Prevent main Caddy module from upgrading from adding a plugin

[francislavoie]

Fixes #54

For example:

$ xcaddy build v2.3.0 --with github.com/caddy-dns/cloudflare
...
2022/04/06 01:54:25 [INFO] Initializing Go module
2022/04/06 01:54:25 [INFO] exec (timeout=10s): go mod init caddy
2022/04/06 01:54:26 [INFO] Pinning versions
2022/04/06 01:54:26 [INFO] exec (timeout=0s): go get -d -v github.com/caddyserver/caddy/v2@v2.3.0
...

2022/04/06 01:54:27 [INFO] exec (timeout=0s): go get -d -v github.com/caddy-dns/cloudflare github.com/caddyserver/caddy/v2@v2.3.0
go: github.com/caddy-dns/cloudflare@upgrade (v0.0.0-20210607183747-91cf700356a1) requires github.com/caddyserver/caddy/v2@v2.4.0, not github.com/caddyserver/caddy/v2@v2.3.0
2022/04/06 01:54:28 [FATAL] exit status 1

The key being that the go get to add the cloudflare plugin also specifies the Caddy module, which puts a constraint that the Caddy module should not upgrade past that version from pulling in the plugin.

This means that some builds that would have previously worked might fail now, but I think this is least-surprise, because it's weird that if you specify a version of Caddy that it would get upgraded higher because of a plugin.

This can be solved by specifying a version of the plugin that does build with the older version of Caddy specified:

$ xcaddy build v2.3.0 --with github.com/caddy-dns/cloudflare@eda8e5aa22232e9c279b0df7531f20c331b331c6
...
2022/04/06 02:12:26 [INFO] Initializing Go module
2022/04/06 02:12:26 [INFO] exec (timeout=10s): go mod init caddy
go: creating new go.mod: module caddy
go: to add module requirements and sums:
        go mod tidy
2022/04/06 02:12:26 [INFO] Pinning versions
2022/04/06 02:12:26 [INFO] exec (timeout=0s): go get -d -v github.com/caddyserver/caddy/v2@v2.3.0
...

2022/04/06 02:12:27 [INFO] exec (timeout=0s): go get -d -v github.com/caddy-dns/cloudflare@eda8e5aa22232e9c279b0df7531f20c331b331c6 github.com/caddyserver/caddy/v2@v2.3.0
go: added github.com/caddy-dns/cloudflare v0.0.0-20210105070211-eda8e5aa2223
go: added github.com/libdns/cloudflare v0.0.0-20200528144945-97886e7873b1
2022/04/06 02:12:29 [INFO] exec (timeout=0s): go get -d -v
2022/04/06 02:12:31 [INFO] Build environment ready
2022/04/06 02:12:31 [INFO] Building Caddy
2022/04/06 02:12:31 [INFO] exec (timeout=0s): go mod tidy
2022/04/06 02:12:31 [INFO] exec (timeout=0s): go build -o ./caddy -ldflags -w -s -trimpath
2022/04/06 02:12:34 [INFO] Build complete: ./caddy

[mholt]

Ah, that's clever (and unfortunate, oh well) -- thanks for the patch!

I think this looks good, except maybe we need a link to this PR or the related issue (#54) for future readers/maintainers.

[mholt]

Thanks for the efforts into investigating this. As unfortunate as the situation is, the fix seems good.