cmd/go: http activity during 'go doc'

[rsc]

I made this change to my repo and did go install cmd/go.

diff --git a/src/net/http/client.go b/src/net/http/client.go
index e0cabc9d4c..9973cdcb23 100644
--- a/src/net/http/client.go
+++ b/src/net/http/client.go
@@ -590,6 +590,7 @@ func urlErrorOp(method string) string {
 // Any returned error will be of type *url.Error. The url.Error
 // value's Timeout method will report true if the request timed out.
 func (c *Client) Do(req *Request) (*Response, error) {
+	panic("HTTP")
 	return c.do(req)
 }

Then I did cd $GOPATH/src/golang.org/x/mod/semver; go doc, and I get:

% go doc
panic: HTTP

goroutine 1 [running]:
net/http.(*Client).Do(...)
	/Users/rsc/go/src/net/http/client.go:593
cmd/go/internal/web.get.func1(0xc00033e3f0)
	/Users/rsc/go/src/cmd/go/internal/web/http.go:112 +0x132
cmd/go/internal/web.get(0x1, 0xc0001c6480)
	/Users/rsc/go/src/cmd/go/internal/web/http.go:127 +0x509
cmd/go/internal/web.Get(...)
	/Users/rsc/go/src/cmd/go/internal/web/api.go:182
cmd/go/internal/modfetch.(*proxyRepo).getBody(0xc000322540, {0xc00030e5a0, 0x2a})
	/Users/rsc/go/src/cmd/go/internal/modfetch/proxy.go:260 +0x13e
cmd/go/internal/modfetch.(*proxyRepo).getBytes(0xc00011c150, {0xc00030e5a0, 0xc0001c65c0})
	/Users/rsc/go/src/cmd/go/internal/modfetch/proxy.go:245 +0x4b
cmd/go/internal/modfetch.(*proxyRepo).Stat(0xc000322540, {0xc00011c150, 0xc00011c150})
	/Users/rsc/go/src/cmd/go/internal/modfetch/proxy.go:353 +0x8b
cmd/go/internal/modfetch.(*cachingRepo).Stat.func1()
	/Users/rsc/go/src/cmd/go/internal/modfetch/cache.go:230 +0x82
cmd/go/internal/par.(*Cache).Do(0xc0003162b0, {0x141d2c0, 0xc00030a340}, 0xc0001c67b8)
	/Users/rsc/go/src/cmd/go/internal/par/work.go:128 +0xc3
cmd/go/internal/modfetch.(*cachingRepo).Stat(0xc0003162a0, {0xc00011c150, 0x22})
	/Users/rsc/go/src/cmd/go/internal/modfetch/cache.go:224 +0x9d
cmd/go/internal/modload.queryProxy({0x1558140, 0xc0000261c8}, {0xc00002c0f8, 0x18}, {0xc00012c040, 0xc0001c6a88}, {0xc00011c150, 0x0}, {0x0, 0x0})
	/Users/rsc/go/src/cmd/go/internal/modload/query.go:139 +0xe35
cmd/go/internal/modload.Query.func1({0xc00002c0f8, 0xc000318e08})
	/Users/rsc/go/src/cmd/go/internal/modload/query.go:71 +0x6d
cmd/go/internal/modfetch.TryProxies(0xc0001c6b38)
	/Users/rsc/go/src/cmd/go/internal/modfetch/proxy.go:163 +0xb2
cmd/go/internal/modload.Query({0x1558140, 0xc0000261c8}, {0xc00012c040, 0x26}, {0xc00011c150, 0xe400000000000003}, {0x0, 0xc0001d20d0}, 0xc0001c6c48)
	/Users/rsc/go/src/cmd/go/internal/modload/query.go:70 +0xc5
cmd/go/internal/modload.moduleInfo.func1(0xc00033c300)
	/Users/rsc/go/src/cmd/go/internal/modload/build.go:250 +0xb0
cmd/go/internal/modload.moduleInfo({0x1558140, 0xc0000261c8}, 0xc000020370, {{0xc00012c040, 0xc0001c6f88}, {0xc00011c150, 0xc0001c6e58}}, 0x0)
	/Users/rsc/go/src/cmd/go/internal/modload/build.go:305 +0x60a
cmd/go/internal/modload.listModules({0x1558140, 0xc0000261c8}, 0xc000020370, {0xc0000200e0, 0x1, 0x0}, 0x0)
	/Users/rsc/go/src/cmd/go/internal/modload/list.go:225 +0x158d
cmd/go/internal/modload.ListModules({0x1558140, 0xc0000261c8}, {0xc0000200e0, 0x1, 0x1}, 0x0)
	/Users/rsc/go/src/cmd/go/internal/modload/list.go:38 +0x93
cmd/go/internal/list.runList({0x1558140, 0xc0000261c8}, 0xc0000284b0, {0xc0000200e0, 0x44, 0x1})
	/Users/rsc/go/src/cmd/go/internal/list/list.go:467 +0x279f
main.invoke(0x17d9aa0, {0xc0000200b0, 0x4, 0x4})
	/Users/rsc/go/src/cmd/go/main.go:216 +0x2f6
main.main()
	/Users/rsc/go/src/cmd/go/main.go:173 +0x78e
package semver // import "golang.org/x/mod/semver"

Package semver implements comparison of semantic version strings. In this
package, semantic version strings must begin with a leading "v", as in
"v1.0.0".
...

It looks like go doc invokes go tool doc which invokes go list.
So far so good.
But go list should not need to use the network:
all I want is the docs for the package in the current directory.
We should remove the network usage.

/cc @bcmills @jayconrod @matloob

[bcmills]

The go list invocation is here:

go/src/cmd/doc/dirs.go

Line 220 in d8f348a

cmd := exec.Command("go", "list", "-m", "-f={{.Path}}\t{{.Dir}}", "all")

cmd/doc is using go list -m because it is doing the same kind of fuzzy searching that gopls and goimports use: it can't just go list a specific package because it doesn't know which package you want.

go list -m needs network access because it collects all of the information about a module needed for the complete JSON output even if the user is only requesting a few locally-available fields (#29666, #29869, #42723). So I think in the general case this is a duplicate of #29666, which is going to require a fair amount of work to fix.

[bcmills]

That said, for the specific case where the exact package directory is known, cmd/doc should probably skip go list -m all (which unnecessarily loads the full module graph), and instead go list only that specific package.