Documentation of algorithm constraints and fail on their violation

[nightlyone]

Without the specific atomicity guarantees of setting a hard link and rename a file in a directory the whole locking beaks. I could be more friendly to the users by documenting these constraints better and being more specific about which errors I ignore.

Also fail properly on unexpected errors when creating the hardlink.

[matjam]

Is the following a correct explanation of the semantics?

1. Create a file at path dir+"."+name called tmplock. Fail if unable to create. (potential race condition here, I suggest it be dir+"."+name+random_stuff_or_pid_or_something)
2. Write the current pid to tmplock, fail if unable to write.
3. Hard link tmplock to dir+name. Ignore failure to link.
4. Stat tmplock by name (should still be dir+"."+name). It should still exist, error if not.
5. Stat the real lock dir+name. Should exist. Error if doesn't.
6. Make sure that both the tmplock and the new lock are the same file.
7. Get the owner of the lock.
   etc

[nightlyone]

@matjam Thanks for taking the time to analyse the code deeply.

Is the following a correct explanation of the semantics?

1. Create a file at path dir+"."+name called tmplock. Fail if unable to create. (potential race condition here, I suggest it be dir+"."+name+random_stuff_or_pid_or_something)

ioutil.TempFile takes care of the "random_stuff_or_pid_or_something" for me. So consider it done 🙂

2. Write the current pid to tmplock, fail if unable to write.

Yes.

3. Hard link tmplock to dir+name. Ignore failure to link.

Your observation of the existing code is correct.

Intention here was to only ignore the failure to create a link due to the destination already existing.
Ignoring every error here was the bug that made analