resolve: ignore current environment vars when discovering go source f…

…iles

When a baur repository was inside the GOPATH and contained a Go Application
using Go-Modules resolving GoSources failed.
It failed because Go-Modules are ignored inside the GOPATH.

A fix is to add "GO111MODULE=on" to Build.Input.GolangSources.environment for
the specific application.

Until now all environment variables of the host were used when resolving go
source files. With this commit only a couple strictly  required environment
variables from the host are inherited.
When a custom GOPATH on the host is used in which the baur repository resides,
resolving gosource files won't fail for this case anymore.
It should also help to reduce other unexpected behaviour that might have
happened because of some env vars set on the host.

Thanks to @joshsteveth for reporting the issue.

resolve/gosource/gosource.go

@@ -22,7 +22,7 @@ type Resolver struct {
 // If empty or nil the default Go environment is used.
 func NewResolver(env []string, goDirs ...string) *Resolver {
 	return &Resolver{
-		env:    append(os.Environ(), env...),
+		env:    env,
 		goDirs: goDirs,
 	}
 }
@@ -44,11 +44,45 @@ func (r *Resolver) Resolve() ([]string, error) {
 	return allFiles, nil
 }
 
+// whitelistedEnvVars returns whitelisted environment variables from the host
+// that are set during resolving.
+func whitelistedEnv() []string {
+	var env []string
+
+	// PATH might be required to locate the "go list" tool on the host
+	// system
+	if path, exist := os.LookupEnv("PATH"); exist {
+		env = append(env, "PATH="+path)
+	}
+
+	// The following variables are required for go list to determine the go
+	// build cache, see: https://github.com/golang/go/blob/release-branch.go1.11/src/cmd/go/internal/cache/default.go#L112.
+	// When those are not set, resolving fails because "go list -compiled" is called internally which requires a gocache dir
+	if gocache, exist := os.LookupEnv("GOCACHE"); exist {
+		env = append(env, "GOCACHE="+gocache)
+	}
+
+	if xdgCacheHome, exist := os.LookupEnv("XDG_CACHE_HOME"); exist {
+		env = append(env, "XDG_CACHE_HOME="+xdgCacheHome)
+	}
+
+	if home, exist := os.LookupEnv("HOME"); exist {
+		env = append(env, "HOME="+home)
+	}
+
+	// plan9 home env variable
+	if home, exist := os.LookupEnv("home"); exist {
+		env = append(env, "home="+home)
+	}
+
+	return env
+}
+
 func (r *Resolver) resolve(path string) ([]string, error) {
 	cfg := &packages.Config{
 		Mode: packages.LoadImports,
 		Dir:  path,
-		Env:  r.env,
+		Env:  append(whitelistedEnv(), r.env...),
 	}
 
 	lpkgs, err := packages.Load(cfg, "./...")