Fixed another subtle corner cases with noop sync broadcasting

This is an extension of the noop-sync after unrelated write-sync after
desync corner case:

  op                 a state         b state
                     in-sync         in-sync
  desync(b)          in-sync         desync
  write(a)           unsync          desync
  sync(a)            in-sync'        desync
  sync(b)            in-sync         in-sync

But instead of explicitly calling lfsr_file_sync, what if you implicitly
triggered sync through something like a write on a file with the
LFS_O_SYNC flag, but not a normal write, a noop write, write(0)?

If the definition of LFS_O_SYNC is taken literally as "lfsr_file_write
and friends implicitly call lfsr_file_sync after every call", then this
should behave just as if lfsr_file_sync had been called, and
unconditionally broadcast the sync. Since this is the simplest
interpretation, I think this is what we should implement.

Added tests, and adopted this behavior. Fortunately this just involves
some small gotos (https://xkcd.com/292):

            code          stack
  before:  33020           2976
  after:   33026 (+0.0%)   2976 (+0.0%)

tests/test_fsync.toml

@@ -2085,6 +2085,124 @@ code = '''
     lfsr_unmount(&lfs) => 0;
 '''
 
+[cases.test_fsync_desync_wdwdrr_noop_creative]
+# there's several clever ways we can noop...
+# TODO flush()?
+# 0 => sync()
+# 1 => write(0)
+# 2 => truncate(size)
+# 3 => fruncate(size)
+defines.NOOP = [0, 1, 2, 3]
+code = '''
+    lfs_t lfs;
+    lfsr_format(&lfs, CFG) => 0;
+    lfsr_mount(&lfs, CFG) => 0;
+
+    // a - writer
+    // b - syncing writer
+    // c - reader kept open, recvs updates from a
+    // d - reader kept closed, checks disk state
+    lfsr_file_t a;
+    lfsr_file_t b;
+    lfsr_file_t c;
+    lfsr_file_t d;
+    uint8_t rbuf[256];
+    lfsr_file_open(&lfs, &a, "jello",
+            LFS_O_WRONLY | LFS_O_CREAT | LFS_O_EXCL) => 0;
+    lfsr_file_open(&lfs, &b, "jello", LFS_O_WRONLY | LFS_O_SYNC) => 0;
+    lfsr_file_open(&lfs, &c, "jello", LFS_O_RDONLY) => 0;
+
+    // write to a and sync
+    lfsr_file_write(&lfs, &a, "hello!", strlen("hello!"))
+            => strlen("hello!");
+    lfsr_file_sync(&lfs, &a) => 0;
+
+    // our write should show up in c
+    lfsr_file_read(&lfs, &c, rbuf, sizeof(rbuf)) => strlen("hello!");
+    assert(memcmp(rbuf, "hello!", strlen("hello!")) == 0);
+    // and on disk
+    lfsr_file_open(&lfs, &d, "jello", LFS_O_RDONLY) => 0;
+    lfsr_file_read(&lfs, &d, rbuf, sizeof(rbuf)) => strlen("hello!");
+    assert(memcmp(rbuf, "hello!", strlen("hello!")) == 0);
+    lfsr_file_close(&lfs, &d) => 0;
+
+    // mark b as desync, this should freeze its contents
+    lfsr_file_desync(&lfs, &b) => 0;
+    // rewrite a, sync
+    lfsr_file_rewind(&lfs, &a) => 0;
+    lfsr_file_write(&lfs, &a, "bonjour!", strlen("bonjour!"))
+            => strlen("bonjour!");
+    lfsr_file_sync(&lfs, &a) => 0;
+
+    // a should show up in c
+    lfsr_file_rewind(&lfs, &c) => 0;
+    lfsr_file_read(&lfs, &c, rbuf, sizeof(rbuf)) => strlen("bonjour!");
+    assert(memcmp(rbuf, "bonjour!", strlen("bonjour!")) == 0);
+    // and on disk
+    lfsr_file_open(&lfs, &d, "jello", LFS_O_RDONLY) => 0;
+    lfsr_file_read(&lfs, &d, rbuf, sizeof(rbuf)) => strlen("bonjour!");
+    assert(memcmp(rbuf, "bonjour!", strlen("bonjour!")) == 0);
+    lfsr_file_close(&lfs, &d) => 0;
+
+    // sync b, _creatively_, dragons be here
+    if (NOOP == 0) {
+        lfsr_file_sync(&lfs, &b) => 0;
+    } else if (NOOP == 1) {
+        lfsr_file_write(&lfs, &b, NULL, 0) => 0;
+    } else if (NOOP == 2) {
+        lfsr_file_size(&lfs, &b) => strlen("hello!");
+        lfsr_file_truncate(&lfs, &b, strlen("hello!")) => 0;
+    } else if (NOOP == 3) {
+        lfsr_file_size(&lfs, &b) => strlen("hello!");
+        lfsr_file_fruncate(&lfs, &b, strlen("hello!")) => 0;
+    }
+
+    // b should show up in c, without a's changes
+    lfsr_file_rewind(&lfs, &c) => 0;
+    lfsr_file_read(&lfs, &c, rbuf, sizeof(rbuf)) => strlen("hello!");
+    assert(memcmp(rbuf, "hello!", strlen("hello!")) == 0);
+    // and on disk
+    lfsr_file_open(&lfs, &d, "jello", LFS_O_RDONLY) => 0;
+    lfsr_file_read(&lfs, &d, rbuf, sizeof(rbuf)) => strlen("hello!");
+    assert(memcmp(rbuf, "hello!", strlen("hello!")) == 0);
+    lfsr_file_close(&lfs, &d) => 0;
+
+    // rewrite b, close, desync flag should have been cleared
+    lfsr_file_rewind(&lfs, &b) => 0;
+    lfsr_file_write(&lfs, &b, "ohayo!", strlen("ohayo!"))
+            => strlen("ohayo!");
+    lfsr_file_close(&lfs, &b) => 0;
+
+    // our write should show up in c
+    lfsr_file_rewind(&lfs, &c) => 0;
+    lfsr_file_read(&lfs, &c, rbuf, sizeof(rbuf)) => strlen("ohayo!");
+    assert(memcmp(rbuf, "ohayo!", strlen("ohayo!")) == 0);
+    // and on disk
+    lfsr_file_open(&lfs, &d, "jello", LFS_O_RDONLY) => 0;
+    lfsr_file_read(&lfs, &d, rbuf, sizeof(rbuf)) => strlen("ohayo!");
+    assert(memcmp(rbuf, "ohayo!", strlen("ohayo!")) == 0);
+    lfsr_file_close(&lfs, &d) => 0;
+
+    // rewrite a, close
+    lfsr_file_rewind(&lfs, &a) => 0;
+    lfsr_file_write(&lfs, &a, "zdrasti!", strlen("zdrasti!"))
+            => strlen("zdrasti!");
+    lfsr_file_close(&lfs, &a) => 0;
+
+    // our write should show up in c
+    lfsr_file_rewind(&lfs, &c) => 0;
+    lfsr_file_read(&lfs, &c, rbuf, sizeof(rbuf)) => strlen("zdrasti!");
+    assert(memcmp(rbuf, "zdrasti!", strlen("zdrasti!")) == 0);
+    // and on disk
+    lfsr_file_open(&lfs, &d, "jello", LFS_O_RDONLY) => 0;
+    lfsr_file_read(&lfs, &d, rbuf, sizeof(rbuf)) => strlen("zdrasti!");
+    assert(memcmp(rbuf, "zdrasti!", strlen("zdrasti!")) == 0);
+    lfsr_file_close(&lfs, &d) => 0;
+
+    lfsr_file_close(&lfs, &c) => 0;
+    lfsr_unmount(&lfs) => 0;
+'''
+
 [cases.test_fsync_desync_wdwdrr_append]
 code = '''
     lfs_t lfs;