module: Re-implement using library constructors/destructors (#2018)

By using constructors/destructors we unify the case of static linking with
that of dynamic linking, and we reuse the build flag FEATURE_INIT_FINI. Using
constructors/destructors also allows us to cover the case where library
constructor/destructor functionality is unavailable, because we can expose the
module registration/unregistration functions as global symbols, to be called
explicitly from within the application.

Fixes https://github.com/jerryscript-project/jerryscript/issues/1952

JerryScript-DCO-1.0-Signed-off-by: Gabriel Schulhof gabriel.schulhof@intel.com

docs/12.EXT-REFERENCE-MODULE.md

@@ -13,14 +13,10 @@ type of module is requested.
 
 Additionally, this extension provides a means of easily defining so-called "native" JerryScript modules which can be
 resolved using the JerryScript native module resolver `jerryx_module_native_resolver()`, which can be passed to
-`jerryx_module_resolve()`. Note, however, that native JerryScript modules are only supported and
-`jerryx_module_native_resolver()` is only compiled in if compiler support for `__attribute__` extensions is present. In
-effect this means that native JerryScript modules are available only when this extension is built with GCC or
-LLVM/clang. In the absence of such support, you may construct alternative module systems and provide your own resolver
-to `jerryx_module_resolve()`.
-
-`jerryscript-ext/module.h` defines the preprocessor directive `JERRYX_NATIVE_MODULES_SUPPORTED` only if support for
-native JerryScript modules is available.
+`jerryx_module_resolve()`. Native modules are registered during application startup and by calling `dlopen()` by means
+of library constructors, support for which can be turned on using the `FEATURE_INIT_FINI` build flag. In the absence of
+such a flag, the module registration and unregistration functions are exposed as global symbols which can be called
+explicitly.
 
 ## jerryx_module_resolve
 
@@ -52,9 +48,8 @@ jerryx_module_resolve (const jerry_char_t *name,
 
 **Summary**
 
-The resolver for JerryScript modules. A pointer to this function can be passed in the second parameter to
-`jerryx_module_resolve` to search for the module among the JerryScript modules built into the binary. This function is
-available only if the preprocessor directive `JERRYX_NATIVE_MODULES_SUPPORTED` is defined.
+The resolver for native JerryScript modules. A pointer to this function can be passed in the second parameter to
+`jerryx_module_resolve` to search for the module among the native JerryScript modules loaded so far.
 
 **Prototype**
 
@@ -74,8 +69,7 @@ jerryx_module_native_resolver (const jerry_char_t *name,
 
 **Summary**
 
-Function pointer type for a function that will create an instance of a native module. This type is only defined if the
-preprocessor directive `JERRYX_NATIVE_MODULES_SUPPORTED` is defined.
+Function pointer type for a function that will create an instance of a native module.
 
 **Prototype**
 
@@ -136,12 +130,15 @@ We can now load JavaScript files:
 ```c
 static const jerryx_module_resolver_t resolvers[] =
 {
-  /* Consult the JerryScript module resolver first, in case the requested module is a compiled-in JerryScript module. */
+  /*
+   * Consult the JerryScript native module resolver first, in case the requested module is a native JerryScript
+   * module.
+   */
   jerryx_module_native_resolver,
 
   /*
-   * If the requested module is not a JerryScript module, assume it is a JavaScript file on disk and use the above-
-   * defined JavaScript file loader to load it.
+   * If the requested module is not a native JerryScript module, assume it is a JavaScript file on disk and use the
+   * above-defined JavaScript file loader to load it.
    */
   load_and_evaluate_js_file
 };
@@ -154,9 +151,11 @@ jerry_value_t js_module = jerryx_module_resolve (requested_module, resolvers, 2)
 
 **Summary**
 
-Helper macro to define a JerryScript module. Currently stores the name of the module and its initializer in an
-executable linker section. This macro is available only if the preprocessor directive `JERRYX_NATIVE_MODULES_SUPPORTED`
-is defined.
+Helper macro to define a native JerryScript module. Currently declares a global static structure of type
+`jerryx_native_module_t` and a constructor/destructor pair that calls `jerryx_native_module_register()` resp.
+`jerryx_native_module_unregister()`. If the extension is built without the FEATURE_INIT_FINI flag, indicating that
+support for library constructors and destructors is absent, the constructor and destructor are declared as global
+symbols so that they may be called explicitly from within the application.
 
 **Note**: The helper macro must appear at the bottom of a source file, and no semicolon must follow it.
 
@@ -165,7 +164,8 @@ is defined.
 #define JERRYX_NATIVE_MODULE(module_name, on_resolve_cb)
 ```
 
-- `module_name` - the name of the module without quotes
+- `module_name` - the name of the module without quotes. This value is used as the prefix for the registration and unregistration funtions. For example, when `module_name` is `example_module`, this results in the declaration of two functions `example_module_register()` and `example_module_unregister()`. These functions are declared global if support for library constructors/destructors is absent, allowing you to call them from other parts of the code by
+first forward-declaring them.
 - `on_resolve_cb` - the function of type `jerryx_native_module_on_resolve_t` that will be called when the module needs to be
 loaded.
 
@@ -184,3 +184,38 @@ my_module_on_resolve (void)
 /* Note that there is no semicolon at the end of the next line. This is how it must be. */
 JERRYX_NATIVE_MODULE (my_module, my_module_on_resolve)
 ```
+
+**Example Usage When Library Constructors Are Unavailable**
+
+```c
+#include "jerryscript.h"
+#include "jerryscript-ext/module.h"
+
+/**
+ * Forward-declare the module registration and unregistration function.
+ */
+extern void my_module_register (void);
+extern void my_module_unregister (void);
+int
+main (int argc, char **argv)
+{
+  jerryx_module_resolver_t resolvers[] =
+  {
+    jerryx_native_module_resolver
+  };
+
+  /* This plays the role of the library constructor. */
+  my_module_register ();
+
+  jerry_init (JERRY_INIT_EMPTY);
+  ...
+  jerry_value_t my_module = jerryx_module_resolve ("my_module", resolvers, 1);
+  ...
+  jerry_cleanup ();
+
+  /* This plays the role of the library destructor */
+  my_module_unregister();
+
+  return 0;
+}
+```