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

include/asterisk/http_websocket.h

@@ -22,6 +22,8 @@
 #include "asterisk/http.h"
 #include "asterisk/optional_api.h"
 
+#include <errno.h>
+
 /*!
  * \file http_websocket.h
  * \brief Support for WebSocket connections within the Asterisk HTTP server.
@@ -71,7 +73,7 @@ typedef void (*ast_websocket_callback)(struct ast_websocket *session, struct ast
  * \retval \c NULL on error
  * \since 12
  */
-struct ast_websocket_server *ast_websocket_server_create(void);
+AST_OPTIONAL_API(struct ast_websocket_server *, ast_websocket_server_create, (void), { return NULL; });
 
 /*!
  * \brief Callback suitable for use with a \ref ast_http_uri.
@@ -79,7 +81,7 @@ struct ast_websocket_server *ast_websocket_server_create(void);
  * Set the data field of the ast_http_uri to \ref ast_websocket_server.
  * \since 12
  */
-int ast_websocket_uri_cb(struct ast_tcptls_session_instance *ser, const struct ast_http_uri *urih, const char *uri, enum ast_http_method method, struct ast_variable *get_vars, struct ast_variable *headers);
+AST_OPTIONAL_API(int, ast_websocket_uri_cb, (struct ast_tcptls_session_instance *ser, const struct ast_http_uri *urih, const char *uri, enum ast_http_method method, struct ast_variable *get_vars, struct ast_variable *headers), { return -1; });
 
 /*!
  * \brief Add a sub-protocol handler to the default /ws server
@@ -141,7 +143,7 @@ AST_OPTIONAL_API(int, ast_websocket_server_remove_protocol, (struct ast_websocke
  *
  * \note Once an AST_WEBSOCKET_OPCODE_CLOSE opcode is received the socket will be closed
  */
-AST_OPTIONAL_API(int, ast_websocket_read, (struct ast_websocket *session, char **payload, uint64_t *payload_len, enum ast_websocket_opcode *opcode, int *fragmented), {return -1;});
+AST_OPTIONAL_API(int, ast_websocket_read, (struct ast_websocket *session, char **payload, uint64_t *payload_len, enum ast_websocket_opcode *opcode, int *fragmented), { errno = ENOSYS; return -1;});
 
 /*!
  * \brief Construct and transmit a WebSocket frame
@@ -154,7 +156,7 @@ AST_OPTIONAL_API(int, ast_websocket_read, (struct ast_websocket *session, char *
  * \retval 0 if successfully written
  * \retval -1 if error occurred
  */
-AST_OPTIONAL_API(int, ast_websocket_write, (struct ast_websocket *session, enum ast_websocket_opcode opcode, char *payload, uint64_t actual_length), {return -1;});
+AST_OPTIONAL_API(int, ast_websocket_write, (struct ast_websocket *session, enum ast_websocket_opcode opcode, char *payload, uint64_t actual_length), { errno = ENOSYS; return -1;});
 
 /*!
  * \brief Close a WebSocket session by sending a message with the CLOSE opcode and an optional code
@@ -165,7 +167,7 @@ AST_OPTIONAL_API(int, ast_websocket_write, (struct ast_websocket *session, enum
  * \retval 0 if successfully written
  * \retval -1 if error occurred
  */
-AST_OPTIONAL_API(int, ast_websocket_close, (struct ast_websocket *session, uint16_t reason), {return -1;});
+AST_OPTIONAL_API(int, ast_websocket_close, (struct ast_websocket *session, uint16_t reason), { errno = ENOSYS; return -1;});
 
 /*!
  * \brief Enable multi-frame reconstruction up to a certain number of bytes
@@ -207,7 +209,7 @@ AST_OPTIONAL_API(void, ast_websocket_unref, (struct ast_websocket *session), {re
  *
  * \note You must *not* directly read from or write to this file descriptor. It should only be used for polling.
  */
-AST_OPTIONAL_API(int, ast_websocket_fd, (struct ast_websocket *session), {return -1;});
+AST_OPTIONAL_API(int, ast_websocket_fd, (struct ast_websocket *session), { errno = ENOSYS; return -1;});
 
 /*!
  * \brief Get the remote address for a WebSocket connected session.
@@ -222,7 +224,7 @@ AST_OPTIONAL_API(struct ast_sockaddr *, ast_websocket_remote_address, (struct as
  * \retval 0 if unsecure
  * \retval 1 if secure
  */
-AST_OPTIONAL_API(int, ast_websocket_is_secure, (struct ast_websocket *session), {return -1;});
+AST_OPTIONAL_API(int, ast_websocket_is_secure, (struct ast_websocket *session), { errno = ENOSYS; return -1;});
 
 /*!
  * \brief Set the socket of a WebSocket session to be non-blocking.
@@ -230,6 +232,6 @@ AST_OPTIONAL_API(int, ast_websocket_is_secure, (struct ast_websocket *session),
  * \retval 0 on success
  * \retval -1 on failure
  */
-AST_OPTIONAL_API(int, ast_websocket_set_nonblock, (struct ast_websocket *session), {return -1;});
+AST_OPTIONAL_API(int, ast_websocket_set_nonblock, (struct ast_websocket *session), { errno = ENOSYS; return -1;});
 
 #endif