cmd/go: document that go mod vendor ignores directories that do not contain Go files

[urandom2]

What version of Go are you using (go version)?

$ go version
go1.19.4

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/user/.cache/go-build"
GOENV="/home/user/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/user/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/colin/go"
GOPRIVATE=""
GOPROXY="direct"
GOROOT="/nix/store/a7875alzpnr46z6mv4ssymfdwmvr6xbq-go-1.19.4/share/go"
GOSUMDB="off"
GOTMPDIR=""
GOTOOLDIR="/nix/store/a7875alzpnr46z6mv4ssymfdwmvr6xbq-go-1.19.4/share/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.19.4"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
GOWORK=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build3977569072=/tmp/go-build -gno-record-gcc-switches"

What did you do?

[user@localhost:~]$ git clone https://github.com/ava-labs/avalanchego
[user@localhost:~]$ cd avalanchego
[user@localhost:avalanchego]$ go build -o /dev/null ./main
[user@localhost:avalanchego]$ go mod vendor

What did you expect to see?

[user@localhost:avalanchego]$ go build -o /dev/null ./main

What did you see instead?

[user@localhost:avalanchego]$ go build -o /dev/null ./main
# github.com/supranational/blst/bindings/go
vendor/github.com/supranational/blst/bindings/go/blst.go:16:11: fatal error: blst.h: No such file or directory
   16 | // #include "blst.h"
      |           ^~~~~~~~
compilation terminated.
# github.com/zondax/hid
vendor/github.com/zondax/hid/hid_enabled.go:23:18: fatal error: os/threads_posix.c: No such file or directory
   23 |         #include "os/threads_posix.c"
      |                  ^~~~~~~~~~~~~~~~~~~~
compilation terminated.

[seankhliao]

working as intended.

Unlike many projects, the Go project does not use GitHub Issues for general discussion or asking questions. GitHub Issues are used for tracking bugs and proposals only.

For questions please refer to https://github.com/golang/go/wiki/Questions

[seankhliao]

Duplicate of #26366

[urandom2]

Unlike many projects, the Go project does not use GitHub Issues for general discussion or asking questions. GitHub Issues are used for tracking bugs and proposals only.

For questions please refer to https://github.com/golang/go/wiki/Questions

I am really confused, I am not "asking question" or trying to spark a "general discussion". I encountered an actual bug and was reporting it, as confirmed by your duplicate link. can you clarify what you mean?

---

also, I do think there is an outstanding bug, since the toolchain behaves differently with and without a vendor directory. sorry if I should have been more clear, but note that the project builds correctly before the call to go mod vendor:

[user@localhost:~]$ git clone https://github.com/ava-labs/avalanchego
[user@localhost:~]$ cd avalanchego
[user@localhost:avalanchego]$ go build -o /dev/null ./main
[user@localhost:avalanchego]$ 

[ianlancetaylor]

Sorry for the confusing responses. This is the current expected behavior. See #26366 (comment) .

[urandom2]

all good; I used a full repo as a reproducer, for simplicity, so I get it may have seemed like I was asking about "how do I build this thing".

that said, it really confuses me that we have intentionally chosen to have the module cache behave differently to the vendor directory. considering how big a foot gun this is, and how many other github issues link #26366, can we call this out in some documentation that is searchable? I tried googling around for an issue/faq entry, but ended up making a bug since I could not find one.

[ianlancetaylor]

This should probably be clearly documented at least at https://go.dev/ref/mod#go-mod-vendor. CC @bcmills @matloob