Repository: go
Issue: cmd/go: test reported as cached, but not actually cached
Labels: ToolSpeed,NeedsInvestigation,GoCommand

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### What version of Go are you using (`go version`)?

<pre>
$ go version
go version go1.15.2 linux/amd64
</pre>

### Does this issue reproduce with the latest release?

Yes

### What operating system and processor architecture are you using (`go env`)?

<details><summary><code>go env</code> Output</summary><br><pre>
$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/user/.cache/go-build"
GOENV="/home/user/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/user/.go_workspace/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/user/.go_workspace:/usr/local/go_workspace"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/user/project/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-build013192427=/tmp/go-build -gno-record-gcc-switches"
</pre></details>

### What did you do?

<!--
If possible, provide a recipe for reproducing the error.
A complete runnable program is good.
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We have a large(ish) cache built up from CI builds (approx 2GB in size). We've seen tests slow down fairly dramatically, and it seems that the underlying cause is (possibly partial) cache misses resulting in at least some of the test process to be run every single time (in particular I see the linker being invoked).

```
$ time GODEBUG=gocachetest=1 go test -race ./service/
testcache: github.com/foo/bar/service: test ID 5277766d4236333530556841686b797248445250 => 32e67a3c459a396178c3acafc87928bc9fc94b28d20fa9763dfc1fac9925e9a1
testcache: github.com/foo/bar/service: input list not found: cache entry not found: open /home/user/.cache/go-build/32/32e67a3c459a396178c3acafc87928bc9fc94b28d20fa9763dfc1fac9925e9a1-a: no such file or directory
testcache: github.com/foo/bar/service: test ID 484f364f314d6d6c647371774138555430696467 => ce7e1c46d67997e3d0b493d985ea92e199bad7b2fc4a97e7b405c87a92ac5d18
testcache: github.com/foo/bar/service: test ID ce7e1c46d67997e3d0b493d985ea92e199bad7b2fc4a97e7b405c87a92ac5d18 => input ID de8952dc6e704af61a533169fd1be1877a6c2778382c4f9a0828a177db9401ac => 74e40de3953e4347471748ef1e1ff4aacfa7380968c99f0baa14b0b7b1d24dae
ok      github.com/foo/bar/service  (cached)

real    0m2.185s
user    0m2.687s
sys     0m0.387s

$ time GODEBUG=gocachetest=1 go test -race ./service/
testcache: github.com/foo/bar/service: test ID 5277766d4236333530556841686b797248445250 => 32e67a3c459a396178c3acafc87928bc9fc94b28d20fa9763dfc1fac9925e9a1
testcache: github.com/foo/bar/service: input list not found: cache entry not found: open /home/user/.cache/go-build/32/32e67a3c459a396178c3acafc87928bc9fc94b28d20fa9763dfc1fac9925e9a1-a: no such file or directory
testcache: github.com/foo/bar/service: test ID 484f364f314d6d6c647371774138555430696467 => ce7e1c46d67997e3d0b493d985ea92e199bad7b2fc4a97e7b405c87a92ac5d18
testcache: github.com/foo/bar/service: test ID ce7e1c46d67997e3d0b493d985ea92e199bad7b2fc4a97e7b405c87a92ac5d18 => input ID de8952dc6e704af61a533169fd1be1877a6c2778382c4f9a0828a177db9401ac => 74e40de3953e4347471748ef1e1ff4aacfa7380968c99f0baa14b0b7b1d24dae
ok      github.com/foo/bar/service  (cached)

real    0m2.207s
user    0m2.786s
sys     0m0.397s
```

I'm not familiar with what it is caching, or indeed how the caching works, but it seems removing the cache entry that _is_ being hit then resolves this issue:

```
$ rm ~/.cache/go-build/74/74e40de3953e4347471748ef1e1ff4aacfa7380968c99f0baa14b0b7b1d24dae-a

$ time GODEBUG=gocachetest=1 go test -race ./service/
testcache: github.com/foo/bar/service: test ID 5277766d4236333530556841686b797248445250 => 32e67a3c459a396178c3acafc87928bc9fc94b28d20fa9763dfc1fac9925e9a1
testcache: github.com/foo/bar/service: input list not found: cache entry not found: open /home/user/.cache/go-build/32/32e67a3c459a396178c3acafc87928bc9fc94b28d20fa9763dfc1fac9925e9a1-a: no such file or directory
testcache: github.com/foo/bar/service: test ID 484f364f314d6d6c647371774138555430696467 => ce7e1c46d67997e3d0b493d985ea92e199bad7b2fc4a97e7b405c87a92ac5d18
testcache: github.com/foo/bar/service: test ID ce7e1c46d67997e3d0b493d985ea92e199bad7b2fc4a97e7b405c87a92ac5d18 => input ID de8952dc6e704af61a533169fd1be1877a6c2778382c4f9a0828a177db9401ac => 74e40de3953e4347471748ef1e1ff4aacfa7380968c99f0baa14b0b7b1d24dae
testcache: github.com/foo/bar/service: test output not found: cache entry not found: open /home/user/.cache/go-build/74/74e40de3953e4347471748ef1e1ff4aacfa7380968c99f0baa14b0b7b1d24dae-a: no such file or directory
testcache: github.com/foo/bar/service: save test ID 32e67a3c459a396178c3acafc87928bc9fc94b28d20fa9763dfc1fac9925e9a1 => input ID de8952dc6e704af61a533169fd1be1877a6c2778382c4f9a0828a177db9401ac => c5d26a24287e143e5891845de941e3d6e14ccca91d2e881d31430334a09001a8
testcache: github.com/foo/bar/service: save test ID ce7e1c46d67997e3d0b493d985ea92e199bad7b2fc4a97e7b405c87a92ac5d18 => input ID de8952dc6e704af61a533169fd1be1877a6c2778382c4f9a0828a177db9401ac => 74e40de3953e4347471748ef1e1ff4aacfa7380968c99f0baa14b0b7b1d24dae
ok      github.com/foo/bar/service  0.052s

real    0m2.236s
user    0m2.863s
sys     0m0.370s

$ time GODEBUG=gocachetest=1 go test -race ./service/
testcache: github.com/foo/bar/service: test ID 5277766d4236333530556841686b797248445250 => 32e67a3c459a396178c3acafc87928bc9fc94b28d20fa9763dfc1fac9925e9a1
testcache: github.com/foo/bar/service: test ID 32e67a3c459a396178c3acafc87928bc9fc94b28d20fa9763dfc1fac9925e9a1 => input ID de8952dc6e704af61a533169fd1be1877a6c2778382c4f9a0828a177db9401ac => c5d26a24287e143e5891845de941e3d6e14ccca91d2e881d31430334a09001a8
ok      github.com/foo/bar/service  (cached)

real    0m0.728s
user    0m0.974s
sys     0m0.276s

$ time GODEBUG=gocachetest=1 go test -race ./service/
testcache: github.com/foo/bar/service: test ID 5277766d4236333530556841686b797248445250 => 32e67a3c459a396178c3acafc87928bc9fc94b28d20fa9763dfc1fac9925e9a1
testcache: github.com/foo/bar/service: test ID 32e67a3c459a396178c3acafc87928bc9fc94b28d20fa9763dfc1fac9925e9a1 => input ID de8952dc6e704af61a533169fd1be1877a6c2778382c4f9a0828a177db9401ac => c5d26a24287e143e5891845de941e3d6e14ccca91d2e881d31430334a09001a8
ok      github.com/foo/bar/service  (cached)

real    0m0.725s
user    0m1.034s
sys     0m0.234s
```

Note the `save test ID` lines and the the speed up.

We can of course clear out the cache to help resolve this, but it feels like there is perhaps an issue here: there is a cache miss yet in these circumstances no new cache entry is created, at least without intervention. I'm open to the possibility that our cache has gotten into a bad state, though I'd have a lot of difficulty trying to understand why.