TASK-003: Add httpserver::feature_unavailable exception type

Introduce a new public header `src/httpserver/feature_unavailable.hpp`
defining `class feature_unavailable : public std::runtime_error`. The
constructor takes `(std::string_view feature, std::string_view
build_flag)` and composes a `what()` message that names both, e.g.
`"feature 'tls' unavailable: built without HAVE_GNUTLS"`.

The class is header-only and inline. It has no library dependencies
(only <stdexcept>, <string>, <string_view>), so any TU — including
later tasks like TASK-034 that need to throw it from sites in
build-time-disabled code paths — can include it without circular
header coupling. Keeping it inline also avoids ABI churn for what is
effectively a labelled std::runtime_error and keeps libhttpserver_la
sources untouched.

The header is re-exported from the umbrella `<httpserver.hpp>`
unconditionally (no `#ifdef HAVE_*` wrap): even a build with no
optional features must let consumers name `feature_unavailable` so
they can write `try { ... } catch (const httpserver::feature_unavailable&)`.

The TASK-002 inclusion gate is applied verbatim — direct inclusion of
the header without the umbrella or `HTTPSERVER_COMPILATION` errors
out, and `_HTTPSERVER_HPP_INSIDE_` does not leak post-umbrella (both
verified by the existing check-headers A.1–A.4 recipes).

A new unit test `test/unit/feature_unavailable_test.cpp` provides:
- a TU-scope `static_assert(std::is_base_of_v<std::runtime_error,
  httpserver::feature_unavailable>)` (acceptance criterion 1),
- a test that catches as `std::runtime_error` and asserts both the
  feature name and the build flag appear in `what()` (AC 2),
- a test that catches as the concrete type and confirms it slices to
  `runtime_error` correctly,
- a test with a different (feature, flag) pair to guard against
  hard-coded message text.

Verified locally:
- `make check`: 18/18 PASS (was 17, +1 for feature_unavailable),
- check-headers A.1–A.4 PASS,
- check-install-layout PASS (no details/ leak),
- staged install ships exactly one feature_unavailable.hpp at
  $(prefix)/include/httpserver/feature_unavailable.hpp,
- debug build (--enable-debug, -Werror -Wextra -pedantic) builds and
  tests cleanly.

Refs: PRD-FLG-REQ-004, PRD-FLG-REQ-005; §7 (feature availability).

Author: etr

src/Makefile.am

@@ -24,7 +24,7 @@ libhttpserver_la_SOURCES = string_utilities.cpp webserver.cpp http_utils.cpp fil
 # Detail headers (httpserver/details/*.hpp) live here so they cannot leak to
 # downstream consumers — the public surface comes in through <httpserver.hpp>.
 noinst_HEADERS = httpserver/string_utilities.hpp httpserver/details/modded_request.hpp httpserver/details/http_endpoint.hpp gettext.h
-nobase_include_HEADERS = httpserver.hpp httpserver/create_webserver.hpp httpserver/webserver.hpp httpserver/http_utils.hpp httpserver/file_info.hpp httpserver/http_request.hpp httpserver/http_response.hpp httpserver/http_resource.hpp httpserver/string_response.hpp httpserver/digest_auth_fail_response.hpp httpserver/deferred_response.hpp httpserver/file_response.hpp httpserver/pipe_response.hpp httpserver/empty_response.hpp httpserver/iovec_response.hpp httpserver/http_arg_value.hpp
+nobase_include_HEADERS = httpserver.hpp httpserver/create_webserver.hpp httpserver/webserver.hpp httpserver/http_utils.hpp httpserver/file_info.hpp httpserver/http_request.hpp httpserver/http_response.hpp httpserver/http_resource.hpp httpserver/string_response.hpp httpserver/digest_auth_fail_response.hpp httpserver/deferred_response.hpp httpserver/file_response.hpp httpserver/pipe_response.hpp httpserver/empty_response.hpp httpserver/feature_unavailable.hpp httpserver/iovec_response.hpp httpserver/http_arg_value.hpp
 
 if HAVE_BAUTH
 libhttpserver_la_SOURCES += basic_auth_fail_response.cpp

src/httpserver.hpp

@@ -35,6 +35,7 @@
 #include "httpserver/digest_auth_fail_response.hpp"
 #endif  // HAVE_DAUTH
 #include "httpserver/empty_response.hpp"
+#include "httpserver/feature_unavailable.hpp"
 #include "httpserver/file_response.hpp"
 #include "httpserver/http_arg_value.hpp"
 #include "httpserver/http_request.hpp"

src/httpserver/feature_unavailable.hpp

@@ -0,0 +1,62 @@
+/*
+     This file is part of libhttpserver
+     Copyright (C) 2011-2019 Sebastiano Merlino
+
+     This library is free software; you can redistribute it and/or
+     modify it under the terms of the GNU Lesser General Public
+     License as published by the Free Software Foundation; either
+     version 2.1 of the License, or (at your option) any later version.
+
+     This library is distributed in the hope that it will be useful,
+     but WITHOUT ANY WARRANTY; without even the implied warranty of
+     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+     Lesser General Public License for more details.
+
+     You should have received a copy of the GNU Lesser General Public
+     License along with this library; if not, write to the Free Software
+     Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301
+     USA
+*/
+
+#if !defined (_HTTPSERVER_HPP_INSIDE_) && !defined (HTTPSERVER_COMPILATION)
+#error "Only <httpserver.hpp> or <httpserverpp> can be included directly."
+#endif
+
+#ifndef SRC_HTTPSERVER_FEATURE_UNAVAILABLE_HPP_
+#define SRC_HTTPSERVER_FEATURE_UNAVAILABLE_HPP_
+
+#include <stdexcept>
+#include <string>
+#include <string_view>
+
+namespace httpserver {
+
+// Exception thrown when a build-time-disabled feature is invoked at runtime.
+// The class is unconditionally available regardless of HAVE_* flags so that
+// downstream code can always write
+//     try { ... } catch (const httpserver::feature_unavailable&) { ... }
+// even in builds that compiled out the optional feature in question.
+//
+// The class is header-only (and inline) on purpose: it has no library
+// dependencies, must be cheap to throw from anywhere in the codebase, and
+// avoids ABI churn for what is effectively a labelled std::runtime_error.
+class feature_unavailable : public std::runtime_error {
+ public:
+    feature_unavailable(std::string_view feature, std::string_view build_flag)
+        : std::runtime_error(compose_message(feature, build_flag)) {}
+
+ private:
+    static std::string compose_message(std::string_view feature,
+                                       std::string_view build_flag) {
+        std::string msg;
+        msg.reserve(feature.size() + build_flag.size() + 32);
+        msg.append("feature '");
+        msg.append(feature);
+        msg.append("' unavailable: built without ");
+        msg.append(build_flag);
+        return msg;
+    }
+};
+
+}  // namespace httpserver
+#endif  // SRC_HTTPSERVER_FEATURE_UNAVAILABLE_HPP_

test/Makefile.am

@@ -26,7 +26,7 @@ LDADD += -lcurl
 
 AM_CPPFLAGS = -I$(top_srcdir)/src -I$(top_srcdir)/src/httpserver/ -DHTTPSERVER_COMPILATION
 METASOURCES = AUTO
-check_PROGRAMS = basic file_upload http_utils threaded nodelay string_utilities http_endpoint ban_system ws_start_stop authentication deferred http_resource http_response create_webserver new_response_types daemon_info uri_log
+check_PROGRAMS = basic file_upload http_utils threaded nodelay string_utilities http_endpoint ban_system ws_start_stop authentication deferred http_resource http_response create_webserver new_response_types daemon_info uri_log feature_unavailable
 
 MOSTLYCLEANFILES = *.gcda *.gcno *.gcov
 
@@ -51,6 +51,7 @@ uri_log_SOURCES = unit/uri_log_test.cpp
 # it needs an explicit -lmicrohttpd in its link line on top of the default
 # LDADD (modern ld enforces --no-copy-dt-needed-entries).
 uri_log_LDADD = $(LDADD) -lmicrohttpd
+feature_unavailable_SOURCES = unit/feature_unavailable_test.cpp
 
 noinst_HEADERS = littletest.hpp
 AM_CXXFLAGS += -Wall -fPIC -Wno-overloaded-virtual

test/unit/feature_unavailable_test.cpp

@@ -0,0 +1,84 @@
+/*
+     This file is part of libhttpserver
+     Copyright (C) 2011-2019 Sebastiano Merlino
+
+     This library is free software; you can redistribute it and/or
+     modify it under the terms of the GNU Lesser General Public
+     License as published by the Free Software Foundation; either
+     version 2.1 of the License, or (at your option) any later version.
+
+     This library is distributed in the hope that it will be useful,
+     but WITHOUT ANY WARRANTY; without even the implied warranty of
+     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+     Lesser General Public License for more details.
+
+     You should have received a copy of the GNU Lesser General Public
+     License along with this library; if not, write to the Free Software
+     Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301
+     USA
+*/
+
+#include <stdexcept>
+#include <string>
+#include <type_traits>
+
+#include "./httpserver.hpp"
+#include "./littletest.hpp"
+
+// AC #1: feature_unavailable derives from std::runtime_error. This compile-time
+// assertion runs at TU scope and fires if the inheritance is ever broken.
+static_assert(
+    std::is_base_of_v<std::runtime_error, httpserver::feature_unavailable>,
+    "feature_unavailable must derive from std::runtime_error");
+
+LT_BEGIN_SUITE(feature_unavailable_suite)
+    void set_up() {
+    }
+
+    void tear_down() {
+    }
+LT_END_SUITE(feature_unavailable_suite)
+
+// AC #2: a unit test catches the exception as std::runtime_error and asserts
+// that what() contains both the feature name and the build flag.
+LT_BEGIN_AUTO_TEST(feature_unavailable_suite,
+                   catches_as_runtime_error_with_feature_and_flag)
+    std::string msg;
+    try {
+        throw httpserver::feature_unavailable("tls", "HAVE_GNUTLS");
+    } catch (const std::runtime_error& e) {
+        msg = e.what();
+    }
+    LT_CHECK(msg.find("tls") != std::string::npos);
+    LT_CHECK(msg.find("HAVE_GNUTLS") != std::string::npos);
+LT_END_AUTO_TEST(catches_as_runtime_error_with_feature_and_flag)
+
+// Catching the concrete type still produces a runtime_error-shaped what().
+LT_BEGIN_AUTO_TEST(feature_unavailable_suite, catches_as_feature_unavailable_directly)
+    std::string msg;
+    try {
+        throw httpserver::feature_unavailable("tls", "HAVE_GNUTLS");
+    } catch (const httpserver::feature_unavailable& e) {
+        const std::runtime_error* base = &e;
+        msg = base->what();
+    }
+    LT_CHECK(msg.find("tls") != std::string::npos);
+    LT_CHECK(msg.find("HAVE_GNUTLS") != std::string::npos);
+LT_END_AUTO_TEST(catches_as_feature_unavailable_directly)
+
+// Guard against a hard-coded message: a different (feature, flag) pair must
+// also propagate verbatim into what().
+LT_BEGIN_AUTO_TEST(feature_unavailable_suite, composes_message_for_websocket)
+    std::string msg;
+    try {
+        throw httpserver::feature_unavailable("websocket", "HAVE_WEBSOCKET");
+    } catch (const std::runtime_error& e) {
+        msg = e.what();
+    }
+    LT_CHECK(msg.find("websocket") != std::string::npos);
+    LT_CHECK(msg.find("HAVE_WEBSOCKET") != std::string::npos);
+LT_END_AUTO_TEST(composes_message_for_websocket)
+
+LT_BEGIN_AUTO_TEST_ENV()
+    AUTORUN_TESTS()
+LT_END_AUTO_TEST_ENV()