Implement global/system file search paths

The goal of this work is to expose the search logic for "global",
"system", and "xdg" files through the git_libgit2_opts() interface.

Behind the scenes, I changed the logic for finding files to have a
notion of a git_strarray that represents a search path and to store
a separate search path for each of the three tiers of config file.
For each tier, I implemented a function to initialize it to default
values (generally based on environment variables), and then general
interfaces to get it, set it, reset it, and prepend new directories
to it.

Next, I exposed these interfaces through the git_libgit2_opts
interface, reusing the GIT_CONFIG_LEVEL_SYSTEM, etc., constants
for the user to control which search path they were modifying.
There are alternative designs for the opts interface / argument
ordering, so I'm putting this phase out for discussion.

Additionally, I ended up doing a little bit of clean up regarding
attr.h and attr_file.h, adding a new attrcache.h so the other two
files wouldn't have to be included in so many places.

2 files changed, 66 insertions, 10 deletions

diff --git a/include/git2/common.h b/include/git2/common.h
index 7066d5ea3..137ffa0a4 100644
--- a/include/git2/common.h
+++ b/include/git2/common.h
@@ -128,7 +128,10 @@ enum {
 	GIT_OPT_GET_MWINDOW_SIZE,
 	GIT_OPT_SET_MWINDOW_SIZE,
 	GIT_OPT_GET_MWINDOW_MAPPED_LIMIT,
-	GIT_OPT_SET_MWINDOW_MAPPED_LIMIT
+	GIT_OPT_SET_MWINDOW_MAPPED_LIMIT,
+	GIT_OPT_GET_SEARCH_PATH,
+	GIT_OPT_SET_SEARCH_PATH,
+	GIT_OPT_PREPEND_SEARCH_PATH,
 };
 
 /**
@@ -136,17 +139,44 @@ enum {
  *
  * Available options:
  *
- *	opts(GIT_OPT_MWINDOW_SIZE, size_t):
- *		set the maximum mmap window size
+ *	opts(GIT_OPT_GET_MWINDOW_SIZE, size_t *):
+ *		Get the maximum mmap window size
  *
- *	opts(GIT_OPT_MWINDOW_MAPPED_LIMIT, size_t):
- *		set the maximum amount of memory that can be mapped at any time
+ *	opts(GIT_OPT_SET_MWINDOW_SIZE, size_t):
+ *		Set the maximum mmap window size
+ *
+ *	opts(GIT_OPT_GET_MWINDOW_MAPPED_LIMIT, size_t *):
+ *		Get the maximum memory that will be mapped in total by the library
+ *
+ *	opts(GIT_OPT_SET_MWINDOW_MAPPED_LIMIT, size_t):
+ *		Set the maximum amount of memory that can be mapped at any time
  *		by the library
  *
- *	@param option Option key
- *	@param ... value to set the option
+ *	opts(GIT_OPT_GET_SEARCH_PATH, const git_strarray **, int level)
+ *		Get a strarray of the search path for a given level of config
+ *		data.  "level" must be one of GIT_CONFIG_LEVEL_SYSTEM,
+ *		GIT_CONFIG_LEVEL_GLOBAL, or GIT_CONFIG_LEVEL_XDG.  The search
+ *		path applies to shared attributes and ignore files, too.
+ *
+ *	opts(GIT_OPT_SET_SEARCH_PATH, int level, const git_strarray *)
+ *		Set the search path for a given level of config data.  Passing
+ *		NULL for the git_strarray pointer resets the search path to the
+ *		default (which is generally based on environment variables).
+ *		"level" must be one of GIT_CONFIG_LEVEL_SYSTEM,
+ *		GIT_CONFIG_LEVEL_GLOBAL, or GIT_CONFIG_LEVEL_XDG.  The search
+ *		path applies to shared attributes and ignore files, too.
+ *
+ *	opts(GIT_OPT_PREPEND_SEARCH_PATH, int level, const git_strarray *)
+ *		Prepend new directories to the search path for a given level of
+ *		config data.  "level" must be one of GIT_CONFIG_LEVEL_SYSTEM,
+ *		GIT_CONFIG_LEVEL_GLOBAL, or GIT_CONFIG_LEVEL_XDG.  The search
+ *		path applies to shared attributes and ignore files, too.
+ *
+ * @param option Option key
+ * @param ... value to set the option
+ * @return 0 on success, <0 on failure
  */
-GIT_EXTERN(void) git_libgit2_opts(int option, ...);
+GIT_EXTERN(int) git_libgit2_opts(int option, ...);
 
 /** @} */
 GIT_END_DECL
diff --git a/include/git2/strarray.h b/include/git2/strarray.h
index 6ea570c14..df34a5b88 100644
--- a/include/git2/strarray.h
+++ b/include/git2/strarray.h
@@ -43,8 +43,8 @@ GIT_EXTERN(void) git_strarray_free(git_strarray *array);
 /**
  * Copy a string array object from source to target.
  *
- * Note: target is overwritten and hence should be empty,
- * otherwise its contents are leaked.
+ * Note: target is overwritten and hence should be empty, otherwise its
+ * contents are leaked.  Call git_strarray_free() if necessary.
  *
  * @param tgt target
  * @param src source
@@ -52,6 +52,32 @@ GIT_EXTERN(void) git_strarray_free(git_strarray *array);
  */
 GIT_EXTERN(int) git_strarray_copy(git_strarray *tgt, const git_strarray *src);
 
+/**
+ * Initialize a string array from a list of strings
+ *
+ * Note: target is overwritten and hence should be empty, otherwise its
+ * contents are leaked.  Call git_strarray_free() if necessary.
+ *
+ * @param tgt target
+ * @param count number of strings to follow
+ * @return 0 on success, <0 on allocation failure
+ */
+GIT_EXTERN(int) git_strarray_set(git_strarray *tgt, size_t count, ...);
+
+/**
+ * Insert a strarray into the beginning of another
+ *
+ * In this case, tgt is an existing (initialized) strarray and the result
+ * will be reallocated with all the strings in src inserted before all of
+ * the existing strings in tgt.  Strings in src will be strdup'ed, so
+ * you should still `git_strarray_free()` src when you are done with it.
+ *
+ * @param tgt strarray to update
+ * @param src strarray to copy from
+ * @return 0 on success, <0 on allocation failure (tgt will be unchanged)
+ */
+GIT_EXTERN(int) git_strarray_prepend(git_strarray *tgt, const git_strarray *src);
+
 
 /** @} */
 GIT_END_DECL