Add a LookupMetadata function to the metadata store.

src/api_server.cc

@@ -41,24 +41,22 @@ void MetadataApiServer::Handler::operator()(const HttpServer::request& request,
               << " body: " << request.body;
   }
   if (request.method == "GET" && request.destination.find(kPrefix) == 0) {
-    std::string id = request.destination.substr(kPrefix.size());
-    std::lock_guard<std::mutex> lock(store_.resource_mu_);
-    const auto result = store_.resource_map_.find(id);
-    if (result == store_.resource_map_.end()) {
+    const std::string id = request.destination.substr(kPrefix.size());
+    try {
+      const MonitoredResource& resource = store_.LookupResource(id);
+      if (config_.VerboseLogging()) {
+        LOG(INFO) << "Found resource for " << id << ": " << resource;
+      }
+      conn->set_status(HttpServer::connection::ok);
+      conn->write(resource.ToJSON()->ToString());
+    } catch (const std::out_of_range& e) {
       // TODO: This could be considered log spam.
       // As we add more resource mappings, these will become less and less
       // frequent, and could be promoted to ERROR.
       if (config_.VerboseLogging()) {
         LOG(WARNING) << "No matching resource for " << id;
       }
       conn->set_status(HttpServer::connection::not_found);
-    } else {
-      const MonitoredResource& resource = result->second;
-      if (config_.VerboseLogging()) {
-        LOG(INFO) << "Found resource for " << id << ": " << resource;
-      }
-      conn->set_status(HttpServer::connection::ok);
-      conn->write(resource.ToJSON()->ToString());
     }
   }
 }

src/store.cc

@@ -29,6 +29,12 @@ MetadataStore::Metadata MetadataStore::Metadata::IGNORED() {
 MetadataStore::MetadataStore(const MetadataAgentConfiguration& config)
     : config_(config) {}
 
+const MonitoredResource& MetadataStore::LookupResource(
+    const std::string& resource_id) const throw(std::out_of_range) {
+  std::lock_guard<std::mutex> lock(resource_mu_);
+  return resource_map_.at(resource_id);
+}
+
 void MetadataStore::UpdateResource(const std::vector<std::string>& resource_ids,
                                    const MonitoredResource& resource) {
   std::lock_guard<std::mutex> lock(resource_mu_);

src/store.h

@@ -20,6 +20,7 @@
 
 #include <map>
 #include <mutex>
+#include <stdexcept>
 #include <string>
 
 #include "json.h"
@@ -75,6 +76,11 @@ class MetadataStore {
 
   MetadataStore(const MetadataAgentConfiguration& config);
 
+  // Looks up the local resource map entry for a given resource id.
+  // Throws an exception if the resource is not found.
+  const MonitoredResource& LookupResource(const std::string& resource_id) const
+      throw(std::out_of_range);
+
   // Updates the local resource map entry for a given resource.
   // Each local id in `resource_ids` is effectively an alias for `resource`.
   // Adds a resource mapping from each of the `resource_ids` to the `resource`.
@@ -87,7 +93,6 @@ class MetadataStore {
                       Metadata&& entry);
 
  private:
-  friend class MetadataApiServer;
   friend class MetadataReporter;
 
   std::map<MonitoredResource, Metadata> GetMetadataMap() const;