Go Buildpack cannot find local module. What am I missing?

php Editor Banana encountered a problem when using Go Buildpack: "The local module cannot be found, what am I missing?". Go Buildpack is a tool for building and running Go applications on the Cloud Foundry platform. This problem is usually caused by missing Go dependencies or incorrect configuration. The solution to this problem is to check your Go project's dependencies and ensure that environment variables such as GOPATH and GO111MODULE are set correctly.

Question content

I'm trying to build and start a go application on https://fly.io, but my test and template packages cannot be found when building, as shown below:

.
    ├── cmd
    │   ├── doc
    │   │   ├── go.mod
    │   │   └── main.go
    │   ├── git
    │   │   ├── go.mod
    │   │   └── main.go
    │   ├── imp
    │   │   ├── go.mod
    │   │   └── main.go
    │   ├── log
    │   │   ├── go.mod
    │   │   └── main.go
    │   ├── met
    │   │   ├── go.mod
    │   │   └── main.go
    │   ├── orc
    │   │   ├── go.mod
    │   │   └── main.go
    │   ├── pub
    │   │   ├── go.mod
    │   │   └── main.go
    │   ├── rep
    │   │   ├── go.mod
    │   │   └── main.go
    │   └── web
    │       ├── fly.toml
    │       ├── go.gen
    │       ├── go.mod
    │       ├── go.sum
    │       ├── handlers.go
    │       ├── handlers_test.go
    │       ├── main.go
    │       ├── main_test.go
    │       ├── router.go
    │       └── router_test.go
    ├── contributing.md
    ├── go.mod
    ├── go.work
    ├── internal
    ├── license.txt
    ├── main.go
    ├── pctl
    ├── pkg
    │   ├── **templates**
    │   │   ├── base.qtpl
    │   │   ├── base.qtpl.go
    │   │   ├── go.mod
    │   │   ├── go.sum
    │   │   ├── whoami.qtpl
    │   │   └── whoami.qtpl.go
    │   └── **test**
    │       ├── go.mod
    │       └── test.go
    └── readme.md

https://paketo.io/docs/reference/go-reference/#package-management-with-go-modules Status:

the buildpack will vendor dependencies using go modules if the app source code contains a go.mod file. during the build phase, the go-mod-vendor buildpack(opens in a new tab) checks to see if the application requires any external modules and if it does, runs the go mod vendor command for your app. the resulting vendor directory will exist in the app’s root directory and will contain all packages required for the build.

Looking at the build log, I found that go modvendor was indeed run.

Paketo Buildpack for Go Distribution 2.2.3
Resolving Go version
Candidate version sources (in priority order):
go.mod    -> ">= 1.19"
 -> ""
Selected Go version (using go.mod): 1.19.5
Executing build process
Installing Go 1.19.5
Completed in 35.526s
Generating SBOM for /layers/paketo-buildpacks_go-dist/go
Completed in 0s
Paketo Buildpack for Go Mod Vendor 1.0.7
Checking module graph
Running 'go mod graph'
Completed in 1.166s
Executing build process
Running 'go mod vendor'
Completed in 9.851s
Generating SBOM for /workspace/go.mod
Completed in 21ms
Paketo Buildpack for Go Build 2.0.8
Executing build process
Running 'go build -o /layers/paketo-buildpacks_go-build/targets/bin -buildmode pie -trimpath .'
Failed after 611ms
failed to execute 'go build': exit status 1
handlers.go:5:2: cannot find package "." in:
/workspace/vendor/templates
main.go:8:2: cannot find package "." in:
/workspace/vendor/test
ERROR: failed to build: exit status 1
Error failed to fetch an image or build from source: executing lifecycle: failed with status code: 51

handlers.go:5:2: "Template" main.go:8:2: "Test"

what did I do:

1. I've changed the module name to a URL (i.e. test -> some.com/test) and used a replace directive in go.mod to point to the module.
2. I ran go modvendor and looked at what was generated...some.com is there!
3. I have tried both local and remote builds via flight launch. Both will result in no such file.

I'm trying private repository now.

Workaround

This is not a great solution, but it is still possible to edit the hosts file to make the module point to the local file server.

# vim /etc/hosts
127.0.0.1    <module name>

I will edit this answer if I find out whether the problem is with fly or buildpack. The problem is that when fly runs go modvendor, it doesn't respect the substitution directives in go.mod.

As long as the module name is in url format, the self-running go mod provider will copy everything correctly . It doesn't have to be a valid URL.

Another solution is to simply host the code on a valid url and be done with it. I'm probably missing a flag, but I haven't found it yet. I hope anyone running into this problem might find this answer helpful in the meantime.

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