[THRIFT-2451] Do not use pointers for optional fields with defaults. Do not write such fields if its value set to default. Also, do not use pointers for any optional fields mapped to go map or slice. generate Get accessors

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 0.9.2
• Component/s: Go - Compiler
• Patch Info: Patch Available
• Assignee: Jens Geyer
• Reporter: Aleksey Pesternikov
• Created: 05/Apr/14 21:29
• Updated: 26/May/14 22:42
• Resolved: 01/May/14 22:27

Description

Currently, for optional fields in struct

struct pkt {
 1: optional string s = "DEFAULT",
 2: optional i64 i = 42,
 3: optional bool b = false
}

go compiler generates the following:

type Pkt struct {
        S *string `thrift:"s,1"`
        I *int64  `thrift:"i,2"`
        B *bool   `thrift:"b,3"`
}

func NewPkt() *Pkt {
        rval := &Pkt{
                S: new(string),

                I: new(int64),

                B: new(bool),
        }
        *(rval.S) = "DEFAULT"
        *(rval.I) = 42
        *(rval.B) = false
        return rval
}

func (p *Pkt) IsSetS() bool {
        return p.S != nil
}

func (p *Pkt) IsSetI() bool {
        return p.I != nil
}

func (p *Pkt) IsSetB() bool {
        return p.B != nil
}

which is wrong in multiple ways:
1. Freshly initialized fields returns IsSetField() true
http://play.golang.org/p/T2pIX80ZJp
This results in
a. wrong semantics: freshly created struct has optional fields set
b. excessive payload produced on serialization (writing field value instead of skipping it)
2. Additional load on garbage collector
3. accessing field value is complicated and error prone. even without default value:

     if pkt.IsSetB() && *pkt.B {
        //do something for b==true
     }

would work for false default for field b. However, if I change default value to true, I need to change all occurrences in the code like this:

     if !pkt.IsSetB() || *pkt.B {
        //do something for b==true
     }

How to fix that?
there are two ways:
1. get back to generating inlines instead of pointers for optional fields with default value and compare with "magic value" of default in IsSet*(). could be tricky since not all types are comparable http://golang.org/ref/spec#Comparison_operators . notably, slices and maps are not.
2. approach, used in protobuf: Do not initialize optional fields, generate Get*() accessors like this:

var Pkt_B_Default = false
 func (p *Pkt) GetB() bool {
  if p.B == nil {
    return Pkt_B_Default
  }
  return *p.B
 }

Just to make API uniform, we can also generate accessors for required fields:

 func (p *Pkt) GetB() bool {
  return p.B
 }

I'm inclining to implement second approach, but I would like to collect opinions before I dig into the code.

Issue Links

• is related to: THRIFT-2429 Provide option to not write default values, rely on receiver default construction instead