Support reading attributes from index

Depending on the operation, we need to consider gitattributes
in both the work dir and the index.  This adds a parameter to
all of the gitattributes related functions that allows user
control of attribute reading behavior (i.e. prefer workdir,
prefer index, only use index).

This fix also covers allowing us to check attributes (and
hence do diff and status) on bare repositories.

This was a somewhat larger change that I hoped because it had
to change the cache key used for gitattributes files.

1 files changed, 155 insertions, 11 deletions

diff --git a/include/git2/attr.h b/include/git2/attr.h
index 81d1e517b..dfa1d2778 100644
--- a/include/git2/attr.h
+++ b/include/git2/attr.h
@@ -19,42 +19,186 @@
  */
 GIT_BEGIN_DECL
 
+/**
+ * GIT_ATTR_TRUE checks if an attribute is set on.  In core git
+ * parlance, this the value for "Set" attributes.
+ *
+ * For example, if the attribute file contains:
+ *
+ *    *.c foo
+ *
+ * Then for file `xyz.c` looking up attribute "foo" gives a value for
+ * which `GIT_ATTR_TRUE(value)` is true.
+ */
 #define GIT_ATTR_TRUE(attr)		((attr) == git_attr__true)
+
+/**
+ * GIT_ATTR_FALSE checks if an attribute is set off.  In core git
+ * parlance, this is the value for attributes that are "Unset" (not to
+ * be confused with values that a "Unspecified").
+ *
+ * For example, if the attribute file contains:
+ *
+ *    *.h -foo
+ *
+ * Then for file `zyx.h` looking up attribute "foo" gives a value for
+ * which `GIT_ATTR_FALSE(value)` is true.
+ */
 #define GIT_ATTR_FALSE(attr)	((attr) == git_attr__false)
-#define GIT_ATTR_UNSPECIFIED(attr)	((attr) == NULL)
+
+/**
+ * GIT_ATTR_UNSPECIFIED checks if an attribute is unspecified.  This
+ * may be due to the attribute not being mentioned at all or because
+ * the attribute was explicitly set unspecified via the `!` operator.
+ *
+ * For example, if the attribute file contains:
+ *
+ *    *.c foo
+ *    *.h -foo
+ *    onefile.c !foo
+ *
+ * Then for `onefile.c` looking up attribute "foo" yields a value with
+ * `GIT_ATTR_UNSPECIFIED(value)` of true.  Also, looking up "foo" on
+ * file `onefile.rb` or looking up "bar" on any file will all give
+ * `GIT_ATTR_UNSPECIFIED(value)` of true.
+ */
+#define GIT_ATTR_UNSPECIFIED(attr)	(!(attr) || (attr) == git_attr__unset)
+
+/**
+ * GIT_ATTR_SET_TO_VALUE checks if an attribute is set to a value (as
+ * opposied to TRUE, FALSE or UNSPECIFIED).  This would be the case if
+ * for a file with something like:
+ *
+ *    *.txt eol=lf
+ *
+ * Given this, looking up "eol" for `onefile.txt` will give back the
+ * string "lf" and `GIT_ATTR_SET_TO_VALUE(attr)` will return true.
+ */
+#define GIT_ATTR_SET_TO_VALUE(attr) \
+	((attr) && (attr) != git_attr__unset && \
+	 (attr) != git_attr__true && (attr) != git_attr__false)
 
 GIT_EXTERN(const char *) git_attr__true;
 GIT_EXTERN(const char *) git_attr__false;
+GIT_EXTERN(const char *) git_attr__unset;
 
+/**
+ * Check attribute flags: Reading values from index and working directory.
+ *
+ * When checking attributes, it is possible to check attribute files
+ * in both the working directory (if there is one) and the index (if
+ * there is one).  You can explicitly choose where to check and in
+ * which order using the following flags.
+ *
+ * Core git usually checks the working directory then the index,
+ * except during a checkout when it checks the index first.  It will
+ * use index only for creating archives or for a bare repo (if an
+ * index has been specified for the bare repo).
+ */
+#define GIT_ATTR_CHECK_FILE_THEN_INDEX	0
+#define GIT_ATTR_CHECK_INDEX_THEN_FILE	1
+#define GIT_ATTR_CHECK_INDEX_ONLY		2
 
 /**
- * Lookup attribute for path returning string caller must free
+ * Check attribute flags: Using the system attributes file.
+ *
+ * Normally, attribute checks include looking in the /etc (or system
+ * equivalent) directory for a `gitattributes` file.  Passing this
+ * flag will cause attribute checks to ignore that file.
+ */
+#define GIT_ATTR_CHECK_NO_SYSTEM		(1 << 2)
+
+/**
+ * Look up the value of one git attribute for path.
+ *
+ * @param repo The repository containing the path.
+ * @param flags A combination of GIT_ATTR_CHECK... flags.
+ * @param path The path to check for attributes.  Relative paths are
+ *             interpreted relative to the repo root.  The file does
+ *             not have to exist, but if it does not, then it will be
+ *             treated as a plain file (not a directory).
+ * @param name The name of the attribute to look up.
+ * @param value Output of the value of the attribute.  Use the GIT_ATTR_...
+ *             macros to test for TRUE, FALSE, UNSPECIFIED, etc. or just
+ *             use the string value for attributes set to a value.  You
+ *             should NOT modify or free this value.
  */
 GIT_EXTERN(int) git_attr_get(
-    git_repository *repo, const char *path, const char *name,
+    git_repository *repo,
+	uint32_t flags,
+	const char *path,
+	const char *name,
 	const char **value);
 
 /**
- * Lookup list of attributes for path, populating array of strings
+ * Look up a list of git attributes for path.
+ *
+ * Use this if you have a known list of attributes that you want to
+ * look up in a single call.  This is somewhat more efficient than
+ * calling `git_attr_get()` multiple times.
+ *
+ * For example, you might write:
+ *
+ *     const char *attrs[] = { "crlf", "diff", "foo" };
+ *     const char **values[3];
+ *     git_attr_get_many(repo, 0, "my/fun/file.c", 3, attrs, values);
+ *
+ * Then you could loop through the 3 values to get the settings for
+ * the three attributes you asked about.
+ *
+ * @param repo The repository containing the path.
+ * @param flags A combination of GIT_ATTR_CHECK... flags.
+ * @param path The path inside the repo to check attributes.  This
+ *             does not have to exist, but if it does not, then
+ *             it will be treated as a plain file (i.e. not a directory).
+ * @param num_attr The number of attributes being looked up
+ * @param names An array of num_attr strings containing attribute names.
+ * @param values An array of num_attr entries that will have string
+ *             pointers written into it for the values of the attributes.
+ *             You should not modify or free the values that are written
+ *             into this array (although of course, you should free the
+ *             array itself if you allocated it).
  */
 GIT_EXTERN(int) git_attr_get_many(
-    git_repository *repo, const char *path,
-    size_t num_attr, const char **names,
+    git_repository *repo,
+	uint32_t flags,
+	const char *path,
+    size_t num_attr,
+	const char **names,
 	const char **values);
 
 /**
- * Perform an operation on each attribute of a path.
+ * Loop over all the git attributes for a path.
+ *
+ * @param repo The repository containing the path.
+ * @param flags A combination of GIT_ATTR_CHECK... flags.
+ * @param path The path inside the repo to check attributes.  This
+ *             does not have to exist, but if it does not, then
+ *             it will be treated as a plain file (i.e. not a directory).
+ * @param callback The function that will be invoked on each attribute
+ *             and attribute value.  The name parameter will be the name
+ *             of the attribute and the value will be the value it is
+ *             set to, including possibly NULL if the attribute is
+ *             explicitly set to UNSPECIFIED using the ! sign.  This
+ *             will be invoked only once per attribute name, even if
+ *             there are multiple rules for a given file.  The highest
+ *             priority rule will be used.
+ * @param payload Passed on as extra parameter to callback function.
  */
 GIT_EXTERN(int) git_attr_foreach(
-    git_repository *repo, const char *path,
+    git_repository *repo,
+	uint32_t flags,
+	const char *path,
 	int (*callback)(const char *name, const char *value, void *payload),
 	void *payload);
 
 /**
  * Flush the gitattributes cache.
  *
- * Call this if you have reason to believe that the attributes files
- * on disk no longer match the cached contents of memory.
+ * Call this if you have reason to believe that the attributes files on
+ * disk no longer match the cached contents of memory.  This will cause
+ * the attributes files to be reloaded the next time that an attribute
+ * access function is called.
  */
 GIT_EXTERN(void) git_attr_cache_flush(
 	git_repository *repo);
@@ -62,7 +206,7 @@ GIT_EXTERN(void) git_attr_cache_flush(
 /**
  * Add a macro definition.
  *
- * Macros will automatically be loaded from the top level .gitattributes
+ * Macros will automatically be loaded from the top level `.gitattributes`
  * file of the repository (plus the build-in "binary" macro).  This
  * function allows you to add others.  For example, to add the default
  * macro, you would call: