expression/expressions: replace iconv-go with text/transform

[unknwon]

Remove CGO requirement for compiling iconv-go currently is using.

Replace it with native implementation of Go.

make test has passed, but test cases don't look very detailed on expression, so not sure if everything will go well.

[unknwon]

Thank god, passed this time.

[xiang90]

LGTM. But I cannot press the merge button. :(

[unknwon]

Godep is killing GitHub diff. So, you can't even find where did I changed.

[xiang90]

@unknwon I usually keep them into separate commits:

1. change
2. godep

[unknwon]

godep save doesn't work with tidb out of box, so I changed everything manually 😂 really don't want to go through hell again.

[unknwon]

OK... there is the diff:

diff --git a/expression/expressions/convert.go b/expression/expressions/convert.go
index b636cb5..8fc7918 100644
--- a/expression/expressions/convert.go
+++ b/expression/expressions/convert.go
@@ -17,11 +17,12 @@ import (
        "fmt"
        "strings"

-       iconv "github.com/djimenez/iconv-go"
        "github.com/juju/errors"
        "github.com/ngaut/log"
        "github.com/pingcap/tidb/context"
        "github.com/pingcap/tidb/expression"
+       "golang.org/x/net/html/charset"
+       "golang.org/x/text/transform"
 )

 // FunctionConvert provides a way to convert data between different character sets.
@@ -74,7 +75,13 @@ func (f *FunctionConvert) Eval(ctx context.Context, args map[interface{}]interfa
        } else if strings.ToLower(f.Charset) == "utf8mb4" {
                return value, nil
        }
-       target, err := iconv.ConvertString(str, "utf-8", f.Charset)
+
+       encoding, _ := charset.Lookup(f.Charset)
+       if encoding == nil {
+               return nil, fmt.Errorf("unknow char decoder %s", f.Charset)
+       }
+
+       target, _, err := transform.String(encoding.NewDecoder(), str)
        if err != nil {
                log.Errorf("Convert %s to %s with error: %v", str, f.Charset, err)
                return nil, errors.Trace(err)

[ngaut]

Thanks @unknwon

And @xiang90 is right, keep them into separate commits:

1. change
2. godep

The correct step is:
cd tidb
godep restore
godep save ./...

Could you fix it ? @unknwon Thanks.

[coocood]

@unknwon this PR introduced too much dependencies, can you remove the depedency of "golang.org/x/net/html/charset", all we need is a map from charset name to encoding.
How about copy the code in https://github.com/golang/net/blob/master/html/charset/table.go

[unknwon]

#42