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Code to create Go mocks for bazel targets using mockgen
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BUILD.bazel add WORKSPACE and BUILD.bazel files Feb 11, 2018
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gomock for Bazel

This skylark code allows you to generate code with mockgen (from golang/mock) and use that code as a dependency in your bazel projects. It handles all the GOPATH stuff for you.

bazel_gomock requires a rules_go external to be set up in your WORKSPACE as well as a go_repository setup for com_github_golang_mock (unless you override an argument; see below).

You call it in your BUILD files as

    name = "mock_sess",
    out = "mock_sess_test.go",
    interfaces = ["SessionsClient"],
    library = "//proto/sessions/sessproto:go_default_library",
    package = "main",

where library is a go_library target, interfaces is the list of names of the Go interfaces you'd like mockgen to generate mocks of, package is the name of the Go package at the top of the generated file (in this example, package "main"), and out is the path of generated source file that will be made.

There is also a source parameter described below.

You use this target's out file directly in the srcs parameter in go_test, go_library, and so on. So, when the above example gomock call is used in the same BUILD file, you put mock_sess_test.go in the srcs parameter like so:

    name = "go_default_test",
    srcs = [
    embed = [":go_default_library"]

If you need to generate mocks from a specific Go file instead of a import path (say, because the go_library you have is a main package and is therefore unreflectable by Go tools and specifically unimportable by mockgen), add the source parameter with the location of source file. E.g. source = "//fancy/path:foo.go" or just source = "foo.go" if the file is in the same directory). The library parameter must still be set to the library that source file lives in so that any referenced dependencies can be pulled into the Go path.

Also, gazelle will remove the generated source file from a go_test target's srcs unless you end the generated file name with _test.go.

As a likely unused feature, you can pass in an alternative external for where to find the mockgen tool target using the mockgen_tool parameter. The only rule for the target is that must be a binary. The current default is "@com_github_golang_mock//mockgen".

If you try to use gomock on a go_library that is in the package main (and so probably being immediately used as an embed target for a go_binary), you'll get an annoying error like:

prog.go:13:2: import "your/main/package/deal" is a program, not an importable package

You can resolve that by setting the source parameter to the location of the file with the interfaces you want in it.

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