mingw: add windows_build() for OS build number

Add unsigned windows_build() to read the Windows OS build number from
the registry and cache the result.

This post explains the methods available to read the build number, with
the registry being the most reasonable one:

https://stackoverflow.com/a/47583450/262458

. This post explains how to read the registry in C/C++:

https://stackoverflow.com/a/35717/262458

.

This is used in a subsequent commit to set $env:TERM correctly for newer
OSes.

Signed-off-by: Rafael Kitover <[email protected]>

Author: rkitover

compat/mingw.c

@@ -1,6 +1,7 @@
 #include "../git-compat-util.h"
 #include "win32.h"
 #include <conio.h>
+#include <string.h>
 #include <wchar.h>
 #include <winioctl.h>
 #include "../strbuf.h"
@@ -3337,6 +3338,45 @@ static int is_system32_path(const char *path)
 	return 1;
 }
 
+/*
+ * Extract the Windows OS build number from the registry, this is
+ * best method of those available here:
+ * https://stackoverflow.com/a/47583450/262458
+ *
+ * This post explains how to read the registry using WinAPI:
+ * https://stackoverflow.com/a/35717/262458
+ */
+unsigned windows_build(void)
+{
+	static unsigned is_cached    = 0;
+	static unsigned cached_value = 0;
+
+	if (is_cached)
+		return cached_value;
+
+	HKEY hkey;
+	ULONG res = RegOpenKeyExA(HKEY_LOCAL_MACHINE, "SOFTWARE\\Microsoft\\Windows NT\\CurrentVersion", 0, KEY_READ, &hkey);
+
+	if (res != ERROR_SUCCESS) goto err;
+
+	char buf[512];
+	int bufsize = sizeof(buf);
+
+	res = RegQueryValueExA(hkey, "CurrentBuildNumber", 0, NULL, (LPSTR)buf, &bufsize);
+
+	if (res != ERROR_SUCCESS) goto err;
+
+	cached_value = atoi(buf);
+	is_cached    = 1;
+
+	return cached_value;
+
+err:
+	cached_value = 0;
+	is_cached    = 1;
+	return 0;
+}
+
 static void setup_windows_environment(void)
 {
 	char *tmp = getenv("TMPDIR");

compat/mingw.h

@@ -714,3 +714,8 @@ int err_win_to_posix(DWORD winerr);
  * Check current process is inside Windows Container.
  */
 int is_inside_windows_container(void);
+
+/*
+ * Get Windows build number from the registry.
+ */
+unsigned windows_build(void);