cmd/go: cache is not aware of go build -trimpath flag

[tv42]

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

$ go-tip version
go version devel +dc0388c565 2019-05-07 18:42:17 +0000 linux/amd64

Does this issue reproduce with the latest release?

Latest release does not yet include the feature, 7469564

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

go env Output

$ go-tip env
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/tv/.cache/go-build"
GOENV="/home/tv/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/tv/go"
GOPROXY="direct"
GOROOT="/home/tv/src/go"
GOSUMDB="off"
GOTMPDIR=""
GOTOOLDIR="/home/tv/src/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/tv/src/2019/escape-string/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=/home/tv/tmp/go-build475315987=/tmp/go-build -gno-record-gcc-switches"

What did you do?

• Got excited about 7469564
• Constructed a test case (trivial main.go and go.mod).
• go build
• strings appname | grep /home/, saw paths listed
• go build -trimpath
• strings appname | grep /home/, still saw paths listed
• go clean -cache
• go build -trimpath
• strings appname | grep /home/, was empty as expected

What did you expect to see?

Empty results from grep even before cleaning the cache.

What did you see instead?

Build cache being ignorant about the presence of the -trimpath flag.

[josharian]

I also got excited about -trimpath, found that it didn't improve build times for go-fuzz, and got distracted before investigating further. This would explain it. Thanks for filing the issue, @tv42!

cc @bcmills @rsc @jayconrod

[gopherbot]

Change https://golang.org/cl/176112 mentions this issue: cmd/go: teach the build cache about -trimpath

[cespare]

I was digging around in this code today anyway so I sent a drive-by CL. I wasn't sure how to write tests, though (there are no tests for go build -trimpath) -- if anyone has ideas, please let me know on the CL.