How can we avoid delete/create inconsistency?

This is a file system note: @MIT 6.824 2006 Lecture 6

Think this satiation,

unlink("f1");
create("f2");
Create happens to re-use the i-node freed by the unlink.
suppose only create write goes to disk, but none of the unlink's writes.

Crash.

After re-start, what does recovery see?

The file system looks correct! Nothing to fix!
But file f1 actually has file f2's contents!

Serious *undetected* inconsisency.

This is *not* a state the file system counld have been in if the crash had occured slightly earlier or later. And fsck did not notify the user there was an unfixable problem!

How can we avoid this delete/create inconsistency?

Observation: We only care about what's visible in the file system tree.

Goal: on-disk directory entry must always point to correct on-disk i-node.

Unlink Rule: remove dirent *on disk* before freeing i-node.

Create Rule: initialize new i-node *on disk* before creating directory entry.

In general, directory entry writes should be commit points.
Crash just before leves us with unused allocated i-node.
Crash just after is fine.