From a9aa47586d4d367ff4eca51de03fb800476ea307 Mon Sep 17 00:00:00 2001
From: Tim Smith <tsmith84@gmail.com>
Date: Wed, 29 Jan 2020 16:54:50 -0800
Subject: Support fetching the latest version of Azure metadata

This is inspired by the way we do this in EC2, but it's different in
many ways since Azure presents the latest version in a different way.

Signed-off-by: Tim Smith <tsmith@chef.io>
---
 lib/ohai/mixin/azure_metadata.rb | 87 +++++++++++++++++++++++++++++++++-------
 1 file changed, 72 insertions(+), 15 deletions(-)

diff --git a/lib/ohai/mixin/azure_metadata.rb b/lib/ohai/mixin/azure_metadata.rb
index 5e924b4b..4e3cd771 100644
--- a/lib/ohai/mixin/azure_metadata.rb
+++ b/lib/ohai/mixin/azure_metadata.rb
@@ -1,6 +1,6 @@
 #
 # Author:: Tim Smith (<tsmith@chef.io>)
-# Copyright:: Copyright 2017 Chef Software, Inc.
+# Copyright:: 2017-2020 Chef Software, Inc.
 # License:: Apache License, Version 2.0
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
@@ -19,30 +19,87 @@ require "net/http" unless defined?(Net::HTTP)
 
 module Ohai
   module Mixin
+    ##
+    # This code parses the Azure Instance Metadata API to provide details
+    # of the running instance.
+    #
+    # The code probes the instance metadata endpoint for
+    # available versions, determines the most advanced version known to
+    # work and executes the metadata retrieval using that version.
+    #
+    # If no compatible version is found, an empty hash is returned.
+    #
     module AzureMetadata
 
       AZURE_METADATA_ADDR ||= "169.254.169.254".freeze
-      AZURE_METADATA_URL ||= "/metadata/instance?api-version=2017-08-01".freeze
 
-      # fetch the meta content with a timeout and the required header
+      # it's important that the newer versions are at the end of this array so we can skip sorting it
+      AZURE_SUPPORTED_VERSIONS ||= %w{ 2017-04-02 2017-08-01 2017-12-01 2018-02-01 2018-04-02
+                                   2018-10-01 2019-02-01 2019-03-11 2019-04-30 2019-06-01
+                                   2019-06-04 2019-08-01 2019-08-15 2019-11-01 }.freeze
+
+      def best_api_version
+        @api_version ||= begin
+          logger.trace("Mixin AzureMetadata: Fetching http://#{AZURE_METADATA_ADDR}/metadata/instance to determine the latest supported metadata release")
+          response = http_get("/metadata/instance")
+          if response.code == "404"
+            logger.trace("Mixin AzureMetadata: Received HTTP 404 from metadata server while determining API version, assuming #{AZURE_SUPPORTED_VERSIONS[-1]}")
+            return AZURE_SUPPORTED_VERSIONS[-1]
+          elsif response.code != "400" # 400 is actually what we want
+            raise "Mixin AzureMetadata: Unable to determine Azure metadata version (returned #{response.code} response)"
+          end
+
+          # azure returns a list of the 3 latest versions it supports
+          versions = parse_json(response.body)["newest-versions"]
+          versions.sort!
+
+          until versions.empty? || AZURE_SUPPORTED_VERSIONS.include?(versions.last)
+            pv = versions.pop
+            logger.trace("Mixin AzureMetadata: Azure metadata version #{pv} is not present in the versions provided by the Azure Instance Metadata service")
+          end
+
+          if versions.empty?
+            logger.debug "Mixin AzureMetadata: The short list of supported versions provided by Azure Instance Metadata service doesn't match any known versions to Ohai. Using the latest supported release known to Ohai instead: #{AZURE_SUPPORTED_VERSIONS.last}"
+            return AZURE_SUPPORTED_VERSIONS.last
+          end
+
+          logger.trace("Mixin AzureMetadata: Latest supported Azure metadata version: #{versions.last}")
+          versions.last
+        end
+      end
+
+      # fetch the meta content with a timeout and the required header and a read timeout of 6s
+      #
+      # @param [String] the relative uri to fetch from the Azure Metadata Service URL
+      #
+      # @return [Net::HTTP]
       def http_get(uri)
+        full_uri = URI.join("http://#{AZURE_METADATA_ADDR}", uri)
         conn = Net::HTTP.start(AZURE_METADATA_ADDR)
         conn.read_timeout = 6
-        conn.get(uri, { "Metadata" => "true" })
+        conn.get(full_uri, { "Metadata" => "true" })
       end
 
-      def fetch_metadata
-        logger.trace("Mixin AzureMetadata: Fetching metadata from host #{AZURE_METADATA_ADDR} at #{AZURE_METADATA_URL}")
-        response = http_get(AZURE_METADATA_URL)
+      # parse JSON data from a String to a Hash
+      #
+      # @param [String] response_body json as string to parse
+      #
+      # @return [Hash]
+      def parse_json(response_body)
+        data = StringIO.new(response_body)
+        parser = FFI_Yajl::Parser.new
+        parser.parse(data)
+      rescue FFI_Yajl::ParseError
+        logger.warn("Mixin AzureMetadata: Metadata response is NOT valid JSON")
+        nil
+      end
+
+      def fetch_metadata(api_version = nil)
+        metadata_url = "/metadata/instance?api-version=#{best_api_version}"
+        logger.trace("Mixin AzureMetadata: Fetching metadata from host #{AZURE_METADATA_ADDR} at #{metadata_url}")
+        response = http_get(metadata_url)
         if response.code == "200"
-          begin
-            data = StringIO.new(response.body)
-            parser = FFI_Yajl::Parser.new
-            parser.parse(data)
-          rescue FFI_Yajl::ParseError
-            logger.warn("Mixin AzureMetadata: Metadata response is NOT valid JSON")
-            nil
-          end
+          parse_json(response.body)
         else
           logger.warn("Mixin AzureMetadata: Received response code #{response.code} requesting metadata")
           nil
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