fs: allow global variables to be overriden or set on backend creation

This allows backend config to contain

- `override.var` - set var during remote creation only
- `global.var` - set var in the global config permanently

Fixes #8563

fs/newfs_internal_test.go

@@ -0,0 +1,55 @@
+package fs
+
+import (
+	"context"
+	"testing"
+
+	"github.com/rclone/rclone/fs/config/configmap"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// When no override/global keys exist, ctx must be returned unchanged.
+func TestAddConfigToContext_NoChanges(t *testing.T) {
+	ctx := context.Background()
+	newCtx, err := addConfigToContext(ctx, "unit-test", configmap.Simple{})
+	require.NoError(t, err)
+	assert.Equal(t, newCtx, ctx)
+}
+
+// A single override.key must create a new ctx, but leave the
+// background ctx untouched.
+func TestAddConfigToContext_OverrideOnly(t *testing.T) {
+	override := configmap.Simple{
+		"override.user_agent": "potato",
+	}
+	ctx := context.Background()
+	globalCI := GetConfig(ctx)
+	original := globalCI.UserAgent
+	newCtx, err := addConfigToContext(ctx, "unit-test", override)
+	require.NoError(t, err)
+	assert.NotEqual(t, newCtx, ctx)
+	assert.Equal(t, original, globalCI.UserAgent)
+	ci := GetConfig(newCtx)
+	assert.Equal(t, "potato", ci.UserAgent)
+}
+
+// A single global.key must create a new ctx and update the
+// background/global config.
+func TestAddConfigToContext_GlobalOnly(t *testing.T) {
+	global := configmap.Simple{
+		"global.user_agent": "potato2",
+	}
+	ctx := context.Background()
+	globalCI := GetConfig(ctx)
+	original := globalCI.UserAgent
+	defer func() {
+		globalCI.UserAgent = original
+	}()
+	newCtx, err := addConfigToContext(ctx, "unit-test", global)
+	require.NoError(t, err)
+	assert.NotEqual(t, newCtx, ctx)
+	assert.Equal(t, "potato2", globalCI.UserAgent)
+	ci := GetConfig(newCtx)
+	assert.Equal(t, "potato2", ci.UserAgent)
+}