Configurable storage engine for Netdata agents: step 3 (#12892)

* storage engine: add host context API

Add a new API to allow storage engines to manage host contexts.
* Replace single global context with per-engine global context
* Context is full managed by storage engines: a storage engine
  can use no context, a global engine context, per host contexts,
  or a mix of these.
* Currently, only dbengine uses contexts.
  Following the current logic, legacy hosts use their own context,
  while non-legacy hosts share the global context.

* storage engine: use empty function instead of null for context ops

* rrdhost: don't check return value for void call

* rrdhost: create context with host

* storage engine: move rrddim ops to rrddim_mem.{c,h}

* storage engine: don't use NULL for end-of-list marker

* storage engine: fallback to default engine

4 files changed, 27 insertions, 9 deletions

diff --git a/daemon/analytics.c b/daemon/analytics.c
index 6c02561d0c..c779253485 100644
--- a/daemon/analytics.c
+++ b/daemon/analytics.c
@@ -2,6 +2,9 @@
 
 #include "common.h"
 #include "buildinfo.h"
+#ifdef ENABLE_DBENGINE
+#include "database/engine/rrdengineapi.h"
+#endif
 
 struct analytics_data analytics_data;
 extern void analytics_exporting_connectors (BUFFER *b);
diff --git a/daemon/global_statistics.c b/daemon/global_statistics.c
index b5740b176f..a5f7094d54 100644
--- a/daemon/global_statistics.c
+++ b/daemon/global_statistics.c
@@ -1,6 +1,9 @@
 // SPDX-License-Identifier: GPL-3.0-or-later
 
 #include "common.h"
+#ifdef ENABLE_DBENGINE
+#include "database/engine/rrdengineapi.h"
+#endif
 
 #define GLOBAL_STATS_RESET_WEB_USEC_MAX 0x01
 
@@ -456,7 +459,7 @@ static void dbengine_statistics_charts(void) {
 
         rrdhost_foreach_read(host) {
             if (host->rrd_memory_mode == RRD_MEMORY_MODE_DBENGINE && !rrdhost_flag_check(host, RRDHOST_FLAG_ARCHIVED)) {
-                if (&multidb_ctx == host->rrdeng_ctx) {
+                if (host->rrdeng_ctx == host->rrdeng_ctx->engine->context) {
                     if (counted_multihost_db)
                         continue; /* Only count multi-host DB once */
                     counted_multihost_db = 1;
diff --git a/daemon/main.c b/daemon/main.c
index 6a07add1c9..fd5a42d96c 100644
--- a/daemon/main.c
+++ b/daemon/main.c
@@ -3,6 +3,10 @@
 #include "common.h"
 #include "buildinfo.h"
 #include "static_threads.h"
+#include "database/storage_engine.h"
+#ifdef ENABLE_DBENGINE
+#include "database/engine/rrdengineapi.h"
+#endif
 
 int netdata_zero_metrics_enabled;
 int netdata_anonymous_statistics_enabled;
@@ -54,13 +58,18 @@ void netdata_cleanup_and_exit(int ret) {
 
         // free the database
         info("EXIT: freeing database memory...");
-#ifdef ENABLE_DBENGINE
-        rrdeng_prepare_exit(&multidb_ctx);
-#endif
+        for (STORAGE_ENGINE* eng = storage_engine_foreach_init(); eng; eng = storage_engine_foreach_next(eng)) {
+            if (eng->context)
+                eng->api.engine_ops.exit(eng->context);
+        }
+
         rrdhost_free_all();
-#ifdef ENABLE_DBENGINE
-        rrdeng_exit(&multidb_ctx);
-#endif
+        for (STORAGE_ENGINE* eng = storage_engine_foreach_init(); eng; eng = storage_engine_foreach_next(eng)) {
+            if (eng->context) {
+                eng->api.engine_ops.destroy(eng->context);
+                eng->context = NULL;
+            }
+        }
     }
     sql_close_database();
 
diff --git a/daemon/unit_test.c b/daemon/unit_test.c
index 35f8613a2b..2dc92dcb21 100644
--- a/daemon/unit_test.c
+++ b/daemon/unit_test.c
@@ -1,6 +1,9 @@
 // SPDX-License-Identifier: GPL-3.0-or-later
 
 #include "common.h"
+#ifdef ENABLE_DBENGINE
+#include "database/engine/rrdengineapi.h"
+#endif
 
 static int check_number_printing(void) {
     struct {
@@ -1157,7 +1160,7 @@ int run_test(struct test *test)
     RRDSET *st = rrdset_create_localhost("netdata", name, name, "netdata", NULL, "Unit Testing", "a value", "unittest", NULL, 1
                                          , test->update_every, RRDSET_TYPE_LINE);
     RRDDIM *rd = rrddim_add(st, "dim1", NULL, test->multiplier, test->divisor, test->algorithm);
-    
+
     RRDDIM *rd2 = NULL;
     if(test->feed2)
         rd2 = rrddim_add(st, "dim2", NULL, test->multiplier, test->divisor, test->algorithm);
@@ -1173,7 +1176,7 @@ int run_test(struct test *test)
 
         if(c) {
             time_now += test->feed[c].microseconds;
-            fprintf(stderr, "    > %s: feeding position %lu, after %0.3f seconds (%0.3f seconds from start), delta " CALCULATED_NUMBER_FORMAT ", rate " CALCULATED_NUMBER_FORMAT "\n", 
+            fprintf(stderr, "    > %s: feeding position %lu, after %0.3f seconds (%0.3f seconds from start), delta " CALCULATED_NUMBER_FORMAT ", rate " CALCULATED_NUMBER_FORMAT "\n",
                 test->name, c+1,
                 (float)test->feed[c].microseconds / 1000000.0,
                 (float)time_now / 1000000.0,