Free space of overwritten files in LittleFS (esp8266#7434)

* Free space of overwritten files in LittleFS

Fixes esp8266#7426

LittleFS doesn't update the on-flash data structures when a file is
reopened as O_TRUNC until the file is closed.  This means the space of
the original, inaccessible file cannot be used, causing OOS errors in
cases when a large file is being overwritten.

Explicitly call the file sync operation to update the on-flash metadata
as soon as a file is opened.  For normal files it's a no-op, but for
O_TRUNC modes it will free the space, allowing full overwrite of large
files.

* Add host test case for change

libraries/LittleFS/src/LittleFS.cpp

@@ -87,6 +87,7 @@ FileImplPtr LittleFSImpl::open(const char* path, OpenMode openMode, AccessMode a
         // a directory whose name we are carrying around but which cannot be read or written
         return std::make_shared<LittleFSFileImpl>(this, path, nullptr, flags, creation);
     } else if (rc == 0) {
+        lfs_file_sync(&_lfs, fd.get());
         return std::make_shared<LittleFSFileImpl>(this, path, fd, flags, creation);
     } else {
         DEBUGV("LittleFSDirImpl::openFile: rc=%d fd=%p path=`%s` openMode=%d accessMode=%d err=%d\n",

tests/host/fs/test_fs.inc

@@ -198,6 +198,28 @@ TEST_CASE(TESTPRE "seek() pase EOF returns error (#7323)", TESTPAT)
     f.close();
 }
 
+TEST_CASE(TESTPRE "Rewriting file frees space immediately (#7426)", TESTPAT)
+{
+    FS_MOCK_DECLARE(64, 8, 512, "");
+    REQUIRE(FSTYPE.begin());
+    FSInfo inf;
+    FSTYPE.info(inf);
+    // Calculate the size to write per-FS, due to differing overheads
+    int kbToWrite = (inf.totalBytes - inf.usedBytes - 8192) / 1024;
+    // Create and overwrite a file >50% of spaceA (48/64K)
+    for (auto x = 0; x < 2; x++) {
+        auto f = FSTYPE.open("/file1.bin", "w");
+        REQUIRE(f);
+        uint8_t buff[1024];
+        memset(buff, 0xaa, 1024);
+        for (auto i = 0; i < kbToWrite; i++) {
+            REQUIRE(f.write(buff, 1024));
+        }
+        f.close();
+        FSTYPE.info(inf);
+    }
+}
+
 #ifdef FS_HAS_DIRS
 
 #if FSTYPE != SDFS