[PATCH 4/9] gc: run garbage collection in background

[Vladimir Davydov <vdavydov.dev@gmail.com>]

Currently, garbage collection is executed synchronously by functions
that may trigger it, such as gc_consumer_advance or gc_add_checkpoint.
As a result, one has to be very cautious when using those functions as
they may yield at their will. For example, we can't shoot off stale
consumers right in tx_prio handler - we have to use rather clumsy WAL
watcher interface instead. Besides, in future, when the garbage
collector state is persisted, we will need to call those functions from
on_commit trigger callback, where yielding is not normally allowed.

Actually, there's no reason to remove old files synchronously - we could
as well do it in the background. So this patch introduces a background
garbage collection fiber that executes gc_run when woken up. Now all
functions that might trigger garbage collection wake up this fiber
instead of executing gc_run directly.
---
 src/box/box.cc | 12 +++++++++
 src/box/gc.c   | 79 ++++++++++++++++++++++++++++++++++++++++++++++------------
 src/box/gc.h   | 35 +++++++++++++++++++++-----
 3 files changed, 104 insertions(+), 22 deletions(-)

diff --git a/src/box/box.cc b/src/box/box.cc
index a5e1f07a..bb7c1bb9 100644
--- a/src/box/box.cc
+++ b/src/box/box.cc
@@ -2203,6 +2203,18 @@ end:
 
 	latch_unlock(&schema_lock);
 	box_checkpoint_is_in_progress = false;
+
+	/*
+	 * Wait for background garbage collection that might
+	 * have been triggered by this checkpoint to complete.
+	 * Strictly speaking, it isn't necessary, but it
+	 * simplifies testing as it guarantees that by the
+	 * time box.snapshot() returns, all outdated checkpoint
+	 * files have been removed.
+	 */
+	if (!rc)
+		gc_wait();
+
 	return rc;
 }
 
diff --git a/src/box/gc.c b/src/box/gc.c
index 05773b91..9c049977 100644
--- a/src/box/gc.c
+++ b/src/box/gc.c
@@ -45,8 +45,9 @@
 #include <small/rlist.h>
 
 #include "diag.h"
+#include "fiber.h"
+#include "fiber_cond.h"
 #include "say.h"
-#include "latch.h"
 #include "vclock.h"
 #include "cbus.h"
 #include "engine.h"		/* engine_collect_garbage() */
@@ -54,6 +55,9 @@
 
 struct gc_state gc;
 
+static int
+gc_fiber_f(va_list);
+
 /**
  * Comparator used for ordering gc_consumer objects by signature
  * in a binary tree.
@@ -100,7 +104,13 @@ gc_init(void)
 	vclock_create(&gc.vclock);
 	rlist_create(&gc.checkpoints);
 	gc_tree_new(&gc.consumers);
-	latch_create(&gc.latch);
+	fiber_cond_create(&gc.cond);
+
+	gc.fiber = fiber_new("gc", gc_fiber_f);
+	if (gc.fiber == NULL)
+		panic("failed to start garbage collection fiber");
+
+	fiber_start(gc.fiber);
 }
 
 static void
@@ -202,13 +212,6 @@ gc_run(void)
 		return; /* nothing to do */
 
 	/*
-	 * Engine callbacks may sleep, because they use coio for
-	 * removing files. Make sure we won't try to remove the
-	 * same file multiple times by serializing concurrent gc
-	 * executions.
-	 */
-	latch_lock(&gc.latch);
-	/*
 	 * Run garbage collection.
 	 *
 	 * The order is important here: we must invoke garbage
@@ -220,7 +223,51 @@ gc_run(void)
 		rc = engine_collect_garbage(&checkpoint->vclock);
 	if (run_wal_gc && rc == 0)
 		wal_collect_garbage(vclock);
-	latch_unlock(&gc.latch);
+}
+
+static int
+gc_fiber_f(va_list ap)
+{
+	(void)ap;
+	while (!fiber_is_cancelled()) {
+		int delta = gc.scheduled - gc.completed;
+		if (delta == 0) {
+			/* No pending garbage collection. */
+			fiber_sleep(TIMEOUT_INFINITY);
+			continue;
+		}
+		assert(delta > 0);
+		gc_run();
+		gc.completed += delta;
+		fiber_cond_signal(&gc.cond);
+	}
+	return 0;
+}
+
+/**
+ * Trigger asynchronous garbage collection.
+ */
+static void
+gc_schedule(void)
+{
+	/*
+	 * Do not wake up the background fiber if it's executing
+	 * the garbage collection procedure right now, because
+	 * it may be waiting for a cbus message, which doesn't
+	 * tolerate spurious wakeups. Just increment the counter
+	 * then - it will rerun garbage collection as soon as
+	 * the current round completes.
+	 */
+	if (gc.scheduled++ == gc.completed)
+		fiber_wakeup(gc.fiber);
+}
+
+void
+gc_wait(void)
+{
+	unsigned scheduled = gc.scheduled;
+	while (gc.completed < scheduled)
+		fiber_cond_wait(&gc.cond);
 }
 
 /**
@@ -255,7 +302,7 @@ gc_process_wal_event(struct wal_watcher_msg *msg)
 
 		consumer = next;
 	}
-	gc_run();
+	gc_schedule();
 }
 
 void
@@ -276,7 +323,7 @@ gc_add_checkpoint(const struct vclock *vclock)
 		 * Rerun the garbage collector in this case, just
 		 * in case box.cfg.checkpoint_count has changed.
 		 */
-		gc_run();
+		gc_schedule();
 		return;
 	}
 	assert(last_checkpoint == NULL ||
@@ -295,7 +342,7 @@ gc_add_checkpoint(const struct vclock *vclock)
 	rlist_add_tail_entry(&gc.checkpoints, checkpoint, in_checkpoints);
 	gc.checkpoint_count++;
 
-	gc_run();
+	gc_schedule();
 }
 
 void
@@ -314,7 +361,7 @@ void
 gc_unref_checkpoint(struct gc_checkpoint_ref *ref)
 {
 	rlist_del_entry(ref, in_refs);
-	gc_run();
+	gc_schedule();
 }
 
 struct gc_consumer *
@@ -342,7 +389,7 @@ gc_consumer_unregister(struct gc_consumer *consumer)
 {
 	if (!consumer->is_inactive) {
 		gc_tree_remove(&gc.consumers, consumer);
-		gc_run();
+		gc_schedule();
 	}
 	gc_consumer_delete(consumer);
 }
@@ -376,7 +423,7 @@ gc_consumer_advance(struct gc_consumer *consumer, const struct vclock *vclock)
 	if (update_tree)
 		gc_tree_insert(&gc.consumers, consumer);
 
-	gc_run();
+	gc_schedule();
 }
 
 struct gc_consumer *
diff --git a/src/box/gc.h b/src/box/gc.h
index eab19ba3..6e96d7bb 100644
--- a/src/box/gc.h
+++ b/src/box/gc.h
@@ -34,8 +34,8 @@
 #include <stddef.h>
 #include <small/rlist.h>
 
+#include "fiber_cond.h"
 #include "vclock.h"
-#include "latch.h"
 #include "wal.h"
 #include "trivia/util.h"
 
@@ -43,6 +43,7 @@
 extern "C" {
 #endif /* defined(__cplusplus) */
 
+struct fiber;
 struct gc_consumer;
 
 enum { GC_NAME_MAX = 64 };
@@ -122,15 +123,30 @@ struct gc_state {
 	/** Registered consumers, linked by gc_consumer::node. */
 	gc_tree_t consumers;
 	/**
-	 * Latch serializing concurrent invocations of engine
-	 * garbage collection callbacks.
-	 */
-	struct latch latch;
-	/**
 	 * WAL event watcher. Needed to shoot off stale consumers
 	 * when a WAL file is deleted due to ENOSPC.
 	 */
 	struct wal_watcher wal_watcher;
+	/** Fiber that removes old files in the background. */
+	struct fiber *fiber;
+	/**
+	 * Condition variable signaled by the background fiber
+	 * whenever it completes a round of garbage collection.
+	 * Used to wait for garbage collection to complete.
+	 */
+	struct fiber_cond cond;
+	/**
+	 * The following two members are used for scheduling
+	 * background garbage collection and waiting for it to
+	 * complete. To trigger background garbage collection,
+	 * @scheduled is incremented. Whenever a round of garbage
+	 * collection completes, @completed is incremented. Thus
+	 * to wait for background garbage collection scheduled
+	 * at a particular moment of time to complete, one should
+	 * sleep until @completed reaches the value of @scheduled
+	 * taken at that moment of time.
+	 */
+	unsigned completed, scheduled;
 };
 extern struct gc_state gc;
 
@@ -188,6 +204,13 @@ void
 gc_free(void);
 
 /**
+ * Wait for background garbage collection scheduled prior
+ * to this point to complete.
+ */
+void
+gc_wait(void);
+
+/**
  * Update the minimal number of checkpoints to preserve.
  * Called when box.cfg.checkpoint_count is updated.
  *
-- 
2.11.0