MXS-1220: Make the http.hh header public

As it contains utility functions for formatting time_t values to HTTP-date
values, there's no real need to make it an internal header.

include/maxscale/http.hh

@@ -0,0 +1,89 @@
+#pragma once
+/*
+ * Copyright (c) 2016 MariaDB Corporation Ab
+ *
+ * Use of this software is governed by the Business Source License included
+ * in the LICENSE.TXT file and at www.mariadb.com/bsl11.
+ *
+ * Change Date: 2019-07-01
+ *
+ * On the date above, in accordance with the Business Source License, use
+ * of this software will be governed by version 2 or later of the General
+ * Public License.
+ */
+
+#include <maxscale/cppdefs.hh>
+
+#include <string>
+#include <time.h>
+
+#include <maxscale/debug.h>
+
+/**
+ * @brief Return the current HTTP-date
+ *
+ * @return The RFC 1123 compliant date
+ */
+static inline std::string http_get_date()
+{
+    time_t now = time(NULL);
+    struct tm tm;
+    char buf[200]; // Enough to store all dates
+
+    gmtime_r(&now, &tm);
+    strftime(buf, sizeof(buf), "%a, %d %b %y %T GMT", &tm);
+
+    return std::string(buf);
+}
+
+/**
+ * @brief Convert a time_t value into a HTTP-date string
+ *
+ * @param t Time to convert
+ *
+ * @return The time converted to a HTTP-date string
+ */
+static inline std::string http_to_date(time_t t)
+{
+    struct tm tm;
+    char buf[200]; // Enough to store all dates
+
+    gmtime_r(&t, &tm);
+    strftime(buf, sizeof(buf), "%a, %d %b %Y %T GMT", &tm);
+
+    return std::string(buf);
+}
+
+/**
+ * @brief Convert a HTTP-date string into time_t
+ *
+ * @param str HTTP-date formatted string to convert
+ *
+ * @return The time converted to time_t
+ */
+static inline time_t http_from_date(const std::string& str)
+{
+    struct tm tm = {};
+
+    /** First get the GMT time in time_t format */
+    strptime(str.c_str(), "%a, %d %b %Y %T GMT", &tm);
+    time_t t = mktime(&tm);
+
+    /** Then convert it to local time by calculating the difference between
+     * the local time and the GMT time */
+    struct tm local_tm = {};
+    struct tm gmt_tm = {};
+    time_t epoch = 0;
+
+    /** Call tzset() for the sake of portability */
+    tzset();
+    gmtime_r(&epoch, &gmt_tm);
+    localtime_r(&epoch, &local_tm);
+
+    time_t gmt_t = mktime(&gmt_tm);
+    time_t local_t = mktime(&local_tm);
+
+    /** The value of `(gmt_t - local_t)` will be the number of seconds west
+     * from GMT. For timezones east of GMT, it will be negative. */
+    return t - (gmt_t - local_t);
+}