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

$ go version
go version go1.12 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=""
GOCACHE="/home/klauer/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/klauer/src/go"
GOPROXY=""
GORACE=""
GOROOT="/home/klauer/opt/go"
GOTMPDIR=""
GOTOOLDIR="/home/klauer/opt/go/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-build968558791=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Run this test program:

package main

import "os"

func main() {
        file, err := os.OpenFile("foo.dat", os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0600)
        if err != nil { panic("1") }
        file.Close()

        file, err = os.Create("foo.dat")
        if err != nil { panic("2") }
        file.Close()

        file, err = os.Create("bar.dat")
        if err != nil { panic("3") }
        file.Close()

        file, err = os.OpenFile("bar.dat", os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0600)
        if err != nil { panic("4") }
        file.Close()
}

What did you expect to see?

The documentation for os.Create says:

Create creates the named file with mode 0666 (before umask), truncating it if it already exists. […]

This reads as if mode 0666 always applies.
Likewise, the documentation for os.OpenFile says:

[…] It opens the named file with specified […] perm (before umask), if applicable. […]

This might also lead one to expect that perm is applicable with, e. g., O_CREATE and O_TRUNC.

Hence, one would expect:

$ umask
0022
$ ls -l
-rw------- 1 klauer klauer       0 Feb 26 10:53 bar.dat
-rw-r--r-- 1 klauer klauer       0 Feb 26 10:53 foo.dat

What did you see instead?

The actual behaviour is in line with the analogous flags for the open() function from the POSIX standard:

$ umask
0022
$ ls -l
-rw-r--r-- 1 klauer klauer       0 Feb 26 10:53 bar.dat
-rw------- 1 klauer klauer       0 Feb 26 10:53 foo.dat

The POSIX standard makes it explicitly clear that the mode remains unchanged when the file already exists:

O_CREAT If the file exists, this flag has no effect except as noted under O_EXCL below.

O_TRUNC If the file exists and is a regular file, and the file is successfully opened O_RDWR or O_WRONLY, its length shall be truncated to 0, and the mode and owner shall be unchanged.

I suggest the go documentation is likewise explicit. For example I would change the os.Create docs:

Create creates the named file with mode 0666 (before umask), truncating it if it already exists.

to:

Create creates the named file with mode 0666 (before umask) if it does not already exist. If it already exists, it is truncated and the mode remains unchanged.

and the os.OpenFile docs:

It opens the named file with specified flag (O_RDONLY etc.) and perm (before umask), if applicable.

to

It opens the named file with specified flag (O_RDONLY etc.). The perm is applied (before umask) only if a new file is created, and is otherwise ignored.