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Added support for atomically committing custom attributes
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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.
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@geky
geky committed Oct 10, 2018
1 parent 6ffc8d3 commit 6c754c8
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Showing 2 changed files with 218 additions and 118 deletions.
242 changes: 143 additions & 99 deletions lfs.c
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Original file line number Diff line number Diff line change
Expand Up @@ -515,13 +515,20 @@ struct lfs_region {
LFS_FROM_DROP,
LFS_FROM_MEM,
LFS_FROM_REGION,
LFS_FROM_ATTRS,
} type;

lfs_off_t off;
const void *buffer;
lfs_ssize_t size;
};

struct lfs_attrs_region {
const struct lfs_attr *attrs;
int count;
lfs_size_t len;
};

struct lfs_region_region {
lfs_block_t block;
lfs_off_t off;
Expand Down Expand Up @@ -568,6 +575,73 @@ static int lfs_commit_region(lfs_t *lfs,
newoff += regions[i].size;
break;
}
case LFS_FROM_ATTRS: {
const struct lfs_attrs_region *attrs = regions[i].buffer;

// order doesn't matter, so we write new attrs first. this
// is still O(n^2) but only O(n) disk access
for (int j = 0; j < attrs->count; j++) {
if (attrs->attrs[j].size == 0) {
continue;
}

lfs_entry_attr_t attr = {
.d.type = attrs->attrs[j].type,
.d.len = attrs->attrs[j].size,
};

lfs_crc(crc, &attr.d, sizeof(attr.d));
int err = lfs_bd_prog(lfs, newblock, newoff,
&attr.d, sizeof(attr.d));
if (err) {
return err;
}

lfs_crc(crc,
attrs->attrs[j].buffer, attrs->attrs[j].size);
err = lfs_bd_prog(lfs, newblock, newoff+sizeof(attr.d),
attrs->attrs[j].buffer, attrs->attrs[j].size);
if (err) {
return err;
}

newoff += sizeof(attr.d) + attrs->attrs[j].size;
}

// copy over attributes without updates
lfs_entry_attr_t attr;
for (lfs_off_t k = 0; k < attrs->len; k += 2+attr.d.len) {
int err = lfs_bd_read(lfs, oldblock, oldoff,
&attr.d, sizeof(attr.d));
if (err) {
return err;
}

bool updating = false;
for (int j = 0; j < attrs->count; j++) {
if (attr.d.type == attrs->attrs[j].type) {
updating = true;
}
}

if (!updating) {
err = lfs_commit_region(lfs,
oldblock, oldoff,
newblock, newoff,
0, NULL, 0,
attr.d.len, crc);
if (err) {
return err;
}

newoff += 2+attr.d.len;
}

oldoff += 2+attr.d.len;
}

break;
}
}

i += 1;
Expand All @@ -590,6 +664,8 @@ static int lfs_commit_region(lfs_t *lfs,
}
}

// sanity check our commit math
LFS_ASSERT(newoff == end);
return 0;
}

Expand Down Expand Up @@ -1044,118 +1120,102 @@ static int lfs_dir_getinfo(lfs_t *lfs,
return 0;
}

static int lfs_dir_getattr(lfs_t *lfs,
static int lfs_dir_getattrs(lfs_t *lfs,
lfs_dir_t *dir, const lfs_entry_t *entry,
uint8_t type, void *buffer, lfs_size_t size) {
const struct lfs_attr *attrs, int count) {
// set to zero in case we can't find the attributes or size mismatch
for (int j = 0; j < count; j++) {
memset(attrs[j].buffer, 0, attrs[j].size);
}

// search for attribute in attribute region
lfs_off_t off = sizeof(dir->d) + lfs_entry_elen(entry);
lfs_off_t i = 0;
while (i < lfs_entry_alen(entry)) {
lfs_attr_t attr;
lfs_off_t off = 4+lfs_entry_elen(entry);
lfs_entry_attr_t attr;
for (lfs_off_t i = 0; i < lfs_entry_alen(entry); i += 2+attr.d.len) {
int err = lfs_dir_get(lfs, dir,
entry->off+off+i, &attr.d, sizeof(attr.d));
if (err) {
return err;
}

if (attr.d.type != type) {
i += attr.d.len;
continue;
}

if (attr.d.len > size) {
return LFS_ERR_RANGE;
}
for (int j = 0; j < count; j++) {
if (attr.d.type == attrs[j].type) {
if (attr.d.len > attrs[j].size) {
return LFS_ERR_RANGE;
}

err = lfs_dir_get(lfs, dir,
entry->off+off+i+sizeof(attr.d), buffer, attr.d.len);
if (err) {
return err;
err = lfs_dir_get(lfs, dir,
entry->off+off+i+sizeof(attr.d),
attrs[j].buffer, attr.d.len);
if (err) {
return err;
}
}
}

return attr.d.len;
}

return LFS_ERR_NODATA;
return 0;
}

static int lfs_dir_setattr(lfs_t *lfs,
static lfs_ssize_t lfs_dir_checkattrs(lfs_t *lfs,
lfs_dir_t *dir, lfs_entry_t *entry,
uint8_t type, const void *buffer, lfs_size_t size) {
// search for attribute in attribute region
lfs_off_t off = sizeof(dir->d) + lfs_entry_elen(entry);
lfs_off_t i = 0;
lfs_size_t oldlen = 0;
while (i < lfs_entry_alen(entry)) {
lfs_attr_t attr;
const struct lfs_attr *attrs, int count) {
// check that attributes fit
lfs_size_t nsize = 0;
for (int j = 0; j < count; j++) {
nsize += 2+attrs[j].size;
}

