Add a QueryParameters class.

This wraps libevent's evhttp_parse_query_str and friends. It's easier to use
than the raw libevent stuff because it handles initialization (formerly not
done properly in profiler.cc) and cleans up with RAII.

src/http.cc

@@ -36,7 +36,6 @@
 #include <fcntl.h>
 #include <stdlib.h>
 #include <string.h>
-#include <sys/queue.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 

src/http.h

@@ -36,6 +36,7 @@
 
 #include <dirent.h>
 #include <stdarg.h>
+#include <sys/queue.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 
@@ -45,6 +46,7 @@
 #include <string>
 
 #include <event2/buffer.h>
+#include <event2/keyvalq_struct.h>
 #include <event2/http.h>
 #include <glog/logging.h>
 #include <re2/stringpiece.h>
@@ -53,6 +55,30 @@
 
 namespace moonfire_nvr {
 
+// Single-use object to represent a set of HTTP query parameters.
+class QueryParameters {
+ public:
+  // Parse parameters from the given URI.
+  // Caller should check ok() afterward.
+  QueryParameters(const char *uri) {
+    TAILQ_INIT(&me_);
+    ok_ = evhttp_parse_query_str(uri, &me_) == 0;
+  }
+  QueryParameters(const QueryParameters &) = delete;
+  void operator=(const QueryParameters &) = delete;
+
+  ~QueryParameters() { evhttp_clear_headers(&me_); }
+
+  bool ok() const { return ok_; }
+  const char *Get(const char *param) const {
+    return evhttp_find_header(&me_, param);
+  }
+
+ private:
+  struct evkeyvalq me_;
+  bool ok_ = false;
+};
+
 // Wrapped version of libevent's "struct evbuffer" which uses RAII and simply
 // aborts the process if allocations fail. (Moonfire NVR is intended to run on
 // Linux systems with the default vm.overcommit_memory=0, so there's probably

src/profiler.cc

@@ -45,7 +45,6 @@
 
 #include <event2/buffer.h>
 #include <event2/event.h>
-#include <event2/keyvalq_struct.h>
 #include <event2/http.h>
 #include <gperftools/profiler.h>
 #include <glog/logging.h>
@@ -112,12 +111,10 @@ void StartProfileCallback(struct evhttp_request *req, void *arg) {
                              "Profiling already in progress");
   }
   struct timeval timeout = {0, 0};
-  struct evkeyvalq query_options;
-  evhttp_parse_query(evhttp_request_get_uri(req), &query_options);
-  const char *seconds_value = evhttp_find_header(&query_options, "seconds");
+  QueryParameters params(evhttp_request_get_uri(req));
+  const char *seconds_value = params.Get("seconds");
   timeout.tv_sec =
       seconds_value == nullptr ? kDefaultProfileSeconds : atoi(seconds_value);
-  evhttp_clear_headers(&query_options);
   if (timeout.tv_sec <= 0) {
     return evhttp_send_error(req, HTTP_BADREQUEST, "invalid seconds");
   }