cmd/go: preceding tags not always fetched when downloading a pseudo-version

[bcmills]

From a report from @hyangah:

$ go1.17rc2 mod init example.com/m
go: creating new go.mod: module example.com/m

$ export GOPRIVATE=github.com/go-delve/delve

$ go1.17rc2 get -d github.com/go-delve/delve/cmd/dlv@v1.7.1-0.20210804080032-f95340ae1bf9
go: downloading github.com/go-delve/delve v1.7.1-0.20210804080032-f95340ae1bf9
go get: github.com/go-delve/delve/cmd/dlv@v1.7.1-0.20210804080032-f95340ae1bf9: invalid pseudo-version: preceding tag (v1.7.0) not found

$ go1.17rc2 get -d github.com/go-delve/delve/cmd/dlv@f95340ae1bf9
go: downloading github.com/sirupsen/logrus v1.6.0
go: downloading github.com/spf13/cobra v1.1.3
go: downloading github.com/mattn/go-isatty v0.0.3
go: downloading gopkg.in/yaml.v2 v2.4.0
go: downloading github.com/cosiner/argv v0.1.0
go: downloading github.com/mattn/go-colorable v0.0.9
go: downloading github.com/peterh/liner v0.0.0-20170317030525-88609521dc4b
go: downloading github.com/google/go-dap v0.5.0
go: downloading golang.org/x/sys v0.0.0-20210514084401-e8d321eab015
go: downloading go.starlark.net v0.0.0-20200821142938-949cc6f4b097
go: downloading github.com/hashicorp/golang-lru v0.5.4
go: downloading golang.org/x/arch v0.0.0-20190927153633-4e8777c89be4
go: downloading github.com/aquasecurity/libbpfgo v0.1.2-0.20210708203834-4928d36fafac
go: downloading github.com/konsorten/go-windows-terminal-sequences v1.0.3
go: downloading github.com/cpuguy83/go-md2man/v2 v2.0.0
go: downloading github.com/spf13/pflag v1.0.5
go: downloading github.com/inconshreveable/mousetrap v1.0.0
go: downloading github.com/russross/blackfriday/v2 v2.0.1
go: downloading github.com/shurcooL/sanitized_anchor_name v1.0.0
go get: added github.com/aquasecurity/libbpfgo v0.1.2-0.20210708203834-4928d36fafac
go get: added github.com/cosiner/argv v0.1.0
go get: added github.com/cpuguy83/go-md2man/v2 v2.0.0
go get: added github.com/go-delve/delve v1.7.1-0.20210804080032-f95340ae1bf9
go get: added github.com/google/go-dap v0.5.0
go get: added github.com/hashicorp/golang-lru v0.5.4
go get: added github.com/inconshreveable/mousetrap v1.0.0
go get: added github.com/konsorten/go-windows-terminal-sequences v1.0.3
go get: added github.com/mattn/go-colorable v0.0.9
go get: added github.com/mattn/go-isatty v0.0.3
go get: added github.com/peterh/liner v0.0.0-20170317030525-88609521dc4b
go get: added github.com/russross/blackfriday/v2 v2.0.1
go get: added github.com/shurcooL/sanitized_anchor_name v1.0.0
go get: added github.com/sirupsen/logrus v1.6.0
go get: added github.com/spf13/cobra v1.1.3
go get: added github.com/spf13/pflag v1.0.5
go get: added go.starlark.net v0.0.0-20200821142938-949cc6f4b097
go get: added golang.org/x/arch v0.0.0-20190927153633-4e8777c89be4
go get: added golang.org/x/sys v0.0.0-20210514084401-e8d321eab015
go get: added gopkg.in/yaml.v2 v2.4.0

[bcmills]

$ git version
git version 2.33.0.rc1.237.g0d66db33f3-goog

(because it isn't clear to me whether this is actually a bug in git or cmd/go.)

[bcmills]

Interestingly, go1.17rc2 mod download -x github.com/go-delve/delve@v1.7.1-0.20210804080032-f95340ae1bf9 doesn't reproduce the error. So I suspect that this has something to do with looking for nested modules before the root module. (Perhaps we're fetching the nested-module tags but caching them as though we had downloaded the root-module tags?)