fix: tmpl always copy before modifying (#5200)

this is just to prevent some gotchas when using `templ` internally.

internal/tmpl/tmpl.go

@@ -4,6 +4,7 @@ package tmpl
 import (
 	"bytes"
 	"fmt"
+	"maps"
 	"path/filepath"
 	"regexp"
 	"strings"
@@ -141,6 +142,16 @@ func New(ctx *context.Context) *Template {
 	}
 }
 
+// WithExtraFields allows to add new more custom fields to the template.
+// It will override fields with the same name.
+func (t *Template) WithExtraFields(f Fields) *Template {
+	tt := t.copying()
+	for k, v := range f {
+		tt.fields[k] = v
+	}
+	return tt
+}
+
 // WithEnvS overrides template's env field with the given KEY=VALUE list of
 // environment variables.
 func (t *Template) WithEnvS(envs []string) *Template {
@@ -157,35 +168,28 @@ func (t *Template) WithEnvS(envs []string) *Template {
 
 // WithEnv overrides template's env field with the given environment map.
 func (t *Template) WithEnv(e map[string]string) *Template {
-	t.fields[env] = context.Env(e)
-	return t
-}
-
-// WithExtraFields allows to add new more custom fields to the template.
-// It will override fields with the same name.
-func (t *Template) WithExtraFields(f Fields) *Template {
-	for k, v := range f {
-		t.fields[k] = v
-	}
-	return t
+	return t.WithExtraFields(Fields{
+		env: context.Env(e),
+	})
 }
 
 // WithArtifact populates Fields from the artifact.
 func (t *Template) WithArtifact(a *artifact.Artifact) *Template {
-	t.fields[osKey] = a.Goos
-	t.fields[arch] = a.Goarch
-	t.fields[amd64] = a.Goamd64
-	t.fields[go386] = a.Go386
-	t.fields[arm] = a.Goarm
-	t.fields[arm64] = a.Goarm64
-	t.fields[mips] = a.Gomips
-	t.fields[ppc64] = a.Goppc64
-	t.fields[riscv64] = a.Goriscv64
-	t.fields[binary] = artifact.ExtraOr(*a, binary, t.fields[projectName].(string))
-	t.fields[artifactName] = a.Name
-	t.fields[artifactExt] = artifact.ExtraOr(*a, artifact.ExtraExt, "")
-	t.fields[artifactPath] = a.Path
-	return t
+	return t.WithExtraFields(Fields{
+		osKey:        a.Goos,
+		arch:         a.Goarch,
+		amd64:        a.Goamd64,
+		go386:        a.Go386,
+		arm:          a.Goarm,
+		arm64:        a.Goarm64,
+		mips:         a.Gomips,
+		ppc64:        a.Goppc64,
+		riscv64:      a.Goriscv64,
+		binary:       artifact.ExtraOr(*a, binary, t.fields[projectName].(string)),
+		artifactName: a.Name,
+		artifactExt:  artifact.ExtraOr(*a, artifact.ExtraExt, ""),
+		artifactPath: a.Path,
+	})
 }
 
 func (t *Template) WithBuildOptions(opts build.Options) *Template {
@@ -284,6 +288,14 @@ func (t *Template) envOrDefault(name, value string) string {
 	return s
 }
 
+func (t *Template) copying() *Template {
+	tpl := &Template{
+		fields: Fields{},
+	}
+	maps.Copy(tpl.fields, t.fields)
+	return tpl
+}
+
 type ExpectedSingleEnvErr struct{}
 
 func (e ExpectedSingleEnvErr) Error() string {

internal/tmpl/tmpl_test.go

@@ -471,3 +471,20 @@ func TestWithBuildOptions(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, "name_./path_.ext_target_os_arch_amd64_arm_mips", out)
 }
+
+func TestReuseTpl(t *testing.T) {
+	tp := New(testctx.New()).WithExtraFields(Fields{
+		"foo": "bar",
+	})
+	s1, err := tp.Apply("{{.foo}}")
+	require.NoError(t, err)
+	require.Equal(t, "bar", s1)
+
+	s2, err := tp.WithExtraFields(Fields{"foo": "not-bar"}).Apply("{{.foo}}")
+	require.NoError(t, err)
+	require.Equal(t, "not-bar", s2)
+
+	s3, err := tp.Apply("{{.foo}}")
+	require.NoError(t, err)
+	require.Equal(t, "bar", s3)
+}