From 18eebe1d7db35cf82d172db46fc5a592f122f231 Mon Sep 17 00:00:00 2001
From: Dmitry Kolesnikov <dmkolesnikov@gmail.com>
Date: Wed, 16 Jan 2013 23:31:17 +0200
Subject: [PATCH] fix crash on erlcloud_ec2:describe_instance_attribute if
 attribute is empty

---
 .gitignore           |  2 ++
 Makefile             |  6 +++---
 src/erlcloud_ec2.erl | 30 +++++++++++++++++++-----------
 3 files changed, 24 insertions(+), 14 deletions(-)

diff --git a/.gitignore b/.gitignore
index a3db9623..8b3b4f0c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -4,4 +4,6 @@ _build
 ebin/*.beam
 ebin/*.app
 .eunit/
+rebar
+*.sublime-*
 deps/
diff --git a/Makefile b/Makefile
index 9d732e0c..7430175f 100644
--- a/Makefile
+++ b/Makefile
@@ -1,10 +1,10 @@
 REBAR=$(shell which rebar || echo ./rebar)
 
+all: get-deps compile
+
 get-deps:
 	@$(REBAR) get-deps
 
-all: compile
-
 clean:
 	@$(REBAR) clean
 
@@ -12,4 +12,4 @@ compile:
 	@$(REBAR) compile
 
 eunit: compile
-	@$(REBAR) eunit skip_deps=true
\ No newline at end of file
+	@$(REBAR) eunit skip_deps=true
diff --git a/src/erlcloud_ec2.erl b/src/erlcloud_ec2.erl
index c77498be..08dc2671 100644
--- a/src/erlcloud_ec2.erl
+++ b/src/erlcloud_ec2.erl
@@ -621,19 +621,27 @@ describe_instance_attribute(InstanceID, Attribute, Config)
         block_device_mapping -> "blockDeviceMapping"
     end,
     Doc = ec2_query(Config, "DescribeInstanceAttribute", [{"InstanceId", InstanceID}, {"Attribute", AttributeName}]),
-    Node = case Attribute of
-        block_device_mapping -> hd(xmerl_xpath:string("/DescribeInstanceAttributeResponse/" ++ AttributeName, Doc));
-        _ -> hd(xmerl_xpath:string("/DescribeInstanceAttributeResponse/" ++ AttributeName ++ "/value", Doc))
-    end,
-    case Attribute of
-        user_data -> base64:decode(get_text(Node));
-        disable_api_termination -> list_to_existing_atom(get_text(Node));
-        instance_initiated_shutdown_behavior -> list_to_existing_atom(get_text(Node));
-        block_device_mapping ->
-            [extract_block_device_mapping_status(Item) || Item <- xmerl_xpath:string("item", Node)];
-        _ -> get_text(Node)
+    case xmerl_xpath:string(attribute_xpath(Attribute, AttributeName), Doc) of
+        % attribute might not be defined
+        [] -> 
+            undefined;
+        [Node | _] ->
+            case Attribute of
+                user_data -> base64:decode(get_text(Node));
+                disable_api_termination -> list_to_existing_atom(get_text(Node));
+                instance_initiated_shutdown_behavior -> list_to_existing_atom(get_text(Node));
+                block_device_mapping ->
+                    [extract_block_device_mapping_status(Item) || Item <- xmerl_xpath:string("item", Node)];
+                _ -> get_text(Node)
+            end
     end.
 
+attribute_xpath(block_device_mapping, AttributeName) ->
+   "/DescribeInstanceAttributeResponse/" ++ AttributeName;
+attribute_xpath(_, AttributeName) ->
+   "/DescribeInstanceAttributeResponse/" ++ AttributeName ++ "/value".
+
+
 -spec(describe_instances/0 :: () -> proplist()).
 describe_instances() -> describe_instances([]).