Golang Build Constraints Random - build

Question

I have two go files with different build constraints in the header.
constants_production.go:
// +build production,!staging
package main
const (
URL = "production"
)
constants_staging.go:
// +build staging,!production
package main
const (
URL = "staging"
)
main.go:
package main
func main() {
fmt.Println(URL)
}
When I do a go install -tags "staging", sometimes, it prints production; Sometimes, it prints staging. Similarly, when I do go install -tags "production",...
How do I get a consistent output on every build? How do I make it print staging when I specify staging as a build flag? How do I make it print production when I specify production as a build flag? Am I doing something wrong here?

Answer 1

go build and go install will not rebuild the package (binary) if it looks like nothing has changed -- and it's not sensitive to changes in command-line build tags.
One way to see this is to add -v to print the packages as they are built:
$ go install -v -tags "staging"
my/server
$ go install -v -tags "production"
(no output)
You can force a rebuild by adding the -a flag, which tends to be overkill:
$ go install -a -v -tags "production"
my/server
...or by touching a server source file before the build:
$ touch main.go
$ go install -a -tags "staging"
...or manually remove the binary before the build:
$ rm .../bin/server
$ go install -a -tags "production"