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net/rpc: API Call() is not working as expected #29360

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SagarPuneria opened this Issue Dec 20, 2018 · 3 comments

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@SagarPuneria
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commented Dec 20, 2018

What version of Go are you using (go version)?

$ go version
go version go1.11.4 windows/386

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

Windows 10, 64 bit

What did you do?

I am using client.Call(serviceMethod string, args interface{}, reply interface{}) in my code for implementing HTTP RPC client program, where i have passed reference address of variable in both second parameter args(interface type) and third parameter reply(interface type). So that changes made to the receiver's methods in the DefaultServer get reflected outside the client.Call() method.
HTTP RPC client example

Output:
arg:"go version is go1.11.3 is not latest"
reply:"go version is go1.11.4 is latest"

HTTP RPC server example

Output:
Inside StringModify

What did you expect to see?

I expected to see the same string values to args and reply(both are interface type) after successfully calling client.Call(). Since i assigned same value at line number 20 as shown in HTTP RPC server example

What did you see instead?

After calling client.Call() args and reply string values are different. I see only reply get modified but not args.

@SagarPuneria

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commented Dec 20, 2018

@ALTree ALTree added this to the Go1.13 milestone Dec 21, 2018

@as

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commented Dec 23, 2018

I don't think this is an implementation bug, the documentation says the method should be of the form:

func (t *T) MethodName(argType T1, replyType *T2) error

I interpret that as T1 being immutable through the RPC. I would be surprised if T1 was mutated even when it is a pointer. The Arith example uses both arguments as pointer types, perhaps this suggests mutability.

	type Arith int

	func (t *Arith) Multiply(args *Args, reply *int) error {
		*reply = args.A * args.B
		return nil
	}

	func (t *Arith) Divide(args *Args, quo *Quotient) error {
		if args.B == 0 {
			return errors.New("divide by zero")
		}
		quo.Quo = args.A / args.B
		quo.Rem = args.A % args.B
		return nil
	}

Nowhere in the example is args actually modified though. It might be less confusing to make the example have a value type for args to make things unambiguous.

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commented Dec 26, 2018

@as, thanks for investigating.

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