Explicit exit from main (#15299)

In a MT environment such as proposed in crystal-lang/rfcs#2, the main thread's fiber may be resumed by any thread, and it may return which would terminate the program... but it might return from _another thread_ that the process' main thread, which may be unexpected by the OS.

This patch instead explicitly exits from `main` and `wmain`.

For backward compatibility reasons (win32 `wmain` and wasi `__main_argc_argv` both call `main` andand are documented to do so), the default `main` still returns, but is being replaced for UNIX targets by one that exits.

Maybe the OS actual entrypoint could merely call `Crystal.main` instead of `main` and explicitely exit (there wouldn't be a global `main` except for `UNIX`), but this is out of scope for this PR.

src/crystal/main.cr

@@ -132,8 +132,8 @@ end
 
 {% if flag?(:win32) %}
   require "./system/win32/wmain"
-{% end %}
-
-{% if flag?(:wasi) %}
+{% elsif flag?(:wasi) %}
   require "./system/wasi/main"
+{% else %}
+  require "./system/unix/main"
 {% end %}

src/crystal/system/unix/main.cr

@@ -0,0 +1,11 @@
+require "c/stdlib"
+
+# Prefer explicit exit over returning the status, so we are free to resume the
+# main thread's fiber on any thread, without occuring a weird behavior where
+# another thread returns from main when the caller might expect the main thread
+# to be the one returning.
+
+fun main(argc : Int32, argv : UInt8**) : Int32
+  status = Crystal.main(argc, argv)
+  LibC.exit(status)
+end

src/crystal/system/wasi/main.cr

@@ -27,7 +27,8 @@ fun _start
   LibWasi.proc_exit(status) if status != 0
 end
 
-# `__main_argc_argv` is called by wasi-libc's `__main_void` with the program arguments.
+# `__main_argc_argv` is called by wasi-libc's `__main_void` with the program
+# arguments.
 fun __main_argc_argv(argc : Int32, argv : UInt8**) : Int32
   main(argc, argv)
 end

src/crystal/system/win32/wmain.cr

@@ -12,17 +12,13 @@ require "c/stdlib"
 lib LibCrystalMain
 end
 
-# The actual entry point for Windows executables. This is necessary because
-# *argv* (and Win32's `GetCommandLineA`) mistranslate non-ASCII characters to
-# Windows-1252, so `PROGRAM_NAME` and `ARGV` would be garbled; to avoid that, we
-# use this Windows-exclusive entry point which contains the correctly encoded
-# UTF-16 *argv*, convert it to UTF-8, and then forward it to the original
-# `main`.
+# The actual entry point for Windows executables.
 #
-# The different main functions in `src/crystal/main.cr` need not be aware that
-# such an alternate entry point exists, nor that the original command line was
-# not UTF-8. Thus all other aspects of program initialization still occur there,
-# and uses of those main functions continue to work across platforms.
+# This is necessary because *argv* (and Win32's `GetCommandLineA`) mistranslate
+# non-ASCII characters to Windows-1252, so `PROGRAM_NAME` and `ARGV` would be
+# garbled; to avoid that, we use this Windows-exclusive entry point which
+# contains the correctly encoded UTF-16 *argv*, convert it to UTF-8, and then
+# forward it to the original `main`.
 #
 # NOTE: we cannot use anything from the standard library here, including the GC.
 fun wmain(argc : Int32, argv : UInt16**) : Int32
@@ -46,5 +42,9 @@ fun wmain(argc : Int32, argv : UInt16**) : Int32
   end
   LibC.free(utf8_argv)
 
-  status
+  # prefer explicit exit over returning the status, so we are free to resume the
+  # main thread's fiber on any thread, without occuring a weird behavior where
+  # another thread returns from main when the caller might expect the main
+  # thread to be the one returning.
+  LibC.exit(status)
 end