config.c: add support for installation-specific config files

If Git is configured to use the absolute '/etc/gitconfig' for system-wide
settings, it may still be useful to have installation-specific settings
such as 'help.format' (which depends on the installed set of help pages).

This is particularly relevant on Windows, where a variety of Git flavors
with different feature sets may be installed in parallel.

If ETC_GITCONFIG is an absolute path (typically '/etc/gitconfig'), load
installation-specific defaults (from '$(prefix)/$(ETC_GITCONFIG)') before
loading the system-wide configuration.

As installation-specific settings will typically be defined when creating
the software package / installer (i.e. *before* git is installed), we don't
need an extra 'git config --installation' option.

Update the documentation accordingly. This removes '$(prefix)' from most
places (which would be meaningless to normal users anyway).

Signed-off-by: Karsten Blees <[email protected]>

Author: kblees

Documentation/git-config.txt

@@ -117,10 +117,10 @@ See also <<FILES>>.
 
 --system::
 	For writing options: write to system-wide
-	`$(prefix)/etc/gitconfig` rather than the repository
+	`/etc/gitconfig` rather than the repository
 	`.git/config`.
 +
-For reading options: read only from system-wide `$(prefix)/etc/gitconfig`
+For reading options: read only from system-wide `/etc/gitconfig`
 rather than from all available files.
 +
 See also <<FILES>>.
@@ -221,11 +221,21 @@ See also <<FILES>>.
 FILES
 -----
 
-If not set explicitly with '--file', there are four files where
+If not set explicitly with '--file', there are five files where
 'git config' will search for configuration options:
 
 $(prefix)/etc/gitconfig::
+	Installation-specific configuration file, where '$(prefix)' is the
+	installation root directory specified via 'make prefix=...'. This allows
+	software distributions to provide installation-specific default values
+	(e.g. 'help.format=html' if the installation only includes html pages).
+
+/etc/gitconfig::
 	System-wide configuration file.
++
+If git was built with relative `$(sysconfdir)`, this file will not be
+used, and the '--system' option refers to the installation-specific
+`$(prefix)/etc/gitconfig` instead.
 
 $XDG_CONFIG_HOME/git/config::
 	Second user-specific configuration file. If $XDG_CONFIG_HOME is not set
@@ -268,11 +278,11 @@ ENVIRONMENT
 GIT_CONFIG::
 	Take the configuration from the given file instead of .git/config.
 	Using the "--global" option forces this to ~/.gitconfig. Using the
-	"--system" option forces this to $(prefix)/etc/gitconfig.
+	"--system" option forces this to /etc/gitconfig.
 
 GIT_CONFIG_NOSYSTEM::
-	Whether to skip reading settings from the system-wide
-	$(prefix)/etc/gitconfig file. See linkgit:git[1] for details.
+	Whether to skip reading settings from the installation-specific	and
+	system-wide /etc/gitconfig files. See linkgit:git[1] for details.
 
 See also <<FILES>>.
 

Documentation/git.txt

@@ -939,8 +939,8 @@ for further details.
 	on the terminal (e.g., when asking for HTTP authentication).
 
 'GIT_CONFIG_NOSYSTEM'::
-	Whether to skip reading settings from the system-wide
-	`$(prefix)/etc/gitconfig` file.  This environment variable can
+	Whether to skip reading settings from the installation-specific and
+	system-wide `/etc/gitconfig` files. This environment variable can
 	be used along with `$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

Documentation/gitattributes.txt

@@ -84,7 +84,7 @@ for a single user should be placed in a file specified by the
 Its default value is $XDG_CONFIG_HOME/git/attributes. If $XDG_CONFIG_HOME
 is either not set or empty, $HOME/.config/git/attributes is used instead.
 Attributes for all users on a system should be placed in the
-`$(prefix)/etc/gitattributes` file.
+`/etc/gitattributes` file.
 
 Sometimes you would need to override an setting of an attribute
 for a path to `Unspecified` state.  This can be done by listing

config.c

@@ -1147,14 +1147,32 @@ static int git_config_from_blob_ref(config_fn_t fn,
 	return git_config_from_blob_sha1(fn, name, sha1, data);
 }
 
+static const char *etc_gitconfig = ETC_GITCONFIG;
+
 const char *git_etc_gitconfig(void)
 {
 	static const char *system_wide;
 	if (!system_wide)
-		system_wide = system_path(ETC_GITCONFIG);
+		system_wide = system_path(etc_gitconfig);
 	return system_wide;
 }
 
+static const char *git_inst_gitconfig(void)
+{
+	static const char *installation_defaults;
+	if (!installation_defaults) {
+		/*
+		 * if ETC_GITCONFIG as configured in the Makefile is an absolute path,
+		 * also load installation-specific defaults (relative to $(prefix))
+		 */
+		if (is_dir_sep(*etc_gitconfig))
+			installation_defaults = system_path(etc_gitconfig + 1);
+		else
+			installation_defaults = "";
+	}
+	return *installation_defaults ? installation_defaults : NULL;
+}
+
 /*
  * Parse environment variable 'k' as a boolean (in various
  * possible spellings); if missing, use the default value 'def'.
@@ -1205,8 +1223,10 @@ int git_config_early(config_fn_t fn, void *data, const char *repo_config)
 
 	home_config_paths(&user_config, &xdg_config, "config");
 
-	if (git_config_system())
+	if (git_config_system()) {
+		cnt = config_early_helper(fn, git_inst_gitconfig(), data, 0, cnt);
 		cnt = config_early_helper(fn, git_etc_gitconfig(), data, 0, cnt);
+	}
 
 	cnt = config_early_helper(fn, xdg_config, data, ACCESS_EACCES_OK, cnt);
 	cnt = config_early_helper(fn, user_config, data, ACCESS_EACCES_OK, cnt);