server: add get first region meta support by using empty search key.

server/cluster.go

@@ -389,10 +389,6 @@ func (c *raftCluster) GetStore(storeID uint64) (*metapb.Store, error) {
 }
 
 func (c *raftCluster) GetRegion(regionKey []byte) (*metapb.Region, error) {
-	if len(regionKey) == 0 {
-		return nil, errors.New("invalid empty region key")
-	}
-
 	// We must use the next region key for search,
 	// e,g, we have two regions 1, 2, and key ranges are ["", "abc"), ["abc", +infinite),
 	// if we use "abc" to search the region, the first key >= "abc" may be

server/cluster_worker_test.go

@@ -732,7 +732,14 @@ func (s *testClusterWorkerSuite) TestSplit(c *C) {
 		{"e", "d", "", "f"},
 	}
 
-	for _, t := range tbl {
+	var firstRegionStartKey, firstRegionEndKey string
+	for i, t := range tbl {
+		// Get first region start and end key.
+		if i == 0 {
+			firstRegionStartKey = t.startKey
+			firstRegionEndKey = t.splitKey
+		}
+
 		regionKey := []byte(t.searchKey)
 		region, err := cluster.GetRegion(regionKey)
 		c.Assert(err, IsNil)
@@ -777,5 +784,12 @@ func (s *testClusterWorkerSuite) TestSplit(c *C) {
 		c.Assert(err, IsNil)
 		c.Assert(right.GetStartKey(), BytesEquals, []byte(t.splitKey))
 		c.Assert(right.GetEndKey(), BytesEquals, []byte(t.endKey))
+
+		// Test get first region.
+		regionKey = []byte{}
+		region, err = cluster.GetRegion(regionKey)
+		c.Assert(err, IsNil)
+		c.Assert(region.GetStartKey(), BytesEquals, []byte(firstRegionStartKey))
+		c.Assert(region.GetEndKey(), BytesEquals, []byte(firstRegionEndKey))
 	}
 }