Added logic to pass build flags when running the generator (#290)

* Added logic to pass build flags when running the generator while bootstrapping

* fixed parsing build args for differenct cases

* cleanup code

README.md

@@ -34,7 +34,9 @@ Usage of easyjson:
   -all
     	generate marshaler/unmarshalers for all structs in a file
   -build_tags string
-    	build tags to add to generated file
+        build tags to add to generated file
+  -gen_build_flags string
+        build flags when running the generator while bootstrapping
   -byte
         use simple bytes instead of Base64Bytes for slice of bytes
   -leave_temps
@@ -88,6 +90,10 @@ Additional option notes:
 
 * `-build_tags` will add the specified build tags to generated Go sources.
 
+* `-gen_build_flags` will execute the easyjson bootstapping code to launch the 
+  actual generator command with provided flags. Multiple arguments should be
+  separated by space e.g. `-gen_build_flags="-mod=mod -x"`.
+
 ## Structure json tag options
 
 Besides standart json tag options like 'omitempty' the following are supported:

bootstrap/bootstrap.go

@@ -12,6 +12,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"regexp"
 	"sort"
 )
 
@@ -19,6 +20,8 @@ const genPackage = "github.com/mailru/easyjson/gen"
 const pkgWriter = "github.com/mailru/easyjson/jwriter"
 const pkgLexer = "github.com/mailru/easyjson/jlexer"
 
+var buildFlagsRegexp = regexp.MustCompile("'.+'|\".+\"|\\S+")
+
 type Generator struct {
 	PkgPath, PkgName string
 	Types            []string
@@ -30,8 +33,9 @@ type Generator struct {
 	DisallowUnknownFields    bool
 	SkipMemberNameUnescaping bool
 
-	OutName   string
-	BuildTags string
+	OutName       string
+	BuildTags     string
+	GenBuildFlags string
 
 	StubsOnly   bool
 	LeaveTemps  bool
@@ -178,7 +182,14 @@ func (g *Generator) Run() error {
 		defer os.Remove(f.Name()) // will not remove after rename
 	}
 
-	cmd := exec.Command("go", "run", "-tags", g.BuildTags, filepath.Base(path))
+	execArgs := []string{"run"}
+	if g.GenBuildFlags != "" {
+		buildFlags := buildFlagsRegexp.FindAllString(g.GenBuildFlags, -1)
+		execArgs = append(execArgs, buildFlags...)
+	}
+	execArgs = append(execArgs, "-tags", g.BuildTags, filepath.Base(path))
+	cmd := exec.Command("go", execArgs...)
+
 	cmd.Stdout = f
 	cmd.Stderr = os.Stderr
 	cmd.Dir = filepath.Dir(path)

easyjson/main.go

@@ -17,6 +17,7 @@ import (
 )
 
 var buildTags = flag.String("build_tags", "", "build tags to add to generated file")
+var genBuildFlags = flag.String("gen_build_flags", "", "build flags when running the generator while bootstrapping")
 var snakeCase = flag.Bool("snake_case", false, "use snake_case names instead of CamelCase by default")
 var lowerCamelCase = flag.Bool("lower_camel_case", false, "use lowerCamelCase names instead of CamelCase by default")
 var noStdMarshalers = flag.Bool("no_std_marshalers", false, "don't generate MarshalJSON/UnmarshalJSON funcs")
@@ -62,8 +63,14 @@ func generate(fname string) (err error) {
 		trimmedBuildTags = strings.TrimSpace(*buildTags)
 	}
 
+	var trimmedGenBuildFlags string
+	if *genBuildFlags != "" {
+		trimmedGenBuildFlags = strings.TrimSpace(*genBuildFlags)
+	}
+
 	g := bootstrap.Generator{
 		BuildTags:                trimmedBuildTags,
+		GenBuildFlags:            trimmedGenBuildFlags,
 		PkgPath:                  p.PkgPath,
 		PkgName:                  p.PkgName,
 		Types:                    p.StructNames,