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

$ go version
go version go1.11.3 linux/amd64

Does this issue reproduce with the latest release?

Yes

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

$ go env
GOARCH="amd64"
GOBIN="/data/go/bin"
GOCACHE="/home/<>/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/data/go"
GOPROXY=""
GORACE=""
GOROOT="/home/<>/.goenv/versions/1.11.3"
GOTMPDIR=""
GOTOOLDIR="/home/<>/.goenv/versions/1.11.3/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
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 -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build956395453=/tmp/go-build -gno-record-gcc-switches"

What did you do?

When using sql.DB.Conn() a bad sql.ErrBadConn error was returned because the connections were terminated by the server (using pg_terminate_backend on postgres).
This by it self is not a bad thing but seems to not confirm with sql.DB.ExecContext() and sql.DB.PingContext() which actually kept working by opening a new connection.

While looking at the code I noticed that sq.DB.ExecContext() did cachedOrNewConn and then alwaysNewConn to ensure a new connection was available.
The code in sql.DB.Conn() seems to be doing a cachedOrNewConn and then cachedOrNewConn causing a cached/bad connection to be returned.

Probably this can be fixed by replacing src/database/sql/sql.go line 1701:

dc, err = db.conn(ctx, cachedOrNewConnalwaysNewConn)" 

What did you expect to see?

I expected sql.DB.Conn() to use the cached connections and otherwise create a new one similar to the rest of the library.

What did you see instead?

sql.DB.Conn() returned a cached/bad connection and did not try to create a new one.