fix panic when getting memory pressure fails

cdc/puller/sorter/backend_pool.go

@@ -112,17 +112,27 @@ func newBackEndPool(dir string, captureAddr string) (*backEndPool, error) {
 
 			// update memPressure
 			m, err := memory.Get()
+
+			failpoint.Inject("getMemoryPressureFails", func() {
+				m = nil
+				err = errors.New("injected get memory pressure failure")
+			})
+
 			if err != nil {
 				failpoint.Inject("sorterDebug", func() {
 					log.Panic("unified sorter: getting system memory usage failed", zap.Error(err))
 				})
 
 				log.Warn("unified sorter: getting system memory usage failed", zap.Error(err))
+				// Reports a 100% memory pressure, so that the backEndPool will allocate fileBackEnds.
+				// We default to fileBackEnds because they are unlikely to cause OOMs. If IO errors are
+				// encountered, we can fail gracefully.
+				atomic.StoreInt32(&ret.memPressure, 100)
+			} else {
+				memPressure := m.Used * 100 / m.Total
+				atomic.StoreInt32(&ret.memPressure, int32(memPressure))
 			}
 
-			memPressure := m.Used * 100 / m.Total
-			atomic.StoreInt32(&ret.memPressure, int32(memPressure))
-
 			if memPressure := ret.memoryPressure(); memPressure > 50 {
 				log.Debug("unified sorter: high memory pressure", zap.Int32("memPressure", memPressure),
 					zap.Int64("usedBySorter", ret.sorterMemoryUsage()))

cdc/puller/sorter/backend_pool_test.go

@@ -107,7 +107,7 @@ func (s *backendPoolSuite) TestDirectoryBadPermission(c *check.C) {
 	err := os.Chmod(dir, 0o311) // no permission to `ls`
 	c.Assert(err, check.IsNil)
 
-	conf := config.GetDefaultServerConfig()
+	conf := config.GetGlobalServerConfig()
 	conf.Sorter.MaxMemoryPressure = 0 // force using files
 
 	backEndPool, err := newBackEndPool(dir, "")
@@ -141,6 +141,7 @@ func (s *backendPoolSuite) TestCleanUpSelf(c *check.C) {
 
 	err = failpoint.Enable("github.com/pingcap/ticdc/cdc/puller/sorter/memoryPressureInjectPoint", "return(100)")
 	c.Assert(err, check.IsNil)
+	defer failpoint.Disable("github.com/pingcap/ticdc/cdc/puller/sorter/memoryPressureInjectPoint") //nolint:errcheck
 
 	backEndPool, err := newBackEndPool("/tmp/sorter", "")
 	c.Assert(err, check.IsNil)
@@ -296,3 +297,33 @@ func (s *backendPoolSuite) TestCleanUpStaleLockNoPermission(c *check.C) {
 
 	mockP.assertFilesExist(c)
 }
+
+// TestGetMemoryPressureFailure verifies that the backendPool can handle gracefully failures that happen when
+// getting the current system memory pressure. Such a failure is usually caused by a lack of file descriptor quota
+// set by the operating system.
+func (s *backendPoolSuite) TestGetMemoryPressureFailure(c *check.C) {
+	defer testleak.AfterTest(c)()
+
+	err := failpoint.Enable("github.com/pingcap/ticdc/cdc/puller/sorter/getMemoryPressureFails", "return(true)")
+	c.Assert(err, check.IsNil)
+	defer failpoint.Disable("github.com/pingcap/ticdc/cdc/puller/sorter/getMemoryPressureFails") //nolint:errcheck
+
+	dir := c.MkDir()
+	backEndPool, err := newBackEndPool(dir, "")
+	c.Assert(err, check.IsNil)
+	c.Assert(backEndPool, check.NotNil)
+	defer backEndPool.terminate()
+
+	after := time.After(time.Second * 20)
+	tick := time.Tick(time.Second * 1)
+	for {
+		select {
+		case <-after:
+			c.Fatal("TestGetMemoryPressureFailure timed out")
+		case <-tick:
+			if backEndPool.memoryPressure() == 100 {
+				return
+			}
+		}
+	}
+}