Added support for atomically committing custom attributes

Although it's simple and probably what most users expect, the previous
custom attributes API suffered from one problem: the inability to update
attributes atomically.

If we consider our timestamp use case, updating a file would require:
1. Update the file
2. Update the timestamp

If a power loss occurs during this sequence of updates, we could end up
with a file with an incorrect timestamp.

Is this a big deal? Probably not, but it could be a surprise only found
after a power-loss. And littlefs was developed with the _specifically_
to avoid suprises during power-loss.

The littlefs is perfectly capable of bundling multiple attribute updates
in a single directory commit. That's kind of what it was designed to do.
So all we need is a new committer opcode for list of attributes, and
then poking that list of attributes through the API.

We could provide the single-attribute functions, but don't, because the
fewer functions makes for a smaller codebase, and these are already the
more advanced functions so we can expect more from users. This also
changes semantics about what happens when we don't find an attribute,
since erroring would throw away all of the other attributes we're
processing.

To atomically commit both custom attributes and file updates, we need a
new API, lfs_file_setattr. Unfortunately the semantics are a bit more
confusing than lfs_setattr, since the attributes aren't written out
immediately.

1 files changed, 75 insertions, 19 deletions

diff --git a/lfs.h b/lfs.h
index adc1c02..9de7a3a 100644
--- a/lfs.h
+++ b/lfs.h
@@ -234,6 +234,18 @@ struct lfs_info {
     char name[LFS_NAME_MAX+1];
 };
 
+// Custom attribute structure
+struct lfs_attr {
+    // Type of attribute, provided by user and used to identify the attribute
+    uint8_t type;
+
+    // Pointer to buffer containing the attribute
+    void *buffer;
+
+    // Size of attribute in bytes, limited to LFS_ATTRS_MAX
+    lfs_size_t size;
+};
+
 
 /// littlefs data structures ///
 typedef struct lfs_entry {
@@ -255,12 +267,12 @@ typedef struct lfs_entry {
     } d;
 } lfs_entry_t;
 
-typedef struct lfs_attr {
-    struct lfs_disk_attr {
+typedef struct lfs_entry_attr {
+    struct lfs_disk_entry_attr {
         uint8_t type;
         uint8_t len;
     } d;
-} lfs_attr_t;
+} lfs_entry_attr_t;
 
 typedef struct lfs_cache {
     lfs_block_t block;
@@ -388,28 +400,25 @@ int lfs_rename(lfs_t *lfs, const char *oldpath, const char *newpath);
 // Returns a negative error code on failure.
 int lfs_stat(lfs_t *lfs, const char *path, struct lfs_info *info);
 
-// Get a custom attribute
+// Get custom attributes
 //
-// Attributes are identified by an 8-bit type and are limited to at
-// most LFS_ATTRS_SIZE bytes.
-// Returns the size of the attribute, or a negative error code on failure.
-int lfs_getattr(lfs_t *lfs, const char *path,
-        uint8_t type, void *buffer, lfs_size_t size);
-
-// Set a custom attribute
+// Attributes are looked up based on the type id. If the stored attribute is
+// smaller than the buffer, it is padded with zeros. It the stored attribute
+// is larger than the buffer, LFS_ERR_RANGE is returned.
 //
-// Attributes are identified by an 8-bit type and are limited to at
-// most LFS_ATTRS_SIZE bytes.
 // Returns a negative error code on failure.
-int lfs_setattr(lfs_t *lfs, const char *path,
-        uint8_t type, const void *buffer, lfs_size_t size);
+int lfs_getattrs(lfs_t *lfs, const char *path,
+        const struct lfs_attr *attrs, int count);
 
-// Remove a custom attribute
+// Set custom attributes
+//
+// The array of attributes will be used to update the attributes stored on
+// disk based on their type id. Unspecified attributes are left unmodified.
+// Specifying an attribute with zero size deletes the attribute.
 //
-// Attributes are identified by an 8-bit type and are limited to at
-// most LFS_ATTRS_SIZE bytes.
 // Returns a negative error code on failure.
-int lfs_removeattr(lfs_t *lfs, const char *path, uint8_t type);
+int lfs_setattrs(lfs_t *lfs, const char *path,
+        const struct lfs_attr *attrs, int count);
 
 
 /// File operations ///
@@ -484,6 +493,30 @@ int lfs_file_rewind(lfs_t *lfs, lfs_file_t *file);
 // Returns the size of the file, or a negative error code on failure.
 lfs_soff_t lfs_file_size(lfs_t *lfs, lfs_file_t *file);
 
+// Get custom attributes attached to a file
+//
+// Attributes are looked up based on the type id. If the stored attribute is
+// smaller than the buffer, it is padded with zeros. It the stored attribute
+// is larger than the buffer, LFS_ERR_RANGE is returned.
+//
+// Returns a negative error code on failure.
+int lfs_file_getattrs(lfs_t *lfs, lfs_file_t *file,
+        const struct lfs_attr *attrs, int count);
+
+// Set custom attributes on a file
+//
+// The array of attributes will be used to update the attributes stored on
+// disk based on their type id. Unspecified attributes are left unmodified.
+// Specifying an attribute with zero size deletes the attribute.
+//
+// Note: Attributes are not written out until a call to lfs_file_sync
+// or lfs_file_close and must be allocated until the file is closed or
+// lfs_file_setattrs is called with a count of zero.
+//
+// Returns a negative error code on failure.
+int lfs_file_setattrs(lfs_t *lfs, lfs_file_t *file,
+        const struct lfs_attr *attrs, int count);
+
 
 /// Directory operations ///
 
@@ -532,6 +565,29 @@ lfs_soff_t lfs_dir_tell(lfs_t *lfs, lfs_dir_t *dir);
 int lfs_dir_rewind(lfs_t *lfs, lfs_dir_t *dir);
 
 
+/// Filesystem filesystem operations ///
+
+// Get custom attributes on the filesystem
+//
+// Attributes are looked up based on the type id. If the stored attribute is
+// smaller than the buffer, it is padded with zeros. It the stored attribute
+// is larger than the buffer, LFS_ERR_RANGE is returned.
+//
+// Returns a negative error code on failure.
+int lfs_fs_getattrs(lfs_t *lfs, const struct lfs_attr *attrs, int count);
+
+// Set custom attributes on the filesystem
+//
+// The array of attributes will be used to update the attributes stored on
+// disk based on their type id. Unspecified attributes are left unmodified.
+// Specifying an attribute with zero size deletes the attribute.
+//
+// Note: Filesystem level attributes are not available for wear-leveling
+//
+// Returns a negative error code on failure.
+int lfs_fs_setattrs(lfs_t *lfs, const struct lfs_attr *attrs, int count);
+
+
 /// Miscellaneous littlefs specific operations ///
 
 // Traverse through all blocks in use by the filesystem