blog: Building Go Plugins inside Docker

content/blog/2019/2019-11-24-build-go-plugins-using-docker.md

++++
+title = "Building Go Plugins inside Docker"
+date = 2019-11-24T15:30:03+05:30
+draft = false
+tags = ["golang", "docker"]
+categories = ["tutorials"]
+type = "post"
+url = "blog/2019/11/15/building-go-plugins-using-docker"
+author = "Rohan Verma"
++++
+
+Using Go plugins in your projects comes with a lot of caveats. As of writing, 
+there hasn't been much development on the feature recently. The 
+[commit history](https://github.com/golang/go/commits/master/src/plugin/plugin.go)
+shows us that the last commit happened nearly 2 years ago. On the gopher slack,
+the sentiment, more or less, is that this is not a priority anymore. Along with 
+this, there are multiple issues that come up with maintaining projects that use
+it:
+
+- The go version for both host and plugin should match exactly
+- External dependencies should match
+- Host and plugin `GOPATH` needs to exactly match while building
+- Plugins cannot depend on interfaces or structs of the host
+
+To learn more you can refer [this issue](https://github.com/golang/go/issues/20481#issuecomment-326832200) detailing some of the problems with
+go plugins
+
+It works well if the projects bundles all the plugins in its own source tree and
+both the host and plugins are built together at the same time. But, that limits
+the scope of the project. Externally maintained plugins are impossible to
+build independent of the host program.
+
+To solve this, we can use Docker to build both the host program, the bundled plugins,
+and the custom plugin together. Then this image can be distributed instead
+of distributing the host and loading the plugins separately in production.
+
+```Dockerfile
+FROM golang:1.12-alpine AS builder
+RUN apk update && apk add gcc libc-dev make git
+WORKDIR /myproject-plugin/
+# Clone and build myproject and myproject-plugin together
+# prevent version conflict for go plugins
+RUN git clone https://github.com/rhnvrm/myproject.git && \
+    mkdir -p myproject/bundled_plugins/myplugin
+# Load our custom plugin from disk
+COPY ./myplugin.go ./myproject/bundled_plugins/myplugin/myplugin.go
+# CGO_ENABLED=1 is required
+ENV CGO_ENABLED=1 GOOS=linux
+# `make build` builds the host program and bundled plugins
+# `go build` our custom plugin 
+RUN cd myproject && \
+    make deps && make build && \
+    go build -ldflags="-s -w" -buildmode=plugin -o myplugin.prov \
+    bundled_plugins/myplugin/myplugin.go
+
+FROM alpine:latest AS deploy
+RUN apk --no-cache add ca-certificates
+WORKDIR /myproject/
+# Copy the assets from the builder image
+COPY --from=builder /myproject-plugin/myproject/myproject /myproject-plugin/myproject-plugin/myproject/smtp.prov /myproject-plugin/myproject/bundled1.prov /myproject-plugin/myproject/bundled2.prov /myproject-plugin/myproject/myplugin.prov ./
+
+CMD ["./myproject", "--config", "/etc/myproject/config.toml", "--prov", "smtp.prov", "--prov", "bundled1.prov", "--prov", "bundled2.prov", "--prov", "myplugin.prov"]
+```
+
+This image can then be pushed and used where it needs to be deployed.