'unused' recipe in Makefile produces confusing messages for users

Host operating system: output of `uname -a`

Linux legec-laptop 5.4.0-74-generic #83-Ubuntu SMP Sat May 8 02:35:39 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

mysqld_exporter version: output of `mysqld_exporter --version`

mysqld_exporter, version 0.13.0 (branch: master, revision: ad2847c7fa67b9debafccd5a08bacb12fc9031f1) build date: 20210616-12:56:48 go version: go1.16.3 platform: linux/amd64

MySQL server version

n.a.

What did you do that produced an error?

make

What did you expect to see?

a successful build

What did you see instead?

>> running check for unused/missing packages in go.mod
GO111MODULE=on go mod tidy
...
<a long diff, see below>
...
make: *** [Makefile.common:214: common-unused] Error 1

The default make target includes unused :

https://github.com/prometheus/mysqld_exporter/blob/ad2847c7fa67b9debafccd5a08bacb12fc9031f1/Makefile.common#L120-L121

The unused target runs go mod tidy (when GO11MODULE is set, which is the default choice), and runs a git diff check on files go.mod, go.sum and vendor/ to confirm that nothing has changed :

https://github.com/prometheus/mysqld_exporter/blob/ad2847c7fa67b9debafccd5a08bacb12fc9031f1/Makefile.common#L205-L222

The issues are :

there is no message explaining to the user what to make of this output
this action does modify go.mod and go.sum, so running make build afterwards, for example, will not produce the same binary as what the end user would expect
interestingly, this makes the make action depend on the presence of git (not a big dependency issue, but still), and this action will not work if you aren't within a git clone of the project (no git archive tarball for you)

About the git part : a colleague of mine had deleted the .git/ directory after cloning, which turned the command git diff --exit-code -- go.sum go.mod into a diff between the two files go.sum and go.mod :

GO111MODULE=on go mod tidy
diff --git a/go.sum b/go.mod
index 079bfb9..11c6d5e 100644
--- a/go.sum
+++ b/go.mod
@@ -1,444 +1,17 @@
-cloud.google.com/go v0.26.0/go.mod h1:aQUYkXzVsufM+DwF1aE+0xfcU+56JwCaLick0ClmMTw=
-cloud.google.com/go v0.34.0/go.mod h1:aQUYkXzVsufM+DwF1aE+0xfcU+56JwCaLick0ClmMTw=
-github.com/BurntSushi/toml v0.3.1/go.mod h1:xHWCNGjB5oqiDr8zfno3MHue2Ht5sIBksp03qcyfWMU=
...
-honnef.co/go/tools v0.0.1-2019.2.3/go.mod h1:a3bituU0lyd329TUQxRnasdCoJDkEUEAqEt0JzvZhAg=
-sigs.k8s.io/yaml v1.1.0/go.mod h1:UJmg0vDUVViEyp3mgSv9WPwZCDxu4rQW1olrI1uml+o=
-sourcegraph.com/sourcegraph/appdash v0.0.0-20190731080439-ebfcffb1b5c0/go.mod h1:hI742Nqp5OhwiqlzhgfbWU4mW4yO10fP+LoT9WOswdU=
+module github.com/prometheus/mysqld_exporter
+
+require (
+   github.com/DATA-DOG/go-sqlmock v1.5.0
+   github.com/go-kit/kit v0.10.0
+   github.com/go-sql-driver/mysql v1.6.0
+   github.com/prometheus/client_golang v1.10.0
+   github.com/prometheus/client_model v0.2.0
+   github.com/prometheus/common v0.24.0
+   github.com/prometheus/exporter-toolkit v0.5.1
+   github.com/satori/go.uuid v1.2.0
+   github.com/smartystreets/goconvey v1.6.4
+   gopkg.in/alecthomas/kingpin.v2 v2.2.6
+   gopkg.in/ini.v1 v1.62.0
+)
+
+go 1.13
make: *** [Makefile.common:214: common-unused] Error 1

which lead to some unhelpful head scratching before figuring out what it was.

From what I understand, this action is useful for maintainers of the project, but pretty unhelpful for users who just wish to clone and use it, and makes for a weird blocker.

Possible suggestions :

remove unused from default make targets, add some other action to check for unused (another make target ? a script ? a pre-push hook ? ...)
keep unused in default targets, but instruct regular users to run make build && make test in Readme.md

additionally :

display a more helpful message when this action fails (non go users will have a hard time understanding what is displayed, and what to make out of it)

prometheus / mysqld_exporter