gomod fixes for go1.16 (#121)

* gomod fixes for go1.16

patch from: https://github.com/techknowlogick/xgo/issues/109#issuecomment-809396389

* Update test_pr.yml

* Update test_pr.yml

* kludge

* re-enable eth smoke

* Update xgo.bats

* apply feedback from Z

.github/workflows/test_pr.yml

@@ -71,5 +71,7 @@ jobs:
           bats-version: 1.2.1
       - name: run BATS tests
         run: |
+          mkdir -p .xgo-cache
+          mkdir -p ~/go/src
           bats xgo.bats
 

xgo.bats

@@ -1,6 +1,7 @@
 #!/usr/bin/env bats
 
 @test "embedded c" {
+  export GO111MODULE=auto
   run go run xgo.go ./tests/embedded_c
   echo "$output"
   [ "$status" -eq 0 ]
@@ -33,8 +34,9 @@
 }
 
 @test "eth smoke" {
-  skip "remotes are temporarily disabled due to gomod"
-  run go run xgo.go --remote github.com/ethereum/go-ethereum --targets "linux/amd64" github.com/ethereum/go-ethereum/cmd/geth
+  # skip "remotes are temporarily disabled due to gomod"
+  git clone https://github.com/ethereum/go-ethereum.git /tmp/eth
+  run go run xgo.go --targets "linux/amd64" /tmp/eth/cmd/geth
   echo "$output"
   [ "$status" -eq 0 ]
 }

xgo.go

@@ -258,13 +258,17 @@ func compile(image string, config *ConfigFlags, flags *BuildFlags, folder string
 	locals, mounts, paths := []string{}, []string{}, []string{}
 	var usesModules bool
 	if strings.HasPrefix(config.Repository, string(filepath.Separator)) || strings.HasPrefix(config.Repository, ".") {
-		// Resolve the repository import path from the file path
-		config.Repository = resolveImportPath(config.Repository)
+		if _, err := os.Stat(config.Repository + "/go.mod"); err == nil {
+			usesModules = true
+		}
+		if !usesModules {
+			// Resolve the repository import path from the file path
+			config.Repository = resolveImportPath(config.Repository)
 
-		// Determine if this is a module-based repository
-		var modFile = config.Repository + "/go.mod"
-		_, err := os.Stat(modFile)
-		usesModules = !os.IsNotExist(err)
+			if _, err := os.Stat(config.Repository + "/go.mod"); err == nil {
+				usesModules = true
+			}
+		}
 
 		gopathEnv := os.Getenv("GOPATH")
 		if gopathEnv == "" && !usesModules {
@@ -277,6 +281,7 @@ func compile(image string, config *ConfigFlags, flags *BuildFlags, folder string
 			log.Fatalf("No $GOPATH is set or forwarded to xgo")
 		}
 		if !usesModules {
+
 			for _, gopath := range strings.Split(gopathEnv, string(os.PathListSeparator)) {
 				// Since docker sandboxes volumes, resolve any symlinks manually
 				sources := filepath.Join(gopath, "src")