adding option to skip provider button sign_in page

main.go

@@ -36,6 +36,7 @@ func main() {
 	flagSet.Bool("pass-access-token", false, "pass OAuth access_token to upstream via X-Forwarded-Access-Token header")
 	flagSet.Bool("pass-host-header", true, "pass the request Host Header to upstream")
 	flagSet.Var(&skipAuthRegex, "skip-auth-regex", "bypass authentication for requests path's that match (may be given multiple times)")
+	flagSet.Bool("skip-provider-button", false, "will skip sign-in-page to directly reach the next step: oauth/start")
 
 	flagSet.Var(&emailDomains, "email-domain", "authenticate emails with the specified domain (may be given multiple times). Use * to authenticate any email")
 	flagSet.String("github-org", "", "restrict logins to members of this organisation")

oauthproxy.go

@@ -43,6 +43,7 @@ type OAuthProxy struct {
 	DisplayHtpasswdForm bool
 	serveMux            http.Handler
 	PassBasicAuth       bool
+	SkipProviderButton  bool
 	BasicAuthPassword   string
 	PassAccessToken     bool
 	CookieCipher        *cookie.Cipher
@@ -168,6 +169,7 @@ func NewOAuthProxy(opts *Options, validator func(string) bool) *OAuthProxy {
 		PassBasicAuth:      opts.PassBasicAuth,
 		BasicAuthPassword:  opts.BasicAuthPassword,
 		PassAccessToken:    opts.PassAccessToken,
+		SkipProviderButton: opts.SkipProviderButton,
 		CookieCipher:       cipher,
 		templates:          loadTemplates(opts.CustomTemplatesDir),
 	}
@@ -484,7 +486,11 @@ func (p *OAuthProxy) Proxy(rw http.ResponseWriter, req *http.Request) {
 		p.ErrorPage(rw, http.StatusInternalServerError,
 			"Internal Error", "Internal Error")
 	} else if status == http.StatusForbidden {
+		if p.SkipProviderButton {
+			p.OAuthStart(rw, req)
+		} else {
 			p.SignInPage(rw, req, http.StatusForbidden)
+		}
 	} else {
 		p.serveMux.ServeHTTP(rw, req)
 	}

options.go

@@ -47,6 +47,7 @@ type Options struct {
 	BasicAuthPassword  string   `flag:"basic-auth-password" cfg:"basic_auth_password"`
 	PassAccessToken    bool     `flag:"pass-access-token" cfg:"pass_access_token"`
 	PassHostHeader     bool     `flag:"pass-host-header" cfg:"pass_host_header"`
+	SkipProviderButton bool     `flag:"skip-provider-button" cfg:"skip_provider_button"`
 
 	// These options allow for other providers besides Google, with
 	// potential overrides.
@@ -81,6 +82,7 @@ func NewOptions() *Options {
 		PassBasicAuth:       true,
 		PassAccessToken:     false,
 		PassHostHeader:      true,
+		SkipProviderButton:  false,
 		ApprovalPrompt:      "force",
 		RequestLogging:      true,
 	}