Check if config location is writeable before authenticating

Fixes #1314

features/authentication.feature

@@ -95,7 +95,7 @@ Feature: OAuth authentication
       Duplicate value for "description"
       """
     And the exit status should be 1
-    And the file "../home/.config/hub" should not exist
+    And the file "../home/.config/hub" should contain ""
 
   Scenario: Credentials from GITHUB_USER & GITHUB_PASSWORD
     Given the GitHub API server:
@@ -172,7 +172,7 @@ Feature: OAuth authentication
 
       """
     And the exit status should be 1
-    And the file "../home/.config/hub" should not exist
+    And the file "../home/.config/hub" should contain ""
 
   Scenario: Personal access token used instead of password
     Given the GitHub API server:
@@ -192,7 +192,7 @@ Feature: OAuth authentication
 
       """
     And the exit status should be 1
-    And the file "../home/.config/hub" should not exist
+    And the file "../home/.config/hub" should contain ""
 
   Scenario: Two-factor authentication, create authorization
     Given the GitHub API server:
@@ -332,3 +332,15 @@ Feature: OAuth authentication
     Then the output should contain "github.com username:"
     And the output should contain "missing user"
     And the file "../home/.config/hub" should not contain "user"
+    
+  Scenario: Config file is not writeable, should exit before asking for credentails
+      Given I set the environment variables to:
+      | variable           | value      |
+      | HUB_CONFIG | /InvalidConfigFile |
+      When I run `hub create` interactively
+      Then the output should contain exactly:
+        """
+        open /InvalidConfigFile: permission denied\n
+        """
+      And the exit status should be 1
+    
\ No newline at end of file

github/config.go

@@ -51,6 +51,7 @@ func (c *Config) PromptForHost(host string) (h *Host, err error) {
 	h = c.Find(host)
 	if h != nil {
 		if h.User == "" {
+			utils.Check(newConfigService().CheckWriteable(configsFile()))
 			// User is missing from the config: this is a broken config probably
 			// because it was created with an old (broken) version of hub. Let's fix
 			// it now. See issue #1007 for details.
@@ -79,6 +80,7 @@ func (c *Config) PromptForHost(host string) (h *Host, err error) {
 	client := NewClientWithHost(h)
 
 	if !tokenFromEnv {
+		utils.Check(newConfigService().CheckWriteable(configsFile()))
 		err = c.authorizeClient(client, host)
 		if err != nil {
 			return

github/config_service.go

@@ -41,3 +41,20 @@ func (s *configService) Load(filename string, c *Config) error {
 
 	return s.Decoder.Decode(r, c)
 }
+
+// CheckWriteable checks if config file is writeable. This should
+// be called before asking for credentials and only if current
+// operation needs to update the file. See issue #1314 for details.
+func (s *configService) CheckWriteable(filename string) error {
+    err := os.MkdirAll(filepath.Dir(filename), 0771)
+    if err != nil {
+        return err
+    }
+
+    w, err := os.OpenFile(filename, os.O_WRONLY|os.O_CREATE|os.O_APPEND, 0600)
+    if err != nil {
+        return err
+    }
+    w.Close()
+    return nil
+}