[PATCH v7 1/3] cache.h: Introduce a generic "xdg_config_home_for(…)" function

["Lénaïc Huard" <lenaic@lhuard.fr>]

Current implementation of `xdg_config_home(filename)` returns
`$XDG_CONFIG_HOME/git/$filename`, with the `git` subdirectory inserted
between the `XDG_CONFIG_HOME` environment variable and the parameter.

This patch introduces a `xdg_config_home_for(subdir, filename)` function
which is more generic. It only concatenates "$XDG_CONFIG_HOME", or
"$HOME/.config" if the former isn’t defined, with the parameters,
without adding `git` in between.

`xdg_config_home(filename)` is now implemented by calling
`xdg_config_home_for("git", filename)` but this new generic function can
be used to compute the configuration directory of other programs.

Signed-off-by: Lénaïc Huard <lenaic@lhuard.fr>
---
 cache.h |  7 +++++++
 path.c  | 13 ++++++++++---
 2 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/cache.h b/cache.h
index ba04ff8bd3..2a0fb3e4ba 100644
--- a/cache.h
+++ b/cache.h
@@ -1286,6 +1286,13 @@ int is_ntfs_dotmailmap(const char *name);
  */
 int looks_like_command_line_option(const char *str);
 
+/**
+ * Return a newly allocated string with the evaluation of
+ * "$XDG_CONFIG_HOME/$subdir/$filename" if $XDG_CONFIG_HOME is non-empty, otherwise
+ * "$HOME/.config/$subdir/$filename". Return NULL upon error.
+ */
+char *xdg_config_home_for(const char *subdir, const char *filename);
+
 /**
  * Return a newly allocated string with the evaluation of
  * "$XDG_CONFIG_HOME/git/$filename" if $XDG_CONFIG_HOME is non-empty, otherwise
diff --git a/path.c b/path.c
index 7bccd830e9..1b1de3be09 100644
--- a/path.c
+++ b/path.c
@@ -1503,21 +1503,28 @@ int looks_like_command_line_option(const char *str)
 	return str && str[0] == '-';
 }
 
-char *xdg_config_home(const char *filename)
+char *xdg_config_home_for(const char *subdir, const char *filename)
 {
 	const char *home, *config_home;
 
+	assert(subdir);
 	assert(filename);
 	config_home = getenv("XDG_CONFIG_HOME");
 	if (config_home && *config_home)
-		return mkpathdup("%s/git/%s", config_home, filename);
+		return mkpathdup("%s/%s/%s", config_home, subdir, filename);
 
 	home = getenv("HOME");
 	if (home)
-		return mkpathdup("%s/.config/git/%s", home, filename);
+		return mkpathdup("%s/.config/%s/%s", home, subdir, filename);
+
 	return NULL;
 }
 
+char *xdg_config_home(const char *filename)
+{
+	return xdg_config_home_for("git", filename);
+}
+
 char *xdg_cache_home(const char *filename)
 {
 	const char *home, *cache_home;
-- 
2.32.0