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 upcoming Git for Windows 2.x, there would be more paths to
search, as we will have 64-bit and 32-bit versions, and the
corresponding config files will be in %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.

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.

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

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

@@ -270,8 +270,16 @@ 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

@@ -567,7 +567,8 @@ 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

@@ -2611,3 +2611,17 @@ int uname(struct utsname *buf)
 		  "%u", (v >> 16) & 0x7fff);
 	return 0;
 }
+
+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);
+		initialized = 1;
+	}
+	return *path.buf ? path.buf : NULL;
+}

compat/mingw.h

@@ -452,6 +452,8 @@ 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

@@ -1674,9 +1674,16 @@ 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, 0))
-		ret += git_config_from_file(fn, git_etc_gitconfig(),
-					    data);
+	if (git_config_system()) {
+		if (git_program_data_config() &&
+		    !access_or_die(git_program_data_config(), R_OK, 0))
+			ret += git_config_from_file(fn,
+						    git_program_data_config(),
+						    data);
+		if (!access_or_die(git_etc_gitconfig(), R_OK, 0))
+			ret += git_config_from_file(fn, git_etc_gitconfig(),
+						    data);
+	}
 
 	current_parsing_scope = CONFIG_SCOPE_GLOBAL;
 	if (xdg_config && !access_or_die(xdg_config, R_OK, ACCESS_EACCES_OK))

git-compat-util.h

@@ -411,6 +411,10 @@ 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