Support fetching the latest version of Azure metadataazure_versions

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>

1 files 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