Added sticky-bit for preventing file syncs after write errors

Short story, files are no longer committed to directories during
file sync/close if the last write did not complete successfully.
This avoids a set of interesting user-experience issues related
to the end-of-life behaviour of the filesystem.

As a filesystem approaches end-of-life, the chances of running into
LFS_ERR_NOSPC grows rather quickly. Since this condition occurs after
at the end of a devices life, it's likely that operating in these
conditions hasn't been tested thoroughly.

In the specific case of file-writes, you can hit an LFS_ERR_NOSPC after
parts of the file have been written out. If the program simply continues
and closes the file, the file is written out half completed. Since
littlefs has a strong garuntee the prevents half-writes, it's unlikely
this state of the file would be expected.

To make things worse, since close is also responsible for memory
cleanup, it's actually _impossible_ to continue working as it was
without leaking memory.

By prevent the file commits, end-of-life behaviour should at least retain
a previous copy of the filesystem without any surprises.

tests/test_alloc.sh

@@ -121,6 +121,7 @@ tests/test.py << TEST
     size = strlen("exhaustion");
     memcpy(buffer, "exhaustion", size);
     lfs_file_write(&lfs, &file[0], buffer, size) => size;
+    lfs_file_sync(&lfs, &file[0]) => 0;
 
     size = strlen("blahblahblahblah");
     memcpy(buffer, "blahblahblahblah", size);
@@ -142,6 +143,7 @@ tests/test.py << TEST
     lfs_mount(&lfs, &cfg) => 0;
     lfs_file_open(&lfs, &file[0], "exhaustion", LFS_O_RDONLY);
     size = strlen("exhaustion");
+    lfs_file_size(&lfs, &file[0]) => size;
     lfs_file_read(&lfs, &file[0], buffer, size) => size;
     memcmp(buffer, "exhaustion", size) => 0;
     lfs_file_close(&lfs, &file[0]) => 0;
@@ -166,6 +168,7 @@ tests/test.py << TEST
     size = strlen("exhaustion");
     memcpy(buffer, "exhaustion", size);
     lfs_file_write(&lfs, &file[0], buffer, size) => size;
+    lfs_file_sync(&lfs, &file[0]) => 0;
 
     size = strlen("blahblahblahblah");
     memcpy(buffer, "blahblahblahblah", size);
@@ -187,6 +190,7 @@ tests/test.py << TEST
     lfs_mount(&lfs, &cfg) => 0;
     lfs_file_open(&lfs, &file[0], "exhaustion", LFS_O_RDONLY);
     size = strlen("exhaustion");
+    lfs_file_size(&lfs, &file[0]) => size;
     lfs_file_read(&lfs, &file[0], buffer, size) => size;
     memcmp(buffer, "exhaustion", size) => 0;
     lfs_file_close(&lfs, &file[0]) => 0;
@@ -196,14 +200,14 @@ TEST
 echo "--- Dir exhaustion test ---"
 tests/test.py << TEST
     lfs_mount(&lfs, &cfg) => 0;
-    lfs_stat(&lfs, "exhaustion", &info) => 0;
-    lfs_size_t fullsize = info.size;
     lfs_remove(&lfs, "exhaustion") => 0;
 
     lfs_file_open(&lfs, &file[0], "exhaustion", LFS_O_WRONLY | LFS_O_CREAT);
     size = strlen("blahblahblahblah");
     memcpy(buffer, "blahblahblahblah", size);
-    for (lfs_size_t i = 0; i < fullsize - 2*512; i += size) {
+    for (lfs_size_t i = 0;
+            i < (cfg.block_count-6)*(cfg.block_size-8);
+            i += size) {
         lfs_file_write(&lfs, &file[0], buffer, size) => size;
     }
     lfs_file_close(&lfs, &file[0]) => 0;
@@ -214,7 +218,11 @@ tests/test.py << TEST
     lfs_file_open(&lfs, &file[0], "exhaustion", LFS_O_WRONLY | LFS_O_APPEND);
     size = strlen("blahblahblahblah");
     memcpy(buffer, "blahblahblahblah", size);
-    lfs_file_write(&lfs, &file[0], buffer, size) => size;
+    for (lfs_size_t i = 0;
+            i < (cfg.block_size-8);
+            i += size) {
+        lfs_file_write(&lfs, &file[0], buffer, size) => size;
+    }
     lfs_file_close(&lfs, &file[0]) => 0;
 
     lfs_mkdir(&lfs, "exhaustiondir") => LFS_ERR_NOSPC;
@@ -224,14 +232,14 @@ TEST
 echo "--- Chained dir exhaustion test ---"
 tests/test.py << TEST
     lfs_mount(&lfs, &cfg) => 0;
-    lfs_stat(&lfs, "exhaustion", &info) => 0;
-    lfs_size_t fullsize = info.size;
-
     lfs_remove(&lfs, "exhaustion") => 0;
+
     lfs_file_open(&lfs, &file[0], "exhaustion", LFS_O_WRONLY | LFS_O_CREAT);
     size = strlen("blahblahblahblah");
     memcpy(buffer, "blahblahblahblah", size);
-    for (lfs_size_t i = 0; i < fullsize - 19*512; i += size) {
+    for (lfs_size_t i = 0;
+            i < (cfg.block_count-24)*(cfg.block_size-8);
+            i += size) {
         lfs_file_write(&lfs, &file[0], buffer, size) => size;
     }
     lfs_file_close(&lfs, &file[0]) => 0;
@@ -247,7 +255,9 @@ tests/test.py << TEST
     lfs_file_open(&lfs, &file[0], "exhaustion", LFS_O_WRONLY | LFS_O_CREAT);
     size = strlen("blahblahblahblah");
     memcpy(buffer, "blahblahblahblah", size);
-    for (lfs_size_t i = 0; i < fullsize - 20*512; i += size) {
+    for (lfs_size_t i = 0;
+            i < (cfg.block_count-26)*(cfg.block_size-8);
+            i += size) {
         lfs_file_write(&lfs, &file[0], buffer, size) => size;
     }
     lfs_file_close(&lfs, &file[0]) => 0;