Manually fetch dependencies from go.mod?

It was an issue #26610, which is fixed now.

So now you can just use:

go mod download

For this to work you need just the go.mod / go.sum files.

For example, here's how to have a cached multistage Docker build: (source)

FROM golang:1.17-alpine as builder
RUN apk --no-cache add ca-certificates git
WORKDIR /build

# Fetch dependencies
COPY go.mod go.sum ./
RUN go mod download

# Build
COPY . ./
RUN CGO_ENABLED=0 go build

# Create final image
FROM alpine
WORKDIR /
COPY --from=builder /build/myapp .
EXPOSE 8080
CMD ["./myapp"]

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Also see the article Containerize Your Go Developer Environment – Part 2, which describes how to leverage the Go compiler cache to speed up builds even further.

I wanted to re-download all the dependencies using go mod, this is what I did:

1. Go to your GOROOT
2. sudo rm -rf pkg/mod/
3. Go to the directory where the go.mod file exists
4. go mod download

You may use the go mod vendor command which will create a vendor folder in the main module's root folder, and copy all dependencies into it. After this you may pass the -mod=vendor param to the go tool, and then dependencies from the vendor folder will be used to build / compile / test your app.

So what you may do to speed up your builds:

1. Run the go mod vendor command to have an actual version of your dependencies.
2. Save / cache this vendor folder.
3. During builds, restore this vendor folder, and build / install your app by passing the -mod=vendor argument to the go tool, so no dependencies will be downloaded, but the content of the vendor folder will be used.

Quoting from go help mod:

Modules and vendoring

When using modules, the go command completely ignores vendor directories.

By default, the go command satisfies dependencies by downloading modules from their sources and using those downloaded copies (after verification, as described in the previous section). To allow interoperation with older versions of Go, or to ensure that all files used for a build are stored together in a single file tree, 'go mod vendor' creates a directory named vendor in the root directory of the main module and stores there all the packages from dependency modules that are needed to support builds and tests of packages in the main module.

To build using the main module's top-level vendor directory to satisfy dependencies (disabling use of the usual network sources and local caches), use 'go build -mod=vendor'. Note that only the main module's top-level vendor directory is used; vendor directories in other locations are still ignored.