fix issues related to source package imports (#507)

Fixes: #505
Fixes: #515

mockgen/internal/tests/extra_import/import.go

@@ -0,0 +1,12 @@
+// Package extra_import makes sure output does not import it. See #515.
+package extra_import
+
+//go:generate mockgen -destination mock.go -package extra_import . Foo
+
+type Message struct {
+	Text string
+}
+
+type Foo interface {
+	Bar(channels []string, message chan<- Message)
+}

mockgen/internal/tests/missing_import/source/source.go

@@ -0,0 +1,10 @@
+//go:generate mockgen -package source -destination=../output/source_mock.go -source=source.go
+
+// Package source makes sure output imports its. See #505.
+package source
+
+type Foo struct{}
+
+type Bar interface {
+	Baz(Foo)
+}

mockgen/mockgen.go

@@ -23,7 +23,6 @@ import (
 	"encoding/json"
 	"flag"
 	"fmt"
-	"go/build"
 	"go/token"
 	"io"
 	"io/ioutil"
@@ -133,15 +132,14 @@ func main() {
 	// "package.X" since "package" is this package). This can happen if the mock
 	// is output into an already existing package.
 	outputPackagePath := *selfPackage
-	if len(outputPackagePath) == 0 && len(*destination) > 0 {
-		dst, _ := filepath.Abs(filepath.Dir(*destination))
-		for _, prefix := range build.Default.SrcDirs() {
-			if strings.HasPrefix(dst, prefix) {
-				if rel, err := filepath.Rel(prefix, dst); err == nil {
-					outputPackagePath = rel
-					break
-				}
-			}
+	if outputPackagePath == "" && *destination != "" {
+		dstPath, err := filepath.Abs(filepath.Dir(*destination))
+		if err != nil {
+			log.Fatalf("Unable to determine destination file path: %v", err)
+		}
+		outputPackagePath, err = parsePackageImport(dstPath)
+		if err != nil {
+			log.Fatalf("Unable to determine destination file path: %v", err)
 		}
 	}
 
@@ -330,7 +328,7 @@ func (g *generator) Generate(pkg *model.Package, outputPkgName string, outputPac
 		}
 
 		// Avoid importing package if source pkg == output pkg
-		if pth == pkg.PkgPath && outputPkgName == pkg.Name {
+		if pth == pkg.PkgPath && outputPackagePath == pkg.PkgPath {
 			continue
 		}