lfs_off_t off = 4+lfs_entry_elen(entry);
lfs_entry_attr_t attr;
for (lfs_off_t i = 0; i < lfs_entry_alen(entry); i += 2+attr.d.len) {
int err = lfs_dir_get(lfs, dir,
entry->off+off+i, &attr.d, sizeof(attr.d));
if (err) {
return err;
}

if (attr.d.type != type) {
i += attr.d.len;
continue;
bool updated = false;
for (int j = 0; j < count; j++) {
if (attr.d.type == attrs[j].type) {
updated = true;
}
}

oldlen = attr.d.len;
break;
if (!updated) {
nsize += 2+attr.d.len;
}
}

// make sure the attribute fits
if (lfs_entry_elen(entry) - oldlen + size > lfs->attrs_size ||
(0x7fffffff & dir->d.size) - oldlen + size > lfs->cfg->block_size) {
if (nsize > lfs->attrs_size || (
(0x7fffffff & dir->d.size) + lfs_entry_size(entry) -
lfs_entry_alen(entry) + nsize > lfs->cfg->block_size)) {
return LFS_ERR_NOSPC;
}

lfs_attr_t attr;
attr.d.type = type;
attr.d.len = size;
int err = lfs_dir_set(lfs, dir, entry, (struct lfs_region[]){
{LFS_FROM_MEM, off+i, &attr.d, sizeof(attr.d)},
{LFS_FROM_MEM, off+i, buffer, size},
{LFS_FROM_DROP, off+i, NULL, -oldlen}}, 3);
if (err) {
return err;
}

return 0;
return nsize;
}

static int lfs_dir_removeattr(lfs_t *lfs,
lfs_dir_t *dir, lfs_entry_t *entry, uint8_t type) {
// search for attribute in attribute region
lfs_off_t off = sizeof(dir->d) + lfs_entry_elen(entry);
lfs_off_t i = 0;
while (i < lfs_entry_alen(entry)) {
lfs_attr_t attr;
int err = lfs_dir_get(lfs, dir,
entry->off+off+i, &attr.d, sizeof(attr.d));
if (err) {
return err;
}

if (attr.d.type != type) {
i += attr.d.len;
continue;
}

err = lfs_dir_set(lfs, dir, entry, (struct lfs_region[]){
{LFS_FROM_DROP, off+i,
NULL, -(sizeof(attr.d)+attr.d.len)}}, 1);
if (err) {
return err;
}

return 0;
static int lfs_dir_setattrs(lfs_t *lfs,
lfs_dir_t *dir, lfs_entry_t *entry,
const struct lfs_attr *attrs, int count) {
// make sure attributes fit
lfs_ssize_t nsize = lfs_dir_checkattrs(lfs, dir, entry, attrs, count);
if (nsize < 0) {
return nsize;
}

return LFS_ERR_NODATA;
// commit to entry, majority of work is in LFS_FROM_ATTRS
lfs_size_t oldlen = lfs_entry_alen(entry);
entry->d.alen = (0xc0 & entry->d.alen) | nsize;
return lfs_dir_set(lfs, dir, entry, (struct lfs_region[]){
{LFS_FROM_MEM, 0, &entry->d, 4},
{LFS_FROM_DROP, 0, NULL, -4},
{LFS_FROM_ATTRS, 4+lfs_entry_elen(entry),
&(struct lfs_attrs_region){attrs, count, oldlen}, nsize}}, 3);
}



/// Top level directory operations ///
int lfs_mkdir(lfs_t *lfs, const char *path) {
// deorphan if we haven't yet, needed at most once after poweron
Expand Down Expand Up @@ -2372,25 +2432,8 @@ int lfs_rename(lfs_t *lfs, const char *oldpath, const char *newpath) {


/// Attribute operations ///
int lfs_getattr(lfs_t *lfs, const char *path,
uint8_t type, void *buffer, lfs_size_t size) {
lfs_dir_t cwd;
int err = lfs_dir_fetch(lfs, &cwd, lfs->root);
if (err) {
return err;
}

lfs_entry_t entry;
err = lfs_dir_find(lfs, &cwd, &entry, &path);
if (err) {
return err;
}

return lfs_dir_getattr(lfs, &cwd, &entry, type, buffer, size);
}

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) {
lfs_dir_t cwd;
int err = lfs_dir_fetch(lfs, &cwd, lfs->root);
if (err) {
Expand All @@ -2403,10 +2446,11 @@ int lfs_setattr(lfs_t *lfs, const char *path,
return err;
}

return lfs_dir_setattr(lfs, &cwd, &entry, type, buffer, size);
return lfs_dir_getattrs(lfs, &cwd, &entry, attrs, count);
}

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) {
lfs_dir_t cwd;
int err = lfs_dir_fetch(lfs, &cwd, lfs->root);
if (err) {
Expand All @@ -2419,7 +2463,7 @@ int lfs_removeattr(lfs_t *lfs, const char *path, uint8_t type) {
return err;
}

return lfs_dir_removeattr(lfs, &cwd, &entry, type);
return lfs_dir_setattrs(lfs, &cwd, &entry, attrs, count);
}


Expand Down
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