[fs][fat] fix false EOF error on cluster-aligned reads

lib/fs/fat/file.cpp

@@ -194,9 +194,11 @@ ssize_t fat_file::read_file_priv(void *_buf, const off_t offset, size_t len) {
         if (offset_within_sector == fs_->info().bytes_per_sector) {
             offset_within_sector = 0;
             // push the iterator forward to next sector
-            err = fbi.next_sector();
-            if (err < 0) {
-                return err;
+            if (buf_offset < len) {
+                err = fbi.next_sector();
+                if (err < 0) {
+                    return err;
+                }
             }
         }
     }

lib/fs/fat/test/mkblk

@@ -31,6 +31,8 @@ for i in blk.bin.*; do
     mmd -i $i dir.c
     mcopy -i $i hello.txt ::dir.a/long_filename_hello.txt
     mcopy -i $i largefile ::largefile
+    mcopy -i $i test_4kb.bin ::test_4kb.bin
+    mcopy -i $i test_8kb.bin ::test_8kb.bin
 
     # add a bunch of entries to the root dir that cause it
     # to spill over to a few extra sectors

lib/fs/fat/test/test.cpp

@@ -26,6 +26,8 @@
 // pull in a few test files into rodata to test against
 INCFILE(test_file_hello, test_file_hello_size, LOCAL_DIR "/hello.txt");
 INCFILE(test_file_license, test_file_license_size, LOCAL_DIR "/LICENSE");
+INCFILE(test_file_4kb, test_file_4kb_size, LOCAL_DIR "/test_4kb.bin");
+INCFILE(test_file_8kb, test_file_8kb_size, LOCAL_DIR "/test_8kb.bin");
 
 namespace {
 
@@ -171,6 +173,8 @@ bool test_fat_read_file() {
     EXPECT_TRUE(test_file_read(test_path "/long_filename_hello.txt", test_file_hello, test_file_hello_size));
     EXPECT_TRUE(test_file_read(test_path "/a_very_long_filename_hello_that_uses_at_least_a_few_entries.txt", test_file_hello, test_file_hello_size));
     EXPECT_TRUE(test_file_read(test_path "/dir.a/long_filename_hello.txt", test_file_hello, test_file_hello_size));
+    EXPECT_TRUE(test_file_read(test_path "/test_4kb.bin", test_file_4kb, test_file_4kb_size));
+    EXPECT_TRUE(test_file_read(test_path "/test_8kb.bin", test_file_8kb, test_file_8kb_size));
 
     // unmount the fs
     unmount_cleanup.cancel();