I tried to follow your reproduction instructions. In order to build the plugin I had to add -tags mock. To avoid version errors I had to use that option with the other builds. Running go test succeeds for me. You suggest running go test plugin_test.go interface.go but there is no file plugin_test.go and that command doesn't make much sense anyhow.

Please show us the exact commands that you type and the exact output that you get. Thanks.

CC @crawshaw

@ianlancetaylor Thanks for your attention. I think you had compiled master branch of the project. I've simplified the code and put them on a branch named plugin_test.

Please show us the exact commands that you type and the exact output that you get. Thanks.

Sorry, I don't describe the problem clearly. Now I record all command and output step by step.

1. Checkout code from https://github.com/opera/terago/tree/plugin_test (plugin_test is a branch, not master)
2. Run go build -buildmode plugin plugin/terago.go
3. Run go build -o plugin_example examples/plugin.go
4. Run go test -v plugin_test.go interface.go

Here is all output:

peilinx@:~/go/src/github.com/opera/terago$ ls
client_mock.go  examples  interface.go  LICENSE  Makefile  plugin  plugin_test.go  README.md
peilinx@:~/go/src/github.com/opera/terago$ go build -buildmode plugin plugin/terago.go
peilinx@:~/go/src/github.com/opera/terago$ go build -o plugin_example examples/plugin.go
peilinx@:~/go/src/github.com/opera/terago$ ls
client_mock.go  examples  interface.go  LICENSE  Makefile  plugin  plugin_example  plugin_test.go  README.md  terago.so
peilinx@:~/go/src/github.com/opera/terago$ ./plugin_example 
new mock client
close mock client
peilinx@:~/go/src/github.com/opera/terago$ go test -v plugin_test.go interface.go
=== RUN   Test
--- FAIL: Test (0.01s)
panic: interface conversion: plugin.Symbol is func(string, string) (terago.ClientI, error), not func(string, string) (terago.ClientI, error) [recovered]
	panic: interface conversion: plugin.Symbol is func(string, string) (terago.ClientI, error), not func(string, string) (terago.ClientI, error)

goroutine 5 [running]:
testing.tRunner.func1(0xc42009c0f0)
	/usr/local/go/src/testing/testing.go:711 +0x2d2
panic(0x5ef860, 0xc4200606c0)
	/usr/local/go/src/runtime/panic.go:491 +0x283
command-line-arguments.Test(0xc42009c0f0)
	/home/peilinx/go/src/github.com/opera/terago/plugin_test.go:23 +0x494
testing.tRunner(0xc42009c0f0, 0x61f000)
	/usr/local/go/src/testing/testing.go:746 +0xd0
created by testing.(*T).Run
	/usr/local/go/src/testing/testing.go:789 +0x2de
exit status 2
FAIL	command-line-arguments	0.013s
peilinx@:~/go/src/github.com/opera/terago$ 

examples/plugin.go and plugin_test.go are almost same code. But unit test cannot pass.

Change your test file to be like this https://pastebin.com/3TuSkHVQ

terago/plugin imports terago and the test file is in package terago. Building the plugin will not include the test file therefore the packages are different on compile time.

Changing the test file to be in package terago_test makes it a different package and therefore makes terago the same package when compiling the plugin and test.

@AlexRouSg Thank you. You are right. Now the unit test is passed.
Verify mechanism of plugins is so strict.

Sounds like this is fixed, more or less.