cmd/go: prebuilt net/http not used

[neild]

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

go version devel +c882f4b6b1 Tue Aug 14 20:40:15 2018 +0000 linux/amd64

Does this issue reproduce with the latest release?

yes

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

GOARCH="amd64"
GOBIN=""
GOCACHE="/tmp/gocache"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/usr/local/google/home/dneil/go"
GOPROXY=""
GORACE=""
GOROOT="/tmp/go"
GOTMPDIR=""
GOTOOLDIR="/tmp/go/pkg/tool/linux_amd64"
GCCGO="/usr/bin/gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/usr/local/google/home/dneil/foo/go.mod"
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 -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build560741844=/tmp/go-build -gno-record-gcc-switches"

What did you do?

$ cat go.mod
module misago.org/foo
$ cat foo.go
package foo
import _ "net/http"
$ rm -rf /tmp/gocache
$ GOCACHE=/tmp/gocache go build -v .

What did you expect to see?

With a cold cache, this should just build the current package.

misago.org/foo

What did you see instead?

The net/http package and a number of its dependencies are rebuilt.

golang_org/x/crypto/cryptobyte/asn1
golang_org/x/net/dns/dnsmessage
golang_org/x/crypto/curve25519
golang_org/x/text/transform
golang_org/x/crypto/poly1305
golang_org/x/crypto/cryptobyte
golang_org/x/crypto/internal/chacha20
golang_org/x/text/unicode/bidi
golang_org/x/net/http2/hpack
golang_org/x/text/unicode/norm
golang_org/x/crypto/chacha20poly1305
net
golang_org/x/text/secure/bidirule
golang_org/x/net/idna
golang_org/x/net/http/httpproxy
net/textproto
crypto/x509
golang_org/x/net/http/httpguts
crypto/tls
net/http/httptrace
net/http
misago.org/foo

This doesn't happen with other packages; e.g., fmt.

[andybons]

@rsc @bcmills

[agnivade]

Possibly related to #26988

[andybons]

@bradfitz @ianlancetaylor

[ianlancetaylor]

It's because of the vendor in the path. When the " golang_org/x/cryto/cryptobyte/asn1" package is built as part of go install std, the build ID computation done by (*Builder).buildActionID writes

import \"vendor/golang_org/x/crypto/cryptobyte/asn1\"\n

to the hash key. When the build of the module is done later, the build ID computation writes

import \"golang_org/x/crypto/cryptobyte/asn1\"\n

That is, the vendor part of the string is present when building the package but not when checking whether the package needs to rebuilt.

This is presumably happening because go install std expands into installing vendor/golang_org/x/crypto/cryptobyte/asn1. For example, go install vendor/golang_org/x/crypto/cryptobyte/asn1 succeeds. go install golang_org/x/crypto/cryptobyte/asn1 fails with

can't load package: package golang_org/x/crypto/cryptobyte/asn1: cannot find package "golang_org/x/crypto/cryptobyte/asn1" in any of:
	/home/iant/go/src/golang_org/x/crypto/cryptobyte/asn1 (from $GOROOT)
	/home/iant/gopath/src/golang_org/x/crypto/cryptobyte/asn1 (from $GOPATH)

[ianlancetaylor]

This seems related to #23970 and https://golang.org/cl/125836. Perhaps the go/build package needs to explicitly remove "vendor/" from the ImportPath field of a package in the standard library.

CC @bcmills @rsc

[rsc]

Duplicate of #26993. I think the rebuild is fine for Go 1.11 and possibly just in general.

[rsc]

Sorry, this is #26993. Duplicate of #26988.

[thepudds]

See also this comment #26988 (comment) for a brief explanation on why this is happening.