gc: fix panic if checkpoint_interval is updated during checkpoint

To wait for a thread writing a snap file memtx_engine_wait_checkpoint()
uses cord_cojoin(), which doesn't tolerate spurious wakeups. Hence we
must not wake up the checkpoint fiber after reconfiguring the checkpoint
interval if it's currently making a checkpoint.

Fixes 4c04808a ("Rewrite checkpoint daemon in C").

Closes #3878

src/box/gc.c

@@ -311,10 +311,15 @@ gc_set_checkpoint_interval(double interval)
 	/*
 	 * Reconfigure the schedule and wake up the checkpoint
 	 * daemon so that it can readjust.
+	 *
+	 * Note, we must not wake up the checkpoint daemon fiber
+	 * if it's waiting for checkpointing to complete, because
+	 * checkpointing code doesn't tolerate spurious wakeups.
 	 */
 	checkpoint_schedule_cfg(&gc.checkpoint_schedule,
 				ev_monotonic_now(loop()), interval);
-	fiber_wakeup(gc.checkpoint_fiber);
+	if (!gc.checkpoint_is_in_progress)
+		fiber_wakeup(gc.checkpoint_fiber);
 }
 
 void

test/xlog/errinj.result

@@ -85,3 +85,71 @@ test:drop()
 errinj = nil
 ---
 ...
+--
+-- gh-3878: Panic when checkpoint_interval is reconfigured while
+-- the checkpoint daemon is making a checkpoint.
+--
+fio = require('fio')
+---
+...
+box.space._schema:delete('test') -- bump LSN
+---
+...
+box.error.injection.set('ERRINJ_SNAP_WRITE_ROW_TIMEOUT', 0.1)
+---
+- ok
+...
+default_checkpoint_interval = box.cfg.checkpoint_interval
+---
+...
+box.cfg{checkpoint_interval = 0.01}
+---
+...
+-- Wait for the checkpoint daemon to start making a checkpoint.
+test_run:cmd("setopt delimiter ';'")
+---
+- true
+...
+test_run:wait_cond(function()
+    local filename = string.format('%020d.snap.inprogress', box.info.signature)
+    return fio.path.exists(fio.pathjoin(box.cfg.memtx_dir, filename))
+end, 10);
+---
+- true
+...
+test_run:cmd("setopt delimiter ''");
+---
+- true
+...
+box.cfg{checkpoint_interval = 0.1}
+---
+...
+box.cfg{checkpoint_interval = 1}
+---
+...
+box.cfg{checkpoint_interval = 10}
+---
+...
+box.cfg{checkpoint_interval = default_checkpoint_interval}
+---
+...
+box.error.injection.set('ERRINJ_SNAP_WRITE_ROW_TIMEOUT', 0)
+---
+- ok
+...
+-- Wait for the checkpoint daemon to finish making a checkpoint.
+test_run:cmd("setopt delimiter ';'")
+---
+- true
+...
+test_run:wait_cond(function()
+    local filename = string.format('%020d.snap', box.info.signature)
+    return fio.path.exists(fio.pathjoin(box.cfg.memtx_dir, filename))
+end, 10);
+---
+- true
+...
+test_run:cmd("setopt delimiter ''");
+---
+- true
+...

test/xlog/errinj.test.lua

@@ -41,3 +41,32 @@ errinj.set('ERRINJ_WAL_WRITE', false)
 -- Cleanup
 test:drop()
 errinj = nil
+
+--
+-- gh-3878: Panic when checkpoint_interval is reconfigured while
+-- the checkpoint daemon is making a checkpoint.
+--
+fio = require('fio')
+box.space._schema:delete('test') -- bump LSN
+box.error.injection.set('ERRINJ_SNAP_WRITE_ROW_TIMEOUT', 0.1)
+default_checkpoint_interval = box.cfg.checkpoint_interval
+box.cfg{checkpoint_interval = 0.01}
+-- Wait for the checkpoint daemon to start making a checkpoint.
+test_run:cmd("setopt delimiter ';'")
+test_run:wait_cond(function()
+    local filename = string.format('%020d.snap.inprogress', box.info.signature)
+    return fio.path.exists(fio.pathjoin(box.cfg.memtx_dir, filename))
+end, 10);
+test_run:cmd("setopt delimiter ''");
+box.cfg{checkpoint_interval = 0.1}
+box.cfg{checkpoint_interval = 1}
+box.cfg{checkpoint_interval = 10}
+box.cfg{checkpoint_interval = default_checkpoint_interval}
+box.error.injection.set('ERRINJ_SNAP_WRITE_ROW_TIMEOUT', 0)
+-- Wait for the checkpoint daemon to finish making a checkpoint.
+test_run:cmd("setopt delimiter ';'")
+test_run:wait_cond(function()
+    local filename = string.format('%020d.snap', box.info.signature)
+    return fio.path.exists(fio.pathjoin(box.cfg.memtx_dir, filename))
+end, 10);
+test_run:cmd("setopt delimiter ''");