Add full mingw-w64-git (i.e. regular MSYS2 ecosystem) support (#5971)

Every once in a while, there are bug reports in Git for Windows' bug
tracker that describe an issue running [inside MSYS2
proper](https://gitforwindows.org/install-inside-msys2-proper), totally
ignoring the big, honking warning on top of [the
page](https://gitforwindows.org/install-inside-msys2-proper) that spells
out clearly that this is an unsupported use case.

At the same time, we cannot easily deflect and say "just use MSYS2
directly" (and leave the "and stop pestering us" out). We cannot do that
because there is only an _MSYS_ `git` package in MSYS2 (i.e. a Git that
uses the quite slow POSIX emulation layer provided by the MSYS2
runtime), but no `mingw-w64-git` package (which would be equivalent in
speed to Git for Windows).

In https://github.com/msys2/MINGW-packages/pull/26470, I am preparing to
change that. As part of that PR, I noticed and fixed a couple of issues
_in `git-for-windows/git` that prevented full support for
`mingw-w64-git` in MSYS2, such as problems with CLANG64 and UCRT64.

While at it, I simplified the entire setup to trust MSYS2's
`MINGW_PREFIX` & related environment variables instead of hard-coding
values like the installation prefix and what `MSYSTEM` to fall back on
if it is unset.

config.mak.uname

@@ -438,14 +438,8 @@ ifeq ($(uname_S),Windows)
 	GIT_VERSION := $(GIT_VERSION).MSVC
 	pathsep = ;
 	# Assume that this is built in Git for Windows' SDK
-        ifeq (MINGW32,$(MSYSTEM))
-		prefix = /mingw32
-        else
-                ifeq (CLANGARM64,$(MSYSTEM))
-			prefix = /clangarm64
-                else
-			prefix = /mingw64
-                endif
+        ifneq (,$(MSYSTEM))
+		prefix = $(MINGW_PREFIX)
         endif
 	# Prepend MSVC 64-bit tool-chain to PATH.
 	#
@@ -499,7 +493,7 @@ ifeq ($(uname_S),Windows)
 	NATIVE_CRLF = YesPlease
 	DEFAULT_HELP_FORMAT = html
 	SKIP_DASHED_BUILT_INS = YabbaDabbaDoo
-ifeq (/mingw64,$(subst 32,64,$(subst clangarm,mingw,$(prefix))))
+ifneq (,$(MINGW_PREFIX))
 	# Move system config into top-level /etc/
 	ETC_GITCONFIG = ../etc/gitconfig
 	ETC_GITATTRIBUTES = ../etc/gitattributes
@@ -730,26 +724,23 @@ ifeq ($(uname_S),MINGW)
         ifneq (,$(findstring -O,$(filter-out -O0 -Og,$(CFLAGS))))
 		BASIC_LDFLAGS += -Wl,--dynamicbase
         endif
-        ifeq (MINGW32,$(MSYSTEM))
-		prefix = /mingw32
-		HOST_CPU = i686
-		BASIC_LDFLAGS += -Wl,--pic-executable,-e,_mainCRTStartup
+        ifneq (,$(MSYSTEM))
+                ifeq ($(MINGW_PREFIX),$(filter-out /%,$(MINGW_PREFIX)))
+			# Override if empty or does not start with a slash
+			MINGW_PREFIX := /$(shell echo '$(MSYSTEM)' | tr A-Z a-z)
+                endif
+		prefix = $(MINGW_PREFIX)
+		HOST_CPU = $(patsubst %-w64-mingw32,%,$(MINGW_CHOST))
+		BASIC_LDFLAGS += -Wl,--pic-executable
+		COMPAT_CFLAGS += -DDETECT_MSYS_TTY
+                ifeq (MINGW32,$(MSYSTEM))
+			BASIC_LDFLAGS += -Wl,--large-address-aware
+                endif
+		# Move system config into top-level /etc/
+		ETC_GITCONFIG = ../etc/gitconfig
+		ETC_GITATTRIBUTES = ../etc/gitattributes
         endif
-        ifeq (MINGW64,$(MSYSTEM))
-		prefix = /mingw64
-		HOST_CPU = x86_64
-		BASIC_LDFLAGS += -Wl,--pic-executable,-e,mainCRTStartup
-        else ifeq (CLANGARM64,$(MSYSTEM))
-		prefix = /clangarm64
-		HOST_CPU = aarch64
-		BASIC_LDFLAGS += -Wl,--pic-executable,-e,mainCRTStartup
-        else
-		COMPAT_CFLAGS += -D_USE_32BIT_TIME_T
-		BASIC_LDFLAGS += -Wl,--large-address-aware
-        endif
-	CC = gcc
-	COMPAT_CFLAGS += -D__USE_MINGW_ANSI_STDIO=0 -DDETECT_MSYS_TTY \
-		-fstack-protector-strong
+	COMPAT_CFLAGS += -D__USE_MINGW_ANSI_STDIO=0 -fstack-protector-strong
 	EXTLIBS += -lntdll
 	EXTRA_PROGRAMS += headless-git$X
 	INSTALL = /bin/install
@@ -759,11 +750,6 @@ ifeq ($(uname_S),MINGW)
 	USE_LIBPCRE = YesPlease
 	USE_MIMALLOC = YesPlease
 	NO_PYTHON =
-        ifeq (/mingw64,$(subst 32,64,$(subst clangarm,mingw,$(prefix))))
-		# Move system config into top-level /etc/
-		ETC_GITCONFIG = ../etc/gitconfig
-		ETC_GITATTRIBUTES = ../etc/gitattributes
-        endif
 endif
 ifeq ($(uname_S),QNX)
 	COMPAT_CFLAGS += -DSA_RESTART=0

contrib/buildsystems/CMakeLists.txt

@@ -281,7 +281,14 @@ if(CMAKE_SYSTEM_NAME STREQUAL "Windows")
 				_CONSOLE DETECT_MSYS_TTY STRIP_EXTENSION=".exe"  NO_SYMLINK_HEAD UNRELIABLE_FSTAT
 				NOGDI OBJECT_CREATION_MODE=1 __USE_MINGW_ANSI_STDIO=0
 				USE_NED_ALLOCATOR OVERRIDE_STRDUP MMAP_PREVENTS_DELETE USE_WIN32_MMAP
-				HAVE_WPGMPTR ENSURE_MSYSTEM_IS_SET HAVE_RTLGENRANDOM)
+				HAVE_WPGMPTR HAVE_RTLGENRANDOM)
+	if(CMAKE_GENERATOR_PLATFORM STREQUAL "x64")
+		add_compile_definitions(ENSURE_MSYSTEM_IS_SET="MINGW64" MINGW_PREFIX="mingw64")
+	elseif(CMAKE_GENERATOR_PLATFORM STREQUAL "arm64")
+		add_compile_definitions(ENSURE_MSYSTEM_IS_SET="CLANGARM64" MINGW_PREFIX="clangarm64")
+	elseif(CMAKE_GENERATOR_PLATFORM STREQUAL "x86")
+		add_compile_definitions(ENSURE_MSYSTEM_IS_SET="MINGW32" MINGW_PREFIX="mingw32")
+	endif()
 	list(APPEND compat_SOURCES
 		compat/mingw.c
 		compat/winansi.c

environment.c

@@ -91,16 +91,22 @@ int max_allowed_tree_depth =
 	 * the stack overflow can occur.
 	 */
 	512;
-#elif defined(GIT_WINDOWS_NATIVE) && defined(__clang__) && defined(__aarch64__)
+#elif defined(GIT_WINDOWS_NATIVE) && defined(__clang__)
 	/*
-	 * Similar to Visual C, it seems that on Windows/ARM64 the clang-based
-	 * builds have a smaller stack space available. When running out of
-	 * that stack space, a `STATUS_STACK_OVERFLOW` is produced. When the
+	 * Similar to Visual C, it seems that clang-based builds on Windows
+	 * have a smaller stack space available. When running out of that
+	 * stack space, a `STATUS_STACK_OVERFLOW` is produced. When the
 	 * Git command was run from an MSYS2 Bash, this unfortunately results
 	 * in an exit code 127. Let's prevent that by lowering the maximal
-	 * tree depth; This value seems to be low enough.
+	 * tree depth; Unfortunately, it seems that the exact limit differs
+	 * for aarch64 vs x86_64, and the difference is too large to simply
+	 * use a single limit.
 	 */
+#if defined(__aarch64__)
 	1280;
+#else
+	1152;
+#endif
 #else
 	2048;
 #endif

meson.build

@@ -1268,7 +1268,6 @@ elif host_machine.system() == 'windows'
 
   libgit_c_args += [
     '-DDETECT_MSYS_TTY',
-    '-DENSURE_MSYSTEM_IS_SET',
     '-DNATIVE_CRLF',
     '-DNOGDI',
     '-DNO_POSIX_GOODIES',
@@ -1278,6 +1277,18 @@ elif host_machine.system() == 'windows'
     '-D__USE_MINGW_ANSI_STDIO=0',
   ]
 
+  msystem = get_option('msystem')
+  if msystem != ''
+    mingw_prefix = get_option('mingw_prefix')
+    if mingw_prefix == ''
+      mingw_prefix = '/' + msystem.to_lower()
+    endif
+    libgit_c_args += [
+      '-DENSURE_MSYSTEM_IS_SET="' + msystem + '"',
+      '-DMINGW_PREFIX="' + mingw_prefix + '"'
+    ]
+  endif
+
   libgit_dependencies += compiler.find_library('ntdll')
   libgit_include_directories += 'compat/win32'
   if compiler.get_id() == 'msvc'

meson_options.txt

@@ -21,6 +21,10 @@ option('runtime_prefix', type: 'boolean', value: false,
   description: 'Resolve ancillary tooling and support files relative to the location of the runtime binary instead of hard-coding them into the binary.')
 option('sane_tool_path', type: 'array', value: [],
   description: 'An array of paths to pick up tools from in case the normal tools are broken or lacking.')
+option('msystem', type: 'string', value: '',
+  description: 'Fall-back on Windows when MSYSTEM is not set.')
+option('mingw_prefix', type: 'string', value: '',
+  description: 'Fall-back on Windows when MINGW_PREFIX is not set.')
 
 # Build information compiled into Git and other parts like documentation.
 option('build_date', type: 'string', value: '',