net/textproto: missing argument to printf formatting in error string

[tomarus]

In net/textproto there is a small error in the formatting of an error string in mustHaveFieldNameColon()

https://github.com/golang/go/blob/master/src/net/textproto/reader.go#L560

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

$ go version
go version go1.14 darwin/amd64

Does this issue reproduce with the latest release?

Yes

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/tommy/Library/Caches/go-build"
GOENV="/Users/tommy/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/tommy/dev/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/tommy/src/fork/textproto/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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/sq/ksylgl5d00jg7f8vrbqvc2mh0000gn/T/go-build121111092=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

I've fed some broken MIME headers into textproto.

What did you expect to see?

malformed MIME header: missing colon: SOMEHEADER

What did you see instead?

malformed MIME header: missing colon: %!q(MISSING).

[gopherbot]

Change https://golang.org/cl/221297 mentions this issue: net/textproto: pass missing argument to fmt.Sprintf

[dgryski]

It seems like go vet should have caught this.

Edit: Probably because the + makes the format string dynamic, and so neither go vet nor staticcheck look at dynamic format strings.

[ianlancetaylor]

Yeah, I mentioned that in the CL comment but not here.

[cagedmantis]

/cc @bradfitz @rsc