mgo: FindAndModify create too many sockets

[andrewrong]

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Replace this text with your description, and please answer the questions below
before submitting your issue to help us out. Thanks!

---

What version of MongoDB are you using (mongod --version)?

3.4

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

go version go1.14.7 

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

GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/Cellar/go/1.14.7/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.14.7/libexec/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/h3/k95txtxn3wq3nvxx5xxcnj280000gn/T/go-build203709446=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

1. init 100 mgo.session (mgo session is not use),
2. Multiple(for example 100) goroutine run command FindAndModify use session that the step created
3. you find that mgo create many socket connects mongos

I look the FindAndModify's source code

globalsign/mgo/session.go::Apply()::5032

		session = session.Clone()
		defer session.Close()
		session.SetMode(Strong, false)

If the concurrency using FindAndModify is high enough, then many connections will be created；
I think it can be modified like this:

	if session.Mode() != Strong {
		session = session.Clone()
		defer session.Close()
		session.SetMode(Strong, false)
	}

Can you reproduce the issue on the latest development branch?

yeah