SCons: Add an option to enable the experimental ninja build backend

With this option turned on, if properly set up, SCons generates a
`build.ninja` file and quits. To actually build the engine, the user can
then call `ninja` with whatever options they might prefer (not
everything is yet transferred properly to this new generated file).

Ideally, the scons file should never be called again, as ninja
automatically detects any SCons build script change and invokes
the required commands to regenerate itself.

This approach speeds up incremental builds considerably, as it limits
SCons to code generation and uses ninja's extremely fast timestamp-based
file change detector.

3 files changed, 24 insertions, 4 deletions

diff --git a/.gitignore b/.gitignore
index d40f4d3948..46dcf84b43 100644
--- a/.gitignore
+++ b/.gitignore
@@ -35,6 +35,10 @@ bin
 compile_commands.json
 platform/windows/godot_res.res
 
+# Ninja build files
+build.ninja
+.ninja
+
 # Generated by Godot binary
 .import/
 /gdextension_interface.h
diff --git a/SConstruct b/SConstruct
index 753cea40e3..73ef420a0d 100644
--- a/SConstruct
+++ b/SConstruct
@@ -203,6 +203,7 @@ opts.Add(BoolVariable("custom_modules_recursive", "Detect custom modules recursi
 opts.Add(BoolVariable("dev_mode", "Alias for dev options: verbose=yes warnings=extra werror=yes tests=yes", False))
 opts.Add(BoolVariable("tests", "Build the unit tests", False))
 opts.Add(BoolVariable("fast_unsafe", "Enable unsafe options for faster rebuilds", False))
+opts.Add(BoolVariable("ninja", "Use the ninja backend for faster rebuilds", False))
 opts.Add(BoolVariable("compiledb", "Generate compilation DB (`compile_commands.json`) for external tools", False))
 opts.Add(BoolVariable("verbose", "Enable verbose output for the compilation", False))
 opts.Add(BoolVariable("progress", "Show a progress indicator during compilation", True))
@@ -956,7 +957,8 @@ if selected_platform in platform_list:
         env.vs_incs = []
         env.vs_srcs = []
 
-    # CompileDB
+    # CompileDB and Ninja are only available with certain SCons versions which
+    # not everybody might have yet, so we have to check.
     from SCons import __version__ as scons_raw_version
 
     scons_ver = env._get_major_minor_revision(scons_raw_version)
@@ -968,6 +970,20 @@ if selected_platform in platform_list:
         env.Tool("compilation_db")
         env.Alias("compiledb", env.CompilationDatabase())
 
+    if env["ninja"]:
+        if scons_ver < (4, 2, 0):
+            print("The `ninja=yes` option requires SCons 4.2 or later, but your version is %s." % scons_raw_version)
+            Exit(255)
+
+        SetOption("experimental", "ninja")
+
+        # By setting this we allow the user to run ninja by themselves with all
+        # the flags they need, as apparently automatically running from scons
+        # is way slower.
+        SetOption("disable_execute_ninja", True)
+
+        env.Tool("ninja")
+
     # Threads
     if env["threads"]:
         env.Append(CPPDEFINES=["THREADS_ENABLED"])
@@ -1041,9 +1057,10 @@ atexit.register(print_elapsed_time)
 
 
 def purge_flaky_files():
+    paths_to_keep = ["ninja.build"]
     for build_failure in GetBuildFailures():
-        path = build_failure.node.abspath
-        if os.path.isfile(path):
+        path = build_failure.node.path
+        if os.path.isfile(path) and path not in paths_to_keep:
             os.remove(path)
 
 
diff --git a/platform_methods.py b/platform_methods.py
index 92aefcc648..56115db4a4 100644
--- a/platform_methods.py
+++ b/platform_methods.py
@@ -66,7 +66,6 @@ def generate_export_icons(platform_path, platform_name):
 
             svg_str += '";\n'
 
-        # NOTE: It is safe to generate this file here, since this is still executed serially.
         wf = export_path + "/" + name + "_svg.gen.h"
 
         methods.write_file_if_needed(wf, svg_str)