1 files changed, 86 insertions, 0 deletions
diff --git a/internal/cached_token_source.go b/internal/cached_token_source.go
new file mode 100644
index 0000000..22bad75
--- /dev/null
+++ b/internal/cached_token_source.go
@@ -0,0 +1,86 @@
+package internal
+
+import (
+	"context"
+	"encoding/json"
+	"expvar"
+	"fmt"
+	"os"
+	"time"
+
+	"golang.org/x/oauth2"
+)
+
+var (
+	tokenRefreshCount = expvar.NewInt("gauge_xesite_token_refresh_count")
+)
+
+type cachingTokenSource struct {
+	base     oauth2.TokenSource
+	filename string
+}
+
+func (c *cachingTokenSource) saveToken(tok *oauth2.Token) error {
+	fout, err := os.Create(c.filename)
+	if err != nil {
+		return fmt.Errorf("error creating %s: %w", c.filename, err)
+	}
+	defer fout.Close()
+
+	return json.NewEncoder(fout).Encode(tok)
+}
+
+func ReadToken(fname string) (*oauth2.Token, error) {
+	fin, err := os.Open(fname)
+	if err != nil {
+		return nil, fmt.Errorf("error opening %s: %w", fname, err)
+	}
+	defer fin.Close()
+
+	var tok oauth2.Token
+	if err := json.NewDecoder(fin).Decode(&tok); err != nil {
+		return nil, fmt.Errorf("error decoding %s: %w", fname, err)
+	}
+
+	return &tok, nil
+}
+
+func FileExists(filename string) bool {
+	_, err := os.Stat(filename)
+	return !os.IsNotExist(err)
+}
+
+func (c *cachingTokenSource) loadToken() (*oauth2.Token, error) {
+	if !FileExists(c.filename) {
+		return nil, nil
+	}
+
+	return ReadToken(c.filename)
+}
+
+func (c *cachingTokenSource) Token() (tok *oauth2.Token, err error) {
+	tok, _ = c.loadToken()
+	if tok != nil && tok.Expiry.Before(time.Now()) {
+		return tok, nil
+	}
+
+	if tok, err = c.base.Token(); err != nil {
+		return nil, err
+	}
+
+	tokenRefreshCount.Add(1)
+
+	if err := c.saveToken(tok); err != nil {
+		return nil, err
+	}
+
+	return tok, err
+}
+
+func CachingTokenSource(filename string, config *oauth2.Config, tok *oauth2.Token) oauth2.TokenSource {
+	orig := config.TokenSource(context.Background(), tok)
+	return oauth2.ReuseTokenSource(nil, &cachingTokenSource{
+		filename: filename,
+		base:     orig,
+	})
+}