Add `custom_modules` build option to compile external user modules

This patch adds ability to include external, user-defined C++ modules
to be compiled as part of Godot via `custom_modules` build option
which can be passed to `scons`.

```
scons platform=x11 tools=yes custom_modules="../project/modules"
```

Features:

- detects all available modules under `custom_modules` directory the
same way as it does for built-in modules (not recursive);
- works with both relative and absolute paths on the filesystem;
- multiple search paths can be specified as a comma-separated list.

Module custom documentation and editor icons collection and generation
process is adapted to work with absolute paths needed by such modules.

Also fixed doctool bug mixing absolute and relative paths respectively.

Implementation details:

- `env.module_list` is a dictionary now, which holds both module name as
  key and either a relative or absolute path to a module as a value.
- `methods.detect_modules` is run twice: once for built-in modules, and
  second for external modules, all combined later.
- `methods.detect_modules` was not doing what it says on the tin. It is
  split into `detect_modules` which collects a list of available modules
  and `write_modules` which generates `register_types` sources for each.
- whether a module is built-in or external is distinguished by relative
  or absolute paths respectively. `custom_modules` scons converter
  ensures that the path is absolute even if relative path is supplied,
  including expanding user paths and symbolic links.
- treats the parent directory as if it was Godot's base directory, so
  that there's no need to change include paths in cases where custom
  modules are included as dependencies in other modules.

6 files changed, 124 insertions, 67 deletions

diff --git a/SConstruct b/SConstruct
index f74940b059..515cad57d0 100644
--- a/SConstruct
+++ b/SConstruct
@@ -50,8 +50,6 @@ for x in sorted(glob.glob("platform/*")):
     sys.path.remove(tmppath)
     sys.modules.pop("detect")
 
-module_list = methods.detect_modules()
-
 methods.save_active_platforms(active_platforms, active_platform_ids)
 
 custom_tools = ["default"]
@@ -123,6 +121,7 @@ opts.Add(BoolVariable("use_precise_math_checks", "Math checks use very precise e
 opts.Add(BoolVariable("deprecated", "Enable deprecated features", True))
 opts.Add(BoolVariable("minizip", "Enable ZIP archive support using minizip", True))
 opts.Add(BoolVariable("xaudio2", "Enable the XAudio2 audio driver", False))
+opts.Add("custom_modules", "A list of comma-separated directory paths containing custom modules to build.", "")
 
 # Advanced options
 opts.Add(BoolVariable("verbose", "Enable verbose output for the compilation", False))
@@ -181,18 +180,41 @@ for k in platform_opts.keys():
     for o in opt_list:
         opts.Add(o)
 
-for x in module_list:
-    module_enabled = True
-    tmppath = "./modules/" + x
-    sys.path.insert(0, tmppath)
+# Detect modules.
+modules_detected = {}
+module_search_paths = ["modules"]  # Built-in path.
+
+if ARGUMENTS.get("custom_modules"):
+    paths = ARGUMENTS.get("custom_modules").split(",")
+    for p in paths:
+        try:
+            module_search_paths.append(methods.convert_custom_modules_path(p))
+        except ValueError as e:
+            print(e)
+            sys.exit(255)
+
+for path in module_search_paths:
+    # Note: custom modules can override built-in ones.
+    modules_detected.update(methods.detect_modules(path))
+    include_path = os.path.dirname(path)
+    if include_path:
+        env_base.Prepend(CPPPATH=[include_path])
+
+# Add module options.
+for name, path in modules_detected.items():
+    enabled = True
+    sys.path.insert(0, path)
     import config
 
-    enabled_attr = getattr(config, "is_enabled", None)
-    if callable(enabled_attr) and not config.is_enabled():
-        module_enabled = False
-    sys.path.remove(tmppath)
+    try:
+        enabled = config.is_enabled()
+    except AttributeError:
+        pass
+    sys.path.remove(path)
     sys.modules.pop("config")
-    opts.Add(BoolVariable("module_" + x + "_enabled", "Enable module '%s'" % (x,), module_enabled))
+    opts.Add(BoolVariable("module_" + name + "_enabled", "Enable module '%s'" % (name,), enabled))
+
+methods.write_modules(modules_detected)
 
 opts.Update(env_base)  # update environment
 Help(opts.GenerateHelpText(env_base))  # generate help
@@ -501,41 +523,41 @@ if selected_platform in platform_list:
     sys.path.remove(tmppath)
     sys.modules.pop("detect")
 
-    env.module_list = []
+    modules_enabled = {}
     env.module_icons_paths = []
     env.doc_class_path = {}
 
-    for x in sorted(module_list):
-        if not env["module_" + x + "_enabled"]:
+    for name, path in sorted(modules_detected.items()):
+        if not env["module_" + name + "_enabled"]:
             continue
-        tmppath = "./modules/" + x
-        sys.path.insert(0, tmppath)
-        env.current_module = x
+        sys.path.insert(0, path)
+        env.current_module = name
         import config
 
         if config.can_build(env, selected_platform):
             config.configure(env)
-            env.module_list.append(x)
-
             # Get doc classes paths (if present)
             try:
                 doc_classes = config.get_doc_classes()
                 doc_path = config.get_doc_path()
                 for c in doc_classes:
-                    env.doc_class_path[c] = "modules/" + x + "/" + doc_path
+                    env.doc_class_path[c] = path + "/" + doc_path
             except:
                 pass
             # Get icon paths (if present)
             try:
                 icons_path = config.get_icons_path()
-                env.module_icons_paths.append("modules/" + x + "/" + icons_path)
+                env.module_icons_paths.append(path + "/" + icons_path)
             except:
                 # Default path for module icons
-                env.module_icons_paths.append("modules/" + x + "/" + "icons")
+                env.module_icons_paths.append(path + "/" + "icons")
+            modules_enabled[name] = path
 
-        sys.path.remove(tmppath)
+        sys.path.remove(path)
         sys.modules.pop("config")
 
+    env.module_list = modules_enabled
+
     methods.update_version(env.module_version_string)
 
     env["PROGSUFFIX"] = suffix + env.module_version_string + env["PROGSUFFIX"]
diff --git a/editor/SCsub b/editor/SCsub
index 13ae85bbf0..651dd5fffd 100644
--- a/editor/SCsub
+++ b/editor/SCsub
@@ -6,6 +6,7 @@ env.editor_sources = []
 
 import os
 import os.path
+import glob
 from platform_methods import run_in_subprocess
 import editor_builders
 
@@ -40,20 +41,21 @@ if env["tools"]:
         f.write(reg_exporters_inc)
         f.write(reg_exporters)
 
-    # API documentation
+    # Core API documentation.
     docs = []
-    doc_dirs = ["doc/classes"]
+    docs += Glob("#doc/classes/*.xml")
 
-    for p in env.doc_class_path.values():
-        if p not in doc_dirs:
-            doc_dirs.append(p)
+    # Module API documentation.
+    module_dirs = []
+    for d in env.doc_class_path.values():
+        if d not in module_dirs:
+            module_dirs.append(d)
 
-    for d in doc_dirs:
-        try:
-            for f in os.listdir(os.path.join(env.Dir("#").abspath, d)):
-                docs.append("#" + os.path.join(d, f))
-        except OSError:
-            pass
+    for d in module_dirs:
+        if not os.path.isabs(d):
+            docs += Glob("#" + d + "/*.xml")  # Built-in.
+        else:
+            docs += Glob(d + "/*.xml")  # Custom.
 
     _make_doc_data_class_path(os.path.join(env.Dir("#").abspath, "editor"))
 
@@ -61,8 +63,6 @@ if env["tools"]:
     env.Depends("#editor/doc_data_compressed.gen.h", docs)
     env.CommandNoCache("#editor/doc_data_compressed.gen.h", docs, run_in_subprocess(editor_builders.make_doc_header))
 
-    import glob
-
     path = env.Dir(".").abspath
 
     # Editor translations
diff --git a/editor/icons/SCsub b/editor/icons/SCsub
index f0d51999f0..e143276259 100644
--- a/editor/icons/SCsub
+++ b/editor/icons/SCsub
@@ -2,6 +2,8 @@
 
 Import("env")
 
+import os
+
 from platform_methods import run_in_subprocess
 import editor_icons_builders
 
@@ -15,7 +17,10 @@ env["BUILDERS"]["MakeEditorIconsBuilder"] = make_editor_icons_builder
 icon_sources = Glob("*.svg")
 
 # Module icons
-for module_icons in env.module_icons_paths:
-    icon_sources += Glob("#" + module_icons + "/*.svg")
+for path in env.module_icons_paths:
+    if not os.path.isabs(path):
+        icon_sources += Glob("#" + path + "/*.svg")  # Built-in.
+    else:
+        icon_sources += Glob(path + "/*.svg")  # Custom.
 
 env.Alias("editor_icons", [env.MakeEditorIconsBuilder("#editor/editor_icons.gen.h", icon_sources)])
diff --git a/main/main.cpp b/main/main.cpp
index f05032e68c..94dd895a26 100644
--- a/main/main.cpp
+++ b/main/main.cpp
@@ -1645,7 +1645,11 @@ bool Main::start() {
 		print_line("Loading docs...");
 
 		for (int i = 0; i < _doc_data_class_path_count; i++) {
-			String path = doc_tool.plus_file(_doc_data_class_paths[i].path);
+			// Custom modules are always located by absolute path.
+			String path = _doc_data_class_paths[i].path;
+			if (path.is_rel_path()) {
+				path = doc_tool.plus_file(path);
+			}
 			String name = _doc_data_class_paths[i].name;
 			doc_data_classes[name] = path;
 			if (!checked_paths.has(path)) {
diff --git a/methods.py b/methods.py
index 805ae256c3..46b58a13cd 100644
--- a/methods.py
+++ b/methods.py
@@ -137,37 +137,47 @@ def parse_cg_file(fname, uniforms, sizes, conditionals):
     fs.close()
 
 
-def detect_modules():
+def detect_modules(at_path):
+    module_list = {}  # name : path
 
-    module_list = []
+    modules_glob = os.path.join(at_path, "*")
+    files = glob.glob(modules_glob)
+    files.sort()  # so register_module_types does not change that often, and also plugins are registered in alphabetic order
+
+    for x in files:
+        if not is_module(x):
+            continue
+        name = os.path.basename(x)
+        path = x.replace("\\", "/")  # win32
+        module_list[name] = path
+
+    return module_list
+
+
+def is_module(path):
+    return os.path.isdir(path) and os.path.exists(path + "/config.py")
+
+
+def write_modules(module_list):
     includes_cpp = ""
+    preregister_cpp = ""
     register_cpp = ""
     unregister_cpp = ""
-    preregister_cpp = ""
 
-    files = glob.glob("modules/*")
-    files.sort()  # so register_module_types does not change that often, and also plugins are registered in alphabetic order
-    for x in files:
-        if not os.path.isdir(x):
-            continue
-        if not os.path.exists(x + "/config.py"):
-            continue
-        x = x.replace("modules/", "")  # rest of world
-        x = x.replace("modules\\", "")  # win32
-        module_list.append(x)
+    for name, path in module_list.items():
         try:
-            with open("modules/" + x + "/register_types.h"):
-                includes_cpp += '#include "modules/' + x + '/register_types.h"\n'
-                register_cpp += "#ifdef MODULE_" + x.upper() + "_ENABLED\n"
-                register_cpp += "\tregister_" + x + "_types();\n"
-                register_cpp += "#endif\n"
-                preregister_cpp += "#ifdef MODULE_" + x.upper() + "_ENABLED\n"
-                preregister_cpp += "#ifdef MODULE_" + x.upper() + "_HAS_PREREGISTER\n"
-                preregister_cpp += "\tpreregister_" + x + "_types();\n"
+            with open(os.path.join(path, "register_types.h")):
+                includes_cpp += '#include "' + path + '/register_types.h"\n'
+                preregister_cpp += "#ifdef MODULE_" + name.upper() + "_ENABLED\n"
+                preregister_cpp += "#ifdef MODULE_" + name.upper() + "_HAS_PREREGISTER\n"
+                preregister_cpp += "\tpreregister_" + name + "_types();\n"
                 preregister_cpp += "#endif\n"
                 preregister_cpp += "#endif\n"
-                unregister_cpp += "#ifdef MODULE_" + x.upper() + "_ENABLED\n"
-                unregister_cpp += "\tunregister_" + x + "_types();\n"
+                register_cpp += "#ifdef MODULE_" + name.upper() + "_ENABLED\n"
+                register_cpp += "\tregister_" + name + "_types();\n"
+                register_cpp += "#endif\n"
+                unregister_cpp += "#ifdef MODULE_" + name.upper() + "_ENABLED\n"
+                unregister_cpp += "\tunregister_" + name + "_types();\n"
                 unregister_cpp += "#endif\n"
         except IOError:
             pass
@@ -202,7 +212,18 @@ void unregister_module_types() {
     with open("modules/register_module_types.gen.cpp", "w") as f:
         f.write(modules_cpp)
 
-    return module_list
+
+def convert_custom_modules_path(path):
+    if not path:
+        return path
+    err_msg = "Build option 'custom_modules' must %s"
+    if not os.path.isdir(path):
+        raise ValueError(err_msg % "point to an existing directory.")
+    if os.path.realpath(path) == os.path.realpath("modules"):
+        raise ValueError(err_msg % "be a directory other than built-in `modules` directory.")
+    if is_module(path):
+        raise ValueError(err_msg % "point to a directory with modules, not a single module.")
+    return os.path.realpath(os.path.expanduser(path))
 
 
 def disable_module(self):
diff --git a/modules/SCsub b/modules/SCsub
index fb46c5f877..9155a53eaf 100644
--- a/modules/SCsub
+++ b/modules/SCsub
@@ -3,6 +3,7 @@
 Import("env")
 
 import modules_builders
+import os
 
 env_modules = env.Clone()
 
@@ -13,16 +14,20 @@ env.CommandNoCache("modules_enabled.gen.h", Value(env.module_list), modules_buil
 
 vs_sources = []
 # libmodule_<name>.a for each active module.
-for module in env.module_list:
+for name, path in env.module_list.items():
     env.modules_sources = []
-    SConscript(module + "/SCsub")
+
+    if not os.path.isabs(path):
+        SConscript(name + "/SCsub")  # Built-in.
+    else:
+        SConscript(path + "/SCsub")  # Custom.
 
     # Some modules are not linked automatically but can be enabled optionally
     # on iOS, so we handle those specially.
-    if env["platform"] == "iphone" and module in ["arkit", "camera"]:
+    if env["platform"] == "iphone" and name in ["arkit", "camera"]:
         continue
 
-    lib = env_modules.add_library("module_%s" % module, env.modules_sources)
+    lib = env_modules.add_library("module_%s" % name, env.modules_sources)
     env.Prepend(LIBS=[lib])
     if env["vsproj"]:
         vs_sources += env.modules_sources