Windows: add support for a Windows-wide configuration

Between the libgit2 and the Git for Windows project, there has been a
discussion how we could share Git configuration to avoid duplication (or
worse: skew).

Earlier, libgit2 was nice enough to just re-use Git for Windows'

	C:\Program Files (x86)\Git\etc\gitconfig

but with the big changes that went into Git for Windows 2.x, there would
have been more paths to search, as there now was a 64-bit version in
addition to a 32-bit one, and the corresponding config files are
`%PROGRAMFILES%\Git\mingw64\etc` and `...\mingw32\etc`, respectively.

Worse: there are portable Git for Windows versions out there which live
in totally unrelated directories, still.

Therefore we came to a consensus to use `%PROGRAMDATA%\Git\config` as the
location for shared Git settings that are of wider interest than just Git
for Windows, read: for other Git implementations.

Of course, the configuration in `%PROGRAMDATA%\Git\config` has the
widest reach, therefore it must take the lowest precedence, i.e. Git for
Windows can still override settings in its `etc/gitconfig` file.

Git for Windows supports this since v2.5.0 (i.e. since the first v2.x),
and libgit2 supports it as of v0.28.1:
libgit2/libgit2@8c7c5fa585

Helped-by: Andreas Heiduk <asheiduk@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

Documentation/config.txt

@@ -7,7 +7,9 @@ the Git commands' behavior. The files `.git/config` and optionally
 repository are used to store the configuration for that repository, and
 `$HOME/.gitconfig` is used to store a per-user configuration as
 fallback values for the `.git/config` file. The file `/etc/gitconfig`
-can be used to store a system-wide default configuration.
+can be used to store a system-wide default configuration. On Windows,
+configuration can also be stored in `C:\ProgramData\Git\config`; This
+file will be used also by libgit2-based software.
 
 The configuration variables are used by both the Git plumbing
 and the porcelains. The variables are divided into sections, wherein

Documentation/git-config.txt

@@ -272,8 +272,16 @@ FILES
 If not set explicitly with `--file`, there are four files where
 'git config' will search for configuration options:
 
+$PROGRAMDATA/Git/config::
+	(Windows-only) System-wide configuration file shared with other Git
+	implementations. Typically `$PROGRAMDATA` points to `C:\ProgramData`.
+
 $(prefix)/etc/gitconfig::
 	System-wide configuration file.
+	(Windows-only) This file contains only the settings which are
+	specific for this installation of Git for Windows and which should
+	not be shared with other Git implementations like JGit, libgit2.
+	`--system` will select this file.
 
 $XDG_CONFIG_HOME/git/config::
 	Second user-specific configuration file. If $XDG_CONFIG_HOME is not set

Documentation/git.txt

@@ -566,7 +566,8 @@ for further details.
 
 `GIT_CONFIG_NOSYSTEM`::
 	Whether to skip reading settings from the system-wide
-	`$(prefix)/etc/gitconfig` file.  This environment variable can
+	`$(prefix)/etc/gitconfig` file (and on Windows, also from the
+	`%PROGRAMDATA%\Git\config` file).  This environment variable can
 	be used along with `$HOME` and `$XDG_CONFIG_HOME` to create a
 	predictable environment for a picky script, or you can set it
 	temporarily to avoid using a buggy `/etc/gitconfig` file while

compat/mingw.c

@@ -2,6 +2,8 @@
 #include "win32.h"
 #include <conio.h>
 #include <wchar.h>
+#include <aclapi.h>
+#include <sddl.h>
 #include "../strbuf.h"
 #include "../run-command.h"
 #include "../cache.h"
@@ -2450,3 +2452,120 @@ int uname(struct utsname *buf)
 		  "%u", (v >> 16) & 0x7fff);
 	return 0;
 }
+
+/*
+ * Verify that the file in question is owned by an administrator or system
+ * account, or at least by the current user.
+ *
+ * This function returns 1 if successful, 0 if the file is not owned by any of
+ * these, or -1 on error.
+ */
+static int validate_system_file_ownership(const char *path)
+{
+	WCHAR wpath[MAX_PATH];
+	PSID owner_sid = NULL;
+	PSECURITY_DESCRIPTOR descriptor = NULL;
+	HANDLE token;
+	TOKEN_USER* info = NULL;
+	DWORD err, len;
+	int ret;
+
+	if (xutftowcs_path(wpath, path) < 0)
+		return -1;
+
+	err = GetNamedSecurityInfoW(wpath, SE_FILE_OBJECT,
+				    OWNER_SECURITY_INFORMATION |
+				    DACL_SECURITY_INFORMATION,
+				    &owner_sid, NULL, NULL, NULL, &descriptor);
+
+	/* if the file does not exist, it does not have a valid owner */
+	if (err == ERROR_FILE_NOT_FOUND || err == ERROR_PATH_NOT_FOUND) {
+		ret = 0;
+		owner_sid = NULL;
+		goto finish_validation;
+	}
+
+	if (err != ERROR_SUCCESS) {
+		ret = error(_("failed to validate '%s' (%ld)"), path, err);
+		owner_sid = NULL;
+		goto finish_validation;
+	}
+
+	if (!IsValidSid(owner_sid)) {
+		ret = error(_("invalid owner: '%s'"), path);
+		goto finish_validation;
+	}
+
+	if (IsWellKnownSid(owner_sid, WinBuiltinAdministratorsSid) ||
+	    IsWellKnownSid(owner_sid, WinLocalSystemSid)) {
+		ret = 1;
+		goto finish_validation;
+	}
+
+	/* Obtain current user's SID */
+	if (OpenProcessToken(GetCurrentProcess(), TOKEN_QUERY, &token) &&
+	    !GetTokenInformation(token, TokenUser, NULL, 0, &len)) {
+		info = xmalloc((size_t)len);
+		if (!GetTokenInformation(token, TokenUser, info, len, &len))
+			FREE_AND_NULL(info);
+	}
+
+	if (!info)
+		ret = 0;
+	else {
+		ret = EqualSid(owner_sid, info->User.Sid) ? 1 : 0;
+		free(info);
+	}
+
+finish_validation:
+	if (!ret && owner_sid) {
+#define MAX_NAME_OR_DOMAIN 256
+		wchar_t owner_name[MAX_NAME_OR_DOMAIN];
+		wchar_t owner_domain[MAX_NAME_OR_DOMAIN];
+		wchar_t *p = NULL;
+		DWORD size = MAX_NAME_OR_DOMAIN;
+		SID_NAME_USE type;
+		char name[3 * MAX_NAME_OR_DOMAIN + 1];
+
+		if (!LookupAccountSidW(NULL, owner_sid, owner_name, &size,
+				       owner_domain, &size, &type) ||
+		    xwcstoutf(name, owner_name, ARRAY_SIZE(name)) < 0) {
+			if (!ConvertSidToStringSidW(owner_sid, &p))
+				strlcpy(name, "(unknown)", ARRAY_SIZE(name));
+			else {
+				if (xwcstoutf(name, p, ARRAY_SIZE(name)) < 0)
+					strlcpy(name, "(some user)",
+						ARRAY_SIZE(name));
+				LocalFree(p);
+			}
+		}
+
+		warning(_("'%s' has a dubious owner: '%s'.\n"
+			  "For security reasons, it is therefore ignored.\n"
+			  "To fix this, please transfer ownership to an "
+			  "admininstrator."),
+			path, name);
+	}
+
+	if (descriptor)
+		LocalFree(descriptor);
+
+	return ret;
+}
+
+const char *program_data_config(void)
+{
+	static struct strbuf path = STRBUF_INIT;
+	static unsigned initialized;
+
+	if (!initialized) {
+		const char *env = mingw_getenv("PROGRAMDATA");
+		if (env) {
+			strbuf_addf(&path, "%s/Git/config", env);
+			if (validate_system_file_ownership(path.buf) != 1)
+				strbuf_setlen(&path, 0);
+		}
+		initialized = 1;
+	}
+	return *path.buf ? path.buf : NULL;
+}

compat/mingw.h

@@ -443,6 +443,8 @@ static inline void convert_slashes(char *path)
 #define PATH_SEP ';'
 extern char *mingw_query_user_email(void);
 #define query_user_email mingw_query_user_email
+extern const char *program_data_config(void);
+#define git_program_data_config program_data_config
 #if !defined(__MINGW64_VERSION_MAJOR) && (!defined(_MSC_VER) || _MSC_VER < 1800)
 #define PRIuMAX "I64u"
 #define PRId64 "I64d"

config.c

@@ -1676,11 +1676,16 @@ static int do_git_config_sequence(const struct config_options *opts,
 		repo_config = NULL;
 
 	current_parsing_scope = CONFIG_SCOPE_SYSTEM;
-	if (git_config_system() && !access_or_die(git_etc_gitconfig(), R_OK,
-						  opts->system_gently ?
-						  ACCESS_EACCES_OK : 0))
-		ret += git_config_from_file(fn, git_etc_gitconfig(),
-					    data);
+	if (git_config_system()) {
+		int flags = opts->system_gently ? ACCESS_EACCES_OK : 0;
+		const char *program_data = git_program_data_config();
+		const char *etc = git_etc_gitconfig();
+
+		if (program_data && !access_or_die(program_data, R_OK, flags))
+			ret += git_config_from_file(fn, program_data, data);
+		if (!access_or_die(etc, R_OK, flags))
+			ret += git_config_from_file(fn, etc, data);
+	}
 
 	current_parsing_scope = CONFIG_SCOPE_GLOBAL;
 	if (xdg_config && !access_or_die(xdg_config, R_OK, ACCESS_EACCES_OK))

git-compat-util.h

@@ -412,6 +412,10 @@ static inline char *git_find_last_dir_sep(const char *path)
 #endif
 #endif
 
+#ifndef git_program_data_config
+#define git_program_data_config() NULL
+#endif
+
 #if defined(__HP_cc) && (__HP_cc >= 61000)
 #define NORETURN __attribute__((noreturn))
 #define NORETURN_PTR