Enable AWS cred file to be read as long as .s3cfg auth fields are

nonexistent or empty.

This commit allows the credentials to be read from ~/.aws/credentials
or user-specified (via envvar) file if .s3cfg exists but does not have
authentication options.

The previous behavior is that s3cmd will *not* read credentials from the
above path as long as .s3cfg exists (even if it contains no
authentication options).

S3/Config.py

@@ -24,7 +24,7 @@
     import http.client as httplib
 import locale
 
-try: 
+try:
  from configparser import NoOptionError, NoSectionError, MissingSectionHeaderError, ConfigParser as PyConfigParser
 except ImportError:
   # Python2 fallback code
@@ -217,8 +217,7 @@ def __init__(self, configfile = None, access_key=None, secret_key=None, access_t
             try:
                 self.read_config_file(configfile)
             except IOError:
-                if 'AWS_CREDENTIAL_FILE' in os.environ or 'AWS_PROFILE' in os.environ:
-                    self.aws_credential_file()
+                pass
 
             # override these if passed on the command-line
             if access_key and secret_key:
@@ -229,7 +228,10 @@ def __init__(self, configfile = None, access_key=None, secret_key=None, access_t
                 # Do not refresh the IAM role when an access token is provided.
                 self._access_token_refresh = False
 
-            if len(self.access_key)==0:
+            # The next few clauses are in descending order of priority.
+            # That is, we will prefer key envvars, then AWS cred file, then IAM auth.
+
+            if not self.is_option_nonempty('access_key'):
                 env_access_key = os.getenv('AWS_ACCESS_KEY') or os.getenv('AWS_ACCESS_KEY_ID')
                 env_secret_key = os.getenv('AWS_SECRET_KEY') or os.getenv('AWS_SECRET_ACCESS_KEY')
                 env_access_token = os.getenv('AWS_SESSION_TOKEN') or os.getenv('AWS_SECURITY_TOKEN')
@@ -241,15 +243,25 @@ def __init__(self, configfile = None, access_key=None, secret_key=None, access_t
                         # Do not refresh the IAM role when an access token is provided.
                         self._access_token_refresh = False
                         self.access_token = config_unicodise(env_access_token)
-                else:
-                    self.role_config()
+
+            if not self.is_option_nonempty('access_key'):
+                self.aws_credential_file()
+
+            if not self.is_option_nonempty('access_key'):
+                self.role_config()
+
+            if not self.is_option_nonempty('access_key'):
+                raise Exception('There is no access key available!')
 
             #TODO check KMS key is valid
             if self.kms_key and self.server_side_encryption == True:
                 warning('Cannot have server_side_encryption (S3 SSE) and KMS_key set (S3 KMS). KMS encryption will be used. Please set server_side_encryption to False')
             if self.kms_key and self.signature_v2 == True:
                 raise Exception('KMS encryption requires signature v4. Please set signature_v2 to False')
 
+    def is_option_nonempty(self, option_name):
+        return hasattr(self, option_name) and bool(str(getattr(self, option_name)))
+
     def role_config(self):
         """
         Get credentials from IAM authentication
@@ -268,10 +280,6 @@ def role_config(self):
                     Config().update_option('access_key', config_unicodise(creds['AccessKeyId']))
                     Config().update_option('secret_key', config_unicodise(creds['SecretAccessKey']))
                     Config().update_option('access_token', config_unicodise(creds['Token']))
-                else:
-                    raise IOError
-            else:
-                raise IOError
         except:
             raise
 
@@ -284,9 +292,11 @@ def role_refresh(self):
 
     def aws_credential_file(self):
         try:
-            aws_credential_file = os.path.expanduser('~/.aws/credentials') 
+            aws_credential_file = os.path.expanduser('~/.aws/credentials')
             if 'AWS_CREDENTIAL_FILE' in os.environ and os.path.isfile(os.environ['AWS_CREDENTIAL_FILE']):
                 aws_credential_file = config_unicodise(os.environ['AWS_CREDENTIAL_FILE'])
+            elif not os.path.isfile(aws_credential_file):
+                return
 
             config = PyConfigParser()