cmd/cgo: static C functions with the same names are not called correctly

[hajimehoshi]

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

$ go version
go version go1.17.2 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/hajimehoshi/Library/Caches/go-build"
GOENV="/Users/hajimehoshi/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/hajimehoshi/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/hajimehoshi/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"
GOVCS=""
GOVERSION="go1.17.2"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/hajimehoshi/test/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 -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/60/khbk2xqn1c5bml1byjn89dwc0000gn/T/go-build114659014=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Compiled these files:

main.go:

package main

func main() {
        foo()
        bar()
}

foo.go:

package main

// static int x() {                                                                                                                                                                                                                                                                   
//   return 0;                                                                                                                                                                                                                                                                        
// }                                                                                                                                                                                                                                                                                  
import "C"

func foo() {
        println(C.x())
}

bar.go:

package main

// static int x() {                                                                                                                                                                                                                                                                   
//   return 1;                                                                                                                                                                                                                                                                        
// }                                                                                                                                                                                                                                                                                  
import "C"

func bar() {
        println(C.x())
}

What did you expect to see?

0
1

is printed

What did you see instead?

1
1

is printed

[hajimehoshi]

CC @dmitshur ?

[bcmills]

Does cgo define what it uses as a compilation unit?

(Certainly the output you observed is not correct, but it's not obvious to me whether the correct behavior is 0 1 or a compile-time error.)

[hajimehoshi]

Does cgo define what it uses as a compilation unit?

Maybe no. At least I want a compile error rather than the current unexpected behavior.

[bcmills]

(See also #19836, #40820.)

[ianlancetaylor]

The correct behavior here is a compile time error. The entire package is compiled together with callbacks to the C code, so there isn't any other reasonable approach that we can take.

The fix is probably not too complicated but it's not trivial. We have to accept duplicate types and duplicate function declarations. We have to detect and reject duplicate function definitions.