fix sequence cache can be negative (pingcap#18071)

ddl/sequence.go

@@ -144,6 +144,10 @@ func validateSequenceOptions(seqInfo *model.SequenceInfo) bool {
 		// Increment shouldn't be set as 0.
 		return false
 	}
+	if seqInfo.CacheValue <= 0 {
+		// Cache value should be bigger than 0.
+		return false
+	}
 	maxIncrement = math2.Abs(seqInfo.Increment)
 
 	return seqInfo.MaxValue >= seqInfo.Start &&

ddl/sequence_test.go

@@ -972,3 +972,30 @@ func (s *testSequenceSuite) TestSequenceDefaultLogic(c *C) {
 	s.tk.MustGetErrMsg("alter table t add column b int default next value for seq", "[ddl:8230]Unsupported using sequence as default value in add column 'b'")
 	s.tk.MustQuery("select * from t").Check(testkit.Rows("-1", "-1", "-1"))
 }
+
+// Close issue #17945, sequence cache shouldn't be negative.
+func (s *testSequenceSuite) TestSequenceCacheShouldNotBeNegative(c *C) {
+	s.tk = testkit.NewTestKit(c, s.store)
+	s.tk.MustExec("use test")
+
+	s.tk.MustExec("drop sequence if exists seq")
+	_, err := s.tk.Exec("create sequence seq cache -1")
+	c.Assert(err, NotNil)
+	c.Assert(err.Error(), Equals, "[ddl:4136]Sequence 'test.seq' values are conflicting")
+
+	_, err = s.tk.Exec("create sequence seq cache 0")
+	c.Assert(err, NotNil)
+	c.Assert(err.Error(), Equals, "[ddl:4136]Sequence 'test.seq' values are conflicting")
+
+	// This will error because
+	// 1: maxvalue = -1 by default
+	// 2: minvalue = -9223372036854775807 by default
+	// 3: increment = -9223372036854775807 by user
+	// `seqInfo.CacheValue < (math.MaxInt64-absIncrement)/absIncrement` will
+	// ensure there is enough value for one cache allocation at least.
+	_, err = s.tk.Exec("create sequence seq INCREMENT -9223372036854775807 cache 1")
+	c.Assert(err, NotNil)
+	c.Assert(err.Error(), Equals, "[ddl:4136]Sequence 'test.seq' values are conflicting")
+
+	s.tk.MustExec("create sequence seq cache 1")
+}