optional_api: Fix linking problems between modules that export global symbols

With the new work in Asterisk 12, there are some uses of the
optional_api that are prone to failure. The details are rather involved,
and captured on [the wiki][1].

This patch addresses the issue by removing almost all of the magic from
the optional API implementation. Instead of relying on weak symbol
resolution, a new optional_api.c module was added to Asterisk core.

For modules providing an optional API, the pointer to the implementation
function is registered with the core. For modules that use an optional
API, a pointer to a stub function, along with a optional_ref function
pointer are registered with the core. The optional_ref function pointers
is set to the implementation function when it's provided, or the stub
function when it's now.

Since the implementation no longer relies on magic, it is now supported
on all platforms. In the spirit of choice, an OPTIONAL_API flag was
added, so we can disable the optional_api if needed (maybe it's buggy on
some bizarre platform I haven't tested on)

The AST_OPTIONAL_API*() macros themselves remained unchanged, so
existing code could remain unchanged. But to help with debugging the
optional_api, the patch limits the #include of optional API's to just
the modules using the API. This also reduces resource waste maintaining
optional_ref pointers that aren't used.

Other changes made as a part of this patch:
 * The stubs for http_websocket that wrap system calls set errno to
   ENOSYS.

 * res_http_websocket now properly increments module use count.

 * In loader.c, the while() wrappers around dlclose() were removed. The
   while(!dlclose()) is actually an anti-pattern, which can lead to
   infinite loops if the module you're attempting to unload exports a
   symbol that was directly linked to.

 * The special handling of nonoptreq on systems without weak symbol
   support was removed, since we no longer rely on weak symbols for
   optional_api.

 [1]: https://wiki.asterisk.org/wiki/x/wACUAQ

(closes issue ASTERISK-22296)
Reported by: Matt Jordan
Review: https://reviewboard.asterisk.org/r/2797/
........

Merged revisions 397989 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@397990 65c4cc65-6c06-0410-ace0-fbb531ad65f3

1 files changed, 365 insertions, 0 deletions

diff --git a/main/optional_api.c b/main/optional_api.c
new file mode 100644
index 000000000..f48fe11a1
--- /dev/null
+++ b/main/optional_api.c
@@ -0,0 +1,365 @@
+/*
+ * Asterisk -- An open source telephony toolkit.
+ *
+ * Copyright (C) 2013, Digium, Inc.
+ *
+ * David M. Lee, II <dlee@digium.com>
+ *
+ * See http://www.asterisk.org for more information about
+ * the Asterisk project. Please do not directly contact
+ * any of the maintainers of this project for assistance;
+ * the project provides a web site, mailing lists and IRC
+ * channels for your use.
+ *
+ * This program is free software, distributed under the terms of
+ * the GNU General Public License Version 2. See the LICENSE file
+ * at the top of the source tree.
+ */
+
+#include "asterisk.h"
+
+ASTERISK_FILE_VERSION(__FILE__, "$Revision$")
+
+#include "asterisk/optional_api.h"
+#include "asterisk/utils.h"
+
+#if defined(OPTIONAL_API)
+
+/*
+ * \file Optional API innards.
+ *
+ * The calls to ast_optional_api_*() happen implicitly from \c __constructor__
+ * calls which are defined in header files. This means that some number of them
+ * happen before main() is called. This makes calling most Asterisk APIs
+ * dangerous, since we could be called before they are initialized. This
+ * includes things like AO2, malloc debug, and static mutexes.
+ *
+ * Another limitation is that most functions are called from the midst of
+ * dlopen() or dlclose(), and there is no opportunity to return a failure code.
+ * The best we can do is log an error, and call ast_do_crash().
+ *
+ * Fortunately, there are some constraints that help us out. The \c
+ * ast_optional_api_*() are called during module loads, which happens either
+ * before main(), or during dlopen() calls. These are already serialized, so we
+ * don't have to lock ourselves.
+ */
+
+/*! \brief A user of an optional API */
+struct optional_api_user {
+	/*! Pointer to function pointer to link */
+	ast_optional_fn *optional_ref;
+	/*! Stub to use when impl is unavailable */
+	ast_optional_fn stub;
+	/*! Name of the module using the API */
+	char module[];
+};
+
+/*! \brief An optional API */
+struct optional_api {
+	/*! Pointer to the implementation function; could be null */
+	ast_optional_fn impl;
+	/*! Variable length array of users of this API */
+	struct optional_api_user **users;
+	/*! Allocated size of the \a users array */
+	size_t users_maxlen;
+	/*! Number of entries in the \a users array */
+	size_t users_len;
+	/*! Name of the optional API function */
+	char symname[];
+};
+
+/*!
+ * \brief Free an \ref optional_api_user.
+ *
+ * \param user User struct to free.
+ */
+static void optional_api_user_destroy(struct optional_api_user *user)
+{
+	*user->optional_ref = user->stub;
+	free(user);
+}
+
+/*!
+ * \brief Create an \ref optional_api_user.
+ *
+ * \param optional_ref Pointer-to-function-pointer to link to impl/stub.
+ * \param stub Stub function to link to when impl is not available.
+ * \param module Name of the module requesting the API.
+ *
+ * \return New \ref optional_api_user.
+ * \return \c NULL on error.
+ */
+static struct optional_api_user *optional_api_user_create(
+	ast_optional_fn *optional_ref, ast_optional_fn stub, const char *module)
+{
+	struct optional_api_user *user;
+	size_t size = sizeof(*user) + strlen(module) + 1;
+
+	user = calloc(1, size);
+	if (!user) {
+		return NULL;
+	}
+
+	user->optional_ref = optional_ref;
+	user->stub = stub;
+	strcpy(user->module, module); /* SAFE */
+
+	return user;
+}
+
+/*!
+ * \brief Free an \ref optional_api.
+ *
+ * \param api API struct to free.
+ */
+static void optional_api_destroy(struct optional_api *api)
+{
+	while (api->users_len--) {
+		optional_api_user_destroy(api->users[api->users_len]);
+	}
+	free(api->users);
+	api->users = NULL;
+	api->users_maxlen = 0;
+	free(api);
+}
+
+/*!
+ * \brief Create an \ref optional_api.
+ *
+ * \param symname Name of the optional function.
+ * \return New \ref optional_api.
+ * \return \c NULL on error.
+ */
+static struct optional_api *optional_api_create(const char *symname)
+{
+	struct optional_api *api;
+	size_t size;
+
+	ast_verb(6, "%s: building api object\n", symname);
+	size = sizeof(*api) + strlen(symname) + 1;
+	api = calloc(1, size);
+	if (!api) {
+		ast_log(LOG_ERROR, "Failed to allocate api\n");
+		return NULL;
+	}
+
+	strcpy(api->symname, symname); /* SAFE */
+
+	return api;
+}
+
+/*! Array of \ref optional_api functions */
+struct {
+	/*! Variable length array of API's */
+	struct optional_api **list;
+	/*! Allocated size of the \a list array */
+	size_t maxlen;
+	/*! Number of entries in the \a list array */
+	size_t len;
+} apis;
+
+void optional_api_cleanup(void)
+{
+	while (apis.len--) {
+		optional_api_destroy(apis.list[apis.len]);
+	}
+	free(apis.list);
+	apis.list = NULL;
+	apis.maxlen = 0;
+}
+
+/*!
+ * \brief Gets (or creates) the \ref optional_api for the give function.
+ *
+ * \param sysname Name of the function to look up.
+ * \return Corresponding \ref optional_api.
+ * \return \c NULL on error.
+ */
+static struct optional_api *get_api(const char *symname)
+{
+	struct optional_api *api;
+	size_t i;
+
+	/* Find one, if we already have it */
+	for (i = 0; i < apis.len; ++i) {
+		if (strcmp(symname, apis.list[i]->symname) == 0) {
+			return apis.list[i];
+		}
+	}
+
+	/* API not found. Build one */
+	api = optional_api_create(symname);
+
+	/* Grow the list, if needed */
+	if (apis.len + 1 > apis.maxlen) {
+		size_t new_maxlen = apis.maxlen ? 2 * apis.maxlen : 1;
+		struct optional_api **new_list =
+			realloc(apis.list, new_maxlen * sizeof(*new_list));
+
+		if (!new_list) {
+			optional_api_destroy(api);
+			ast_log(LOG_ERROR, "Failed to allocate api list\n");
+			return NULL;
+		}
+
+		apis.maxlen = new_maxlen;
+		apis.list = new_list;
+	}
+
+	apis.list[apis.len++] = api;
+
+	return api;
+}
+
+/*!
+ * \brief Re-links a given \a user against its associated \a api.
+ *
+ * If the \a api has an implementation, the \a user is linked to that
+ * implementation. Otherwise, the \a user is linked to its \a stub.
+ *
+ * \param user \ref optional_api_user to link.
+ * \param api \ref optional_api to link.
+ */
+static void optional_api_user_relink(struct optional_api_user *user,
+	struct optional_api *api)
+{
+	if (api->impl && *user->optional_ref != api->impl) {
+		ast_verb(4, "%s: linking for %s\n", api->symname, user->module);
+		*user->optional_ref = api->impl;
+	} else if (!api->impl && *user->optional_ref != user->stub) {
+		ast_verb(4, "%s: stubbing for %s\n", api->symname,
+			user->module);
+		*user->optional_ref = user->stub;
+	}
+}
+
+/*!
+ * \brief Sets the implementation function pointer for an \a api.
+ *
+ * \param api API to implement/stub out.
+ * \param impl Pointer to implementation function. Can be 0 to remove
+ *             implementation.
+ */
+static void optional_api_set_impl(struct optional_api *api,
+	ast_optional_fn impl)
+{
+	size_t i;
+
+	api->impl = impl;
+
+	/* re-link all users */
+	for (i = 0; i < api->users_len; ++i) {
+		optional_api_user_relink(api->users[i], api);
+	}
+}
+
+void ast_optional_api_provide(const char *symname, ast_optional_fn impl)
+{
+	struct optional_api *api;
+
+	ast_verb(4, "%s: providing\n", symname);
+
+	api = get_api(symname);
+	if (!api) {
+		ast_log(LOG_ERROR, "%s: Allocation failed\n", symname);
+		ast_do_crash();
+		return;
+	}
+
+	optional_api_set_impl(api, impl);
+}
+
+void ast_optional_api_unprovide(const char *symname, ast_optional_fn impl)
+{
+	struct optional_api *api;
+
+	ast_verb(4, "%s: un-providing\n", symname);
+
+	api = get_api(symname);
+	if (!api) {
+		ast_log(LOG_ERROR, "%s: Could not find api\n", symname);
+		ast_do_crash();
+		return;
+	}
+
+	optional_api_set_impl(api, 0);
+}
+
+void ast_optional_api_use(const char *symname, ast_optional_fn *optional_ref,
+	ast_optional_fn stub, const char *module)
+{
+	struct optional_api_user *user;
+	struct optional_api *api;
+
+
+	api = get_api(symname);
+	if (!api) {
+		ast_log(LOG_ERROR, "%s: Allocation failed\n", symname);
+		ast_do_crash();
+		return;
+	}
+
+	user = optional_api_user_create(optional_ref, stub, module);
+	if (!user) {
+		ast_log(LOG_ERROR, "%s: Allocation failed\n", symname);
+		ast_do_crash();
+		return;
+	}
+
+	/* Add user to the API */
+	if (api->users_len + 1 > api->users_maxlen) {
+		size_t new_maxlen = api->users_maxlen ?
+			2 * api->users_maxlen : 1;
+		struct optional_api_user **new_list =
+			realloc(api->users, new_maxlen * sizeof(*new_list));
+
+		if (!new_list) {
+			optional_api_user_destroy(user);
+			ast_log(LOG_ERROR, "Failed to allocate api list\n");
+			ast_do_crash();
+			return;
+		}
+
+		api->users_maxlen = new_maxlen;
+		api->users = new_list;
+	}
+
+	api->users[api->users_len++] = user;
+
+	optional_api_user_relink(user, api);
+}
+
+void ast_optional_api_unuse(const char *symname, ast_optional_fn *optional_ref,
+	const char *module)
+{
+	struct optional_api *api;
+	size_t i;
+
+	api = get_api(symname);
+	if (!api) {
+		ast_log(LOG_ERROR, "%s: Could not find api\n", symname);
+		ast_do_crash();
+		return;
+	}
+
+	for (i = 0; i < api->users_len; ++i) {
+		struct optional_api_user *user = api->users[i];
+		if (user->optional_ref == optional_ref) {
+			if (*user->optional_ref != user->stub) {
+				ast_verb(4, "%s: stubbing for %s\n", symname,
+					module);
+				*user->optional_ref = user->stub;
+			}
+
+			/* Remove from the list */
+			api->users[i] = api->users[--api->users_len];
+
+			optional_api_user_destroy(user);
+			return;
+		}
+	}
+
+	ast_log(LOG_ERROR, "%s: Could not find user %s\n", symname, module);
+}
+
+#endif /* defined(OPTIONAL_API) */