Fixes mediumint unsigned overflowing inside uint32

Shopify#299

dml_events.go

@@ -292,7 +292,7 @@ func NewBinlogDMLEvents(table *TableSchema, ev *replication.BinlogEvent, pos, re
 			)
 		}
 		for i, col := range table.Columns {
-			if col.IsUnsigned {
+			if col.IsUnsigned && col.Type != schema.TYPE_MEDIUM_INT {
 				switch v := row[i].(type) {
 				case int64:
 					row[i] = uint64(v)
@@ -305,6 +305,9 @@ func NewBinlogDMLEvents(table *TableSchema, ev *replication.BinlogEvent, pos, re
 				case int:
 					row[i] = uint(v)
 				}
+			} else if col.IsUnsigned && col.Type == schema.TYPE_MEDIUM_INT {
+				val := row[i].(int32)
+				row[i] = NewUint24(val).Uint32()
 			}
 		}
 	}

uint24.go

@@ -0,0 +1,27 @@
+// with the courtesy of this github file: https://github.com/Cryptkeeper/go-fseq/blob/v0.2.6/pkg/uint24/uint24.go
+// used to convert a int32 to uint24 (mediumint)
+package ghostferry
+
+const MaxUint24 = 1<<24 - 1
+
+func NewUint24(val int32) *Uint24 {
+	var u = new(Uint24)
+	u.Set(val)
+	return u
+}
+
+type Uint24 [3]uint8
+
+func (u *Uint24) Set(val int32) {
+	// panic since this is closer to how a compiler/runtime would treat an overflow compared to err returns
+	if val > MaxUint24 {
+		panic("cannot set Uint24 larger than uint24.MaxUint24")
+	}
+	(*u)[0] = uint8(val & 0xFF)
+	(*u)[1] = uint8((val >> 8) & 0xFF)
+	(*u)[2] = uint8((val >> 16) & 0xFF)
+}
+
+func (u Uint24) Uint32() uint32 {
+	return uint32(u[0]) | uint32(u[1])<<8 | uint32(u[2])<<16
+}