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

$ go version
go version devel +54e75e8f9d Thu Jul 30 16:00:05 2020 +0000 darwin/amd64

Does this issue reproduce with the latest release?

no, an in-develpment build issue

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

$ go env
GO111MODULE="off"
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/mtj/Library/Caches/go-build"
GOENV="/Users/mtj/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/mtj/Documents/gocode/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/mtj/Documents/gocode"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/Users/mtj/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/Users/mtj/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/7b/r8p9wkn118bcmz4r3blf8mwc0000gn/T/go-build078910258=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

cd $GOROOT/src; ./all.bash

What did you expect to see?

build run to completion and all tests pass.

What did you see instead?

build worked but one test fails.
this is our old nemesis "too many open files" on high core count (18 +18smt) apple machines.

[failure excerpt]
--- FAIL: TestScript (0.01s)
    --- FAIL: TestScript/mod_gopkg_unstable (0.59s)
        script_test.go:210: 
            > env GO111MODULE=on
            > cp go.mod.empty go.mod
            > go get -d gopkg.in/dummy.v2-unstable
            [stderr]
            go: downloading gopkg.in/dummy.v2-unstable v2.0.0
            go: gopkg.in/dummy.v2-unstable upgrade => v2.0.0
            > cp x.go.txt x.go
            > cp go.mod.empty go.mod
            > go list
            FAIL: testdata/script/mod_gopkg_unstable.txt:8: unexpected error starting command: pipe: too many open files
            
    --- FAIL: TestScript/mod_go_version_mixed (0.23s)
        script_test.go:210: 
            FAIL: testdata/script/mod_go_version_mixed.txt:0: open $WORK/gopath/src/sub/go.mod: too many open files
            
    --- FAIL: TestScript/mod_get_tags (0.09s)
        script_test.go:210: 
            FAIL: testdata/script/mod_get_tags.txt:0: open testdata/script/mod_get_tags.txt: too many open files
            
    --- FAIL: TestScript/mod_get_none (0.14s)
        script_test.go:210: 
            > env GO111MODULE=on
            > go mod init example.com/foo
            FAIL: testdata/script/mod_get_none.txt:3: unexpected error starting command: pipe: too many open files
            
    --- FAIL: TestScript/modfile_flag (5.39s)
        script_test.go:210: 
            # Tests the behavior of the -modfile flag in commands that support it.
            # The go.mod file exists but should not be read or written.
            # Same with go.sum. (0.037s)
            # go mod init should create a new file, even though go.mod already exists. (0.134s)
            # 'go env GOMOD' should print the path to the real file.
            # 'go env' does not recognize the '-modfile' flag. (0.084s)
            # 'go list -m' should print the effective go.mod file as GoMod though. (0.209s)
            # go mod edit should operate on the alternate file (0.200s)
            # other 'go mod' commands should work. 'go mod vendor' is tested later. (1.628s)
            # 'go list' and other commands with build flags should work.
            # They should update the alternate go.mod when a dependency is missing. (1.292s)
            # 'go mod vendor' should work. (0.122s)
            # Automatic vendoring should be broken by editing an explicit requirement
            # in the alternate go.mod file. (1.023s)
            # 'go generate' should use the alternate file when resolving packages.
            # Recursive go commands started with 'go generate' should not get an explicitly
            # passed -modfile, but they should see arguments from GOFLAGS. (0.217s)
            # The original files should not have been modified. (0.010s)
            # If the altnernate mod file does not have a ".mod" suffix, an error
            # should be reported. (0.000s)
            > cp go.alt.mod goaltmod
            FAIL: testdata/script/modfile_flag.txt:78: open $WORK/gopath/src/go.alt.mod: too many open files
            
    --- FAIL: TestScript/mod_goroot_errors (0.82s)
        script_test.go:210: 
            # Regression test for https://golang.org/issue/34769.
            # Missing standard-library imports should refer to GOROOT rather than
            # complaining about a malformed module path.
            # This is especially important when GOROOT is set incorrectly,
            # since such an error will occur for every package in std.
            # Building a nonexistent std package directly should fail usefully. (0.150s)
            # Building a nonexistent std package indirectly should also fail usefully. (0.226s)
            # Building an *actual* std package should fail if GOROOT is set to something bogus. (0.000s)
            > [!short] go build ./importjson  # Prove that it works when GOROOT is valid.
            > env GOROOT=$WORK/not-a-valid-goroot
            > ! go build ./importjson
            FAIL: testdata/script/mod_goroot_errors.txt:38: unexpected error starting command: pipe: too many open files
            
    --- FAIL: TestScript/mod_sumdb_proxy (4.46s)
        script_test.go:210: 
            # basic fetch (through proxy) works (1.514s)
            # can fetch by explicit URL (1.113s)
            # direct access fails (because localhost.localdev does not exist)
            # The text of the error message is hard to predict because some DNS servers
            # will resolve unknown domains like localhost.localdev to a real IP
            # to serve ads. (0.102s)
            # proxy 404 falls back to direct access (which fails) (0.173s)
            # proxy non-200/404/410 stops direct access (0.184s)
            # the error from the last attempted proxy should be returned. (0.070s)
            # if proxies are separated with '|', fallback is allowed on any error. (0.378s)
            > cp go.mod.orig go.mod
            > env GOSUMDB=$sumdb
            > env GOPROXY=$proxy/sumdb-503|https://0.0.0.0|$proxy
            > go get -d rsc.io/fortune@v1.0.0
            [stderr]
            go: finding module for package rsc.io/quote
            go: found rsc.io/quote in rsc.io/quote v1.5.2
            > rm $GOPATH/pkg/mod/cache/download/sumdb
            FAIL: testdata/script/mod_sumdb_proxy.txt:63: readdirnames $WORK/gopath/pkg/mod/cache/download/sumdb/localhost.localdev/sumdb/lookup: fcntl: too many open files
            
    --- FAIL: TestScript/mod_dot (1.19s)
        script_test.go:210: 
            # golang.org/issue/32917 and golang.org/issue/28459: 'go build' and 'go test'
            # in an empty directory should refer to the path '.' and should not attempt
            # to resolve an external module. (0.151s)
            > cd dir
            $WORK/gopath/src/dir
            > ! go get .
            [stderr]
            go get .: path $WORK/gopath/src/dir is not a package in module rooted at $WORK/gopath/src/dir
            [exit status 1]
            > stderr 'go get \.: path .* is not a package in module rooted at .*[/\\]dir$'
            > ! go list
            FAIL: testdata/script/mod_dot.txt:9: unexpected error starting command: pipe: too many open files
            
FAIL
FAIL	cmd/go	62.525s