Add concurrent builder test (#136)

.github/workflows/build.yml

@@ -17,16 +17,13 @@ on:
 # See https://github.com/cristalhq/.github/.github/workflows
 jobs:
   build:
-    uses: cristalhq/.github/.github/workflows/build.yml@main
-
-  codeql:
-    uses: cristalhq/.github/.github/workflows/codeql.yml@main
+    uses: cristalhq/.github/.github/workflows/build.yml@3f2ea32d08cea472a5d81a15769eb911595118af # v0.2.1
 
   vuln:
-    uses: cristalhq/.github/.github/workflows/vuln.yml@main
+    uses: cristalhq/.github/.github/workflows/vuln.yml@3f2ea32d08cea472a5d81a15769eb911595118af # v0.2.1
 
   release:
     if: github.event_name == 'workflow_dispatch'
-    uses: cristalhq/.github/.github/workflows/release.yml@main
+    uses: cristalhq/.github/.github/workflows/release.yml@3f2ea32d08cea472a5d81a15769eb911595118af # v0.2.1
     with:
       tag: ${{ github.event.input.tag }}

build.go

@@ -19,6 +19,7 @@ func WithContentType(cty string) BuilderOption {
 }
 
 // Builder is used to create a new token.
+// Safe to use concurrently.
 type Builder struct {
 	signer    Signer
 	header    Header

build_test.go

@@ -2,6 +2,7 @@ package jwt
 
 import (
 	"errors"
+	"sync"
 	"testing"
 )
 
@@ -226,6 +227,42 @@ func TestBuildMalformed(t *testing.T) {
 	)
 }
 
+func TestBuilderConcurrently(t *testing.T) {
+	signer, err := NewSignerHS(HS256, []byte("test-key"))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	builder := NewBuilder(signer)
+
+	errCh := make(chan error, 10)
+	claims := "string-claims"
+
+	var wg sync.WaitGroup
+	wg.Add(10)
+	for i := 0; i < 10; i++ {
+		go func() {
+			defer wg.Done()
+
+			token, err := builder.Build(claims)
+			errCh <- err
+
+			if token.String() == "" {
+				panic("should not be empty")
+			}
+		}()
+	}
+
+	wg.Wait()
+	close(errCh)
+
+	for err := range errCh {
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+}
+
 type badSigner struct{}
 
 func (badSigner) SignSize() int {