Modify pserver client C API, create better test.

[helinwang]

Please refer to the design doc diff in this PR for what in API have changed.

[jacquesqiao]

It seems that golang's strings#join can do the same thing?
https://golang.org/pkg/strings/#Join

[helinwang]

Thanks! Makes the code much better! Done.

[jacquesqiao]

should we define some error code? When I use this client, I have to search the code to make sure the meaning of the ret code

[helinwang]

Sorry! Added.

[jacquesqiao]

what is the benefit of using const instead of errors?

[helinwang]

Good question, it's pretty tricky here.
Because if we are using

var ErrAlreadyInitialized = errors.New("pserver already initialized")

func Foo(a int b *int) error {
  return ErrAlreadyInitialized
}

The RPC client can not compare the error:

err := client.Call("Service.Foo", 1, nil)
fmt.Println(err == ErrAlreadyInitialized)
// Output:
// false

This is happening because err is of type error, which is an interface.
Interface comparison rule:

A value x of non-interface type X and a value t of interface type T are comparable when values of type X are comparable and X implements T. They are equal if t's dynamic type is identical to X and t's dynamic value is equal to x.

Since the err instance is deserialized by RPC, which is of different instance (dynamic value) than ErrAlreadyInitialized, they are not equal.

Btw, comparing things like io.EOF is typically fine:

_, err := os.Open("...")
if err == io.EOF {} // fine, os.Open return the same io.EOF instance.

if err == errors.New("EOF") {} // always false!

However, compare by string type is fine. Since comparison rule for string is just comparing the bytes. That why here is using string instead of error.

[jacquesqiao]

get it, thanx

[jacquesqiao]

Here we can check the value got from pserver.

[helinwang]

The parameter value could change due to optimization happening on the pserver. So I did not check the value here.

[jacquesqiao]

I see

[jacquesqiao]

LGTM