem_usage"
+                , "Systemd Services Used Memory"
+                , "MiB"
+                , PLUGIN_CGROUPS_NAME
+                , PLUGIN_CGROUPS_MODULE_SYSTEMD_NAME
+                , NETDATA_CHART_PRIO_CGROUPS_SYSTEMD + 10
+                , update_every
+                , RRDSET_TYPE_STACKED
+        );
     }
 
     if(likely(do_mem_detailed)) {
         if(unlikely(!st_mem_detailed_rss)) {
-
             st_mem_detailed_rss = rrdset_create_localhost(
                     "services"
                     , "mem_rss"
@@ -2912,13 +2899,9 @@ void update_systemd_services_charts(
                     , update_every
                     , RRDSET_TYPE_STACKED
             );
-
         }
-        else
-            rrdset_next(st_mem_detailed_rss);
 
         if(unlikely(!st_mem_detailed_mapped)) {
-
             st_mem_detailed_mapped = rrdset_create_localhost(
                     "services"
                     , "mem_mapped"
@@ -2933,13 +2916,9 @@ void update_systemd_services_charts(
                     , update_every
                     , RRDSET_TYPE_STACKED
             );
-
         }
-        else
-            rrdset_next(st_mem_detailed_mapped);
 
         if(unlikely(!st_mem_detailed_cache)) {
-
             st_mem_detailed_cache = rrdset_create_localhost(
                     "services"
                     , "mem_cache"
@@ -2954,13 +2933,9 @@ void update_systemd_services_charts(
                     , update_every
                     , RRDSET_TYPE_STACKED
             );
-
         }
-        else
-            rrdset_next(st_mem_detailed_cache);
 
         if(unlikely(!st_mem_detailed_writeback)) {
-
             st_mem_detailed_writeback = rrdset_create_localhost(
                     "services"
                     , "mem_writeback"
@@ -2977,11 +2952,8 @@ void update_systemd_services_charts(
             );
 
         }
-        else
-            rrdset_next(st_mem_detailed_writeback);
 
         if(unlikely(!st_mem_detailed_pgfault)) {
-
             st_mem_detailed_pgfault = rrdset_create_localhost(
                     "services"
                     , "mem_pgfault"
@@ -2997,11 +2969,8 @@ void update_systemd_services_charts(
                     , RRDSET_TYPE_STACKED
             );
         }
-        else
-            rrdset_next(st_mem_detailed_pgfault);
 
         if(unlikely(!st_mem_detailed_pgmajfault)) {
-
             st_mem_detailed_pgmajfault = rrdset_create_localhost(
                     "services"
                     , "mem_pgmajfault"
@@ -3016,13 +2985,9 @@ void update_systemd_services_charts(
                     , update_every
                     , RRDSET_TYPE_STACKED
             );
-
         }
-        else
-            rrdset_next(st_mem_detailed_pgmajfault);
 
         if(unlikely(!st_mem_detailed_pgpgin)) {
-
             st_mem_detailed_pgpgin = rrdset_create_localhost(
                     "services"
                     , "mem_pgpgin"
@@ -3039,11 +3004,8 @@ void update_systemd_services_charts(
             );
 
         }
-        else
-            rrdset_next(st_mem_detailed_pgpgin);
 
         if(unlikely(!st_mem_detailed_pgpgout)) {
-
             st_mem_detailed_pgpgout = rrdset_create_localhost(
                     "services"
                     , "mem_pgpgout"
@@ -3058,61 +3020,45 @@ void update_systemd_services_charts(
                     , update_every
                     , RRDSET_TYPE_STACKED
             );
-
         }
-        else
-            rrdset_next(st_mem_detailed_pgpgout);
     }
 
-    if(likely(do_mem_failcnt)) {
-        if(unlikely(!st_mem_failcnt)) {
-
-            st_mem_failcnt = rrdset_create_localhost(
-                    "services"
-                    , "mem_failcnt"
-                    , NULL
-                    , "mem"
-                    , "services.mem_failcnt"
-                    , "Systemd Services Memory Limit Failures"
-                    , "failures"
-                    , PLUGIN_CGROUPS_NAME
-                    , PLUGIN_CGROUPS_MODULE_SYSTEMD_NAME
-                    , NETDATA_CHART_PRIO_CGROUPS_SYSTEMD + 110
-                    , update_every
-                    , RRDSET_TYPE_STACKED
-            );
-
-        }
-        else
-            rrdset_next(st_mem_failcnt);
+    if(unlikely(do_mem_failcnt && !st_mem_failcnt)) {
+        st_mem_failcnt = rrdset_create_localhost(
+                "services"
+                , "mem_failcnt"
+                , NULL