swag init treats CGo warnings as fatal

[MrDOS]

Describe the bug
When C source code compiled by CGo produces warnings, swag init treats these errors as fatal, and doc generation fails.

To Reproduce
go.mod:

module swag-1232

go 1.18

main.go:

package main

// #pragma GCC warning "test"
import "C"

func main() {}

Then generate Swagger documentations with dependency parsing:

$ swag init --parseDependency
2022/07/05 18:04:08 Generate swagger docs....
2022/07/05 18:04:08 Generate general API Info, search dir:./
2022/07/05 18:04:08 pkg /.../swag-1232 cannot find all dependencies, <nil>
# swag-1232
./main.go:3:14: warning: test [-W#pragma-messages]

Expected behavior
I would expect the Swagger documentation to be generated successfully, instead of failing due to the C compilation warning.

Your swag version

$ swag --version
swag version v1.8.3

This does not occur in swag v1.8.2 or earlier.

Your go version

$ go version
go version go1.18.3 darwin/arm64

Desktop (please complete the following information):

• OS: macOS 12.4
• Browser: n/a
• Version: n/a

Additional context
As of #1108, released in swag v1.8.3, swag init invokes go list under the hood. When CGo is enabled, that command will invoke the C compiler to evaluate C source; the C compiler, in turn, sends compilation errors and warnings to stderr. However, swag init treats any output to stderr by that command as fatal:

swag/golist.go

Lines 25 to 27 in bd21bb0

	if stderrBuf.Len() > 0 {
	finalErr = fmt.Errorf("%v\n%s", finalErr, stderrBuf.Bytes())
	}

I think the most straightforward solution here is to disable CGo for the invocation of go list by setting the CGO_ENABLED environment variable to 0. I don't think this would break any existing API documentation, because swag init doesn't seem to understand the C pseudo-package anyway:

// @Success 200 {object} C.int
// @Router /some-route [get]
func SomeHandler() {}

$ swag init
2022/07/05 18:21:58 Generate swagger docs....
2022/07/05 18:21:58 Generate general API Info, search dir:./
2022/07/05 18:21:58 ParseComment error in file /.../main.go :cannot find type definition: C.int

This extends to the inclusion of CGo types embedded within Go structs.

I think this issue is separate from #1232. In that issue, go list failed to find any source files; here, go list runs successfully, and produces the expected dependency descriptions, but also includes extraneous output.

[MrDOS]

Another possible solution would be to remove the stderr checking, and rely on the error checking of cmd.Wait() to catch a non-zero exit status from the go list invocation. Conventionally, output to stderr shouldn't be considered fatal by itself, just informative, unless accompanied by a non-zero exit status.

I'm happy to implement either solution. Just let me know which direction you'd like me to go.