gc: rename checkpoint_count to min_checkpoint_count

Because it's the minimal number of checkpoints that must not be deleted,
not the actual number of preserved checkpoints. Do it now, in a separate
patch so as to ease review of the next patch.

While we are at it, fix the comment to gc_set_(min_)checkpoint_count()
which got outdated by commit 5512053f ("box: gc: do not remove files
being backed up").

src/box/box.cc

@@ -826,7 +826,7 @@ box_set_checkpoint_count(void)
 {
 	int checkpoint_count = cfg_geti("checkpoint_count");
 	box_check_checkpoint_count(checkpoint_count);
-	gc_set_checkpoint_count(checkpoint_count);
+	gc_set_min_checkpoint_count(checkpoint_count);
 }
 
 void

src/box/gc.c

@@ -116,8 +116,8 @@ gc_tree_first_checkpoint(gc_tree_t *consumers)
 void
 gc_run(void)
 {
-	int checkpoint_count = gc.checkpoint_count;
-	assert(checkpoint_count > 0);
+	int min_checkpoint_count = gc.min_checkpoint_count;
+	assert(min_checkpoint_count > 0);
 
 	/* Look up the consumer that uses the oldest WAL. */
 	struct gc_consumer *leftmost = gc_tree_first(&gc.consumers);
@@ -127,8 +127,9 @@ gc_run(void)
 
 	/*
 	 * Find the oldest checkpoint that must be preserved.
-	 * We have to maintain @checkpoint_count oldest checkpoints,
-	 * plus we can't remove checkpoints that are still in use.
+	 * We have to preserve @min_checkpoint_count oldest
+	 * checkpoints, plus we can't remove checkpoints that
+	 * are still in use.
 	 */
 	struct vclock gc_checkpoint_vclock;
 	vclock_create(&gc_checkpoint_vclock);
@@ -138,7 +139,7 @@ gc_run(void)
 
 	const struct vclock *vclock;
 	while ((vclock = checkpoint_iterator_prev(&checkpoints)) != NULL) {
-		if (--checkpoint_count > 0)
+		if (--min_checkpoint_count > 0)
 			continue;
 		if (leftmost_checkpoint != NULL &&
 		    vclock_sum(&leftmost_checkpoint->vclock) < vclock_sum(vclock))
@@ -188,9 +189,9 @@ gc_run(void)
 }
 
 void
-gc_set_checkpoint_count(int checkpoint_count)
+gc_set_min_checkpoint_count(int min_checkpoint_count)
 {
-	gc.checkpoint_count = checkpoint_count;
+	gc.min_checkpoint_count = min_checkpoint_count;
 }
 
 struct gc_consumer *

src/box/gc.h

@@ -75,8 +75,11 @@ typedef rb_tree(struct gc_consumer) gc_tree_t;
 
 /** Garbage collection state. */
 struct gc_state {
-	/** Number of checkpoints to maintain. */
-	int checkpoint_count;
+	/**
+	 * Minimal number of checkpoints to preserve.
+	 * Configured by box.cfg.checkpoint_count.
+	 */
+	int min_checkpoint_count;
 	/** Max vclock WAL garbage collection has been called for. */
 	struct vclock wal_vclock;
 	/** Max vclock checkpoint garbage collection has been called for. */
@@ -112,11 +115,15 @@ void
 gc_run(void);
 
 /**
- * Update the checkpoint_count configuration option and
- * rerun garbage collection.
+ * Update the minimal number of checkpoints to preserve.
+ * Called when box.cfg.checkpoint_count is updated.
+ *
+ * Note, this function doesn't run garbage collector so
+ * changes will take effect only after a new checkpoint
+ * is created or a consumer is unregistered.
  */
 void
-gc_set_checkpoint_count(int checkpoint_count);
+gc_set_min_checkpoint_count(int min_checkpoint_count);
 
 /**
  * Register a consumer.