[Tarantool-patches] [PATCH v3 02/10] box: introduce summary RO flag

[Vladislav Shpilevoy <v.shpilevoy@tarantool.org>]

An instance is writable if box.cfg.read_only is false, and it is
not orphan. Update of the final read-only state of the instance
needs to fire read-only update triggers, and notify the engines.
These 2 flags were easy and cheap to check on each operation, and
the triggers were easy to use since both flags are stored and
updated inside box.cc.

That is going to change when Raft is introduced. Raft will add 2
more checks:

  - A flag if Raft is enabled on the node. If it is not, then Raft
    state won't affect whether the instance is writable;

  - When Raft is enabled, it will allow writes on a leader only.

It means a check for being read-only would look like this:

    is_ro || is_orphan || (raft_is_enabled() && !raft_is_leader())

This is significantly slower. Besides, Raft somehow needs to
access the read-only triggers and engine API - this looks wrong.

The patch introduces a new flag is_ro_summary. The flag
incorporates all the read-only conditions into one flag. When some
subsystem may change read-only state of the instance, it needs to
call box_update_ro_summary(), and the function takes care of
updating the summary flag, running the triggers, and notifying the
engines.

Raft will use this function when its state or config will change.

Needed for #1146
---
 src/box/box.cc | 44 +++++++++++++++++++++++++++-----------------
 src/box/box.h  |  6 ++++++
 2 files changed, 33 insertions(+), 17 deletions(-)

diff --git a/src/box/box.cc b/src/box/box.cc
index 0b1f6c237..8de451db5 100644
--- a/src/box/box.cc
+++ b/src/box/box.cc
@@ -129,6 +129,14 @@ static bool is_local_recovery = false;
  */
 static bool is_orphan;
 
+/**
+ * Summary flag incorporating all the instance attributes,
+ * affecting ability to write. Currently these are:
+ * - is_ro;
+ * - is_orphan;
+ */
+static bool is_ro_summary = true;
+
 /**
  * The pool of fibers in the transaction processor thread
  * working on incoming messages from net, wal and other
@@ -144,11 +152,24 @@ static struct fiber_pool tx_fiber_pool;
  */
 static struct cbus_endpoint tx_prio_endpoint;
 
+void
+box_update_ro_summary(void)
+{
+	bool old_is_ro_summary = is_ro_summary;
+	is_ro_summary = is_ro || is_orphan;
+	/* In 99% nothing changes. Filter this out first. */
+	if (is_ro_summary == old_is_ro_summary)
+		return;
+
+	if (is_ro_summary)
+		engine_switch_to_ro();
+	fiber_cond_broadcast(&ro_cond);
+}
+
 static int
 box_check_writable(void)
 {
-	/* box is only writable if box.cfg.read_only == false and */
-	if (is_ro || is_orphan) {
+	if (is_ro_summary) {
 		diag_set(ClientError, ER_READONLY);
 		diag_log();
 		return -1;
@@ -253,20 +274,14 @@ box_check_ro(void);
 void
 box_set_ro(void)
 {
-	bool ro = box_check_ro();
-	if (ro == is_ro)
-		return; /* nothing to do */
-	if (ro)
-		engine_switch_to_ro();
-
-	is_ro = ro;
-	fiber_cond_broadcast(&ro_cond);
+	is_ro = box_check_ro();
+	box_update_ro_summary();
 }
 
 bool
 box_is_ro(void)
 {
-	return is_ro || is_orphan;
+	return is_ro_summary;
 }
 
 bool
@@ -293,13 +308,8 @@ box_wait_ro(bool ro, double timeout)
 void
 box_do_set_orphan(bool orphan)
 {
-	if (is_orphan == orphan)
-		return; /* nothing to do */
-	if (orphan)
-		engine_switch_to_ro();
-
 	is_orphan = orphan;
-	fiber_cond_broadcast(&ro_cond);
+	box_update_ro_summary();
 }
 
 void
diff --git a/src/box/box.h b/src/box/box.h
index f9bd8b98d..5988264a5 100644
--- a/src/box/box.h
+++ b/src/box/box.h
@@ -137,6 +137,12 @@ box_set_orphan(bool orphan);
 void
 box_do_set_orphan(bool orphan);
 
+/**
+ * Update the final RO flag based on the instance flags and state.
+ */
+void
+box_update_ro_summary(void);
+
 /**
  * Iterate over all spaces and save them to the
  * snapshot file.
-- 
2.21.1 (Apple Git-122.3)