Why is this Go build failing on Heroku? It builds locally with no issue - go

Question

Is anyone able to tell me why this build has failed on heroku?
I've tried finding substitute packages and tidying the mod file, even deleting it and redoing it.
here is my build log
Overview Resources Deploy Metrics Activity Access Settings
Activity Feed Build LogID f8e9d5ce-0d8f-4e8e-8b2a-661c4d1f68f7
-----> Building on the Heroku-20 stack
-----> Go app detected
-----> Fetching stdlib.sh.v8... done
----->
Detected go modules via go.mod
----->
Detected Module Name: github.com/xxxxxx/xxxxxx-xxxxxx
----->
!! The go.mod file for this project does not specify a Go version
!!
!! Defaulting to go1.12.17
!!
!! For more details see: https://devcenter.heroku.com/articles/go-apps-with-modules#build-configuration
!!
-----> Using go1.12.17
-----> Determining packages to install
Detected the following main packages to install:
github.com/xxxxxx/xxxxxx-xxxxxx
-----> Running: go install -v -tags heroku github.com/xxxxxx/xxxxxx-companion
github.com/vektah/gqlparser/v2/ast
github.com/agnivade/levenshtein
github.com/mitchellh/mapstructure
github.com/vektah/gqlparser/v2/gqlerror
github.com/99designs/gqlgen/graphql
# github.com/99designs/gqlgen/graphql
../codon/tmp/cache/go-path/pkg/mod/github.com/99designs/gqlgen#v0.13.0/graphql/error.go:21:5: undefined: errors.As
github.com/99designs/gqlgen/graphql/errcode
github.com/vektah/gqlparser/v2/lexer
github.com/hashicorp/golang-lru/simplelru
github.com/vektah/gqlparser/v2/parser
github.com/hashicorp/golang-lru
github.com/gorilla/websocket
github.com/vektah/gqlparser/v2/validator
github.com/99designs/gqlgen/graphql/playground
github.com/xxxxxx/xxxxxx-xxxxxx/graph/model
github.com/xxxxxx/xxxxxx-xxxxxx/database
github.com/99designs/gqlgen/graphql/introspection
github.com/vektah/gqlparser/v2/validator/rules
github.com/go-sql-driver/mysql
github.com/vektah/gqlparser/v2
! Push rejected, failed to compile Go app.
! Push failed
Here is my go.mod file:
module github.com/xxxxxx/xxxxxx-xxxxxx
go 1.15
require (
github.com/99designs/gqlgen v0.13.0
github.com/go-sql-driver/mysql v1.5.0
github.com/hashicorp/golang-lru v0.5.1 // indirect
github.com/mitchellh/mapstructure v0.0.0-20180220230111-00c29f56e238 // indirect
github.com/stretchr/testify v1.5.1 // indirect
github.com/vektah/gqlparser/v2 v2.1.0
gopkg.in/yaml.v2 v2.2.8 // indirect
)
I'm at a complete loss, I suspect it has to do with an incorrect version, but the build log doesn't give me much to go on. How can I diagnose this kind of issue in the future?
Thank you.

Answer 1

You could add a directive // +heroku goVersion go1.15 in your go.mod file.
module somemodule
// +heroku goVersion go1.15
go 1.15
require (
// ...
)
Then it should look like this.
remote: Detected go modules via go.mod
remote: ----->
remote: Detected Module Name: somemodule
remote: ----->
remote: -----> New Go Version, clearing old cache
remote: -----> Installing go1.15
Documentation: https://github.com/heroku/heroku-buildpack-go#go-module-specifics
Go Module Specifics
You can specify specific package spec(s) via the go.mod file's // +heroku install directive (see below).
// +heroku goVersion <version>: the major version of go you would
like Heroku to use when compiling your code. If not specified this
defaults to the buildpack's [DefaultVersion].

Answer 2

Coming back to mark this one complete, I figured out that by specifying in the go mod file
// +heroku goVersion go1.15
Go buildback supports arbitrary comments specifiying versioning before build.
https://github.com/heroku/heroku-buildpack-go#go-module-specifics
Thank you as well to user #JimB for pointing me in the right direction.