add LoadFromStdin (#820)

cmd/validate/main.go

@@ -63,7 +63,12 @@ func main() {
 		loader := openapi3.NewLoader()
 		loader.IsExternalRefsAllowed = *ext
 
-		doc, err := loader.LoadFromFile(filename)
+		var doc *openapi3.T
+		if filename == "-" {
+			doc, err = loader.LoadFromStdin()
+		} else {
+			doc, err = loader.LoadFromFile(filename)
+		}
 		if err != nil {
 			log.Fatalln("Loading error:", err)
 		}

openapi3/loader.go

@@ -5,7 +5,9 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"io"
 	"net/url"
+	"os"
 	"path"
 	"path/filepath"
 	"reflect"
@@ -130,6 +132,15 @@ func (loader *Loader) readURL(location *url.URL) ([]byte, error) {
 	return DefaultReadFromURI(loader, location)
 }
 
+// LoadFromStdin loads a spec from stdin
+func (loader *Loader) LoadFromStdin() (*T, error) {
+	data, err := io.ReadAll(os.Stdin)
+	if err != nil {
+		return nil, fmt.Errorf("read from stdin: %w", err)
+	}
+	return loader.LoadFromData(data)
+}
+
 // LoadFromData loads a spec from a byte array
 func (loader *Loader) LoadFromData(data []byte) (*T, error) {
 	loader.resetVisitedPathItemRefs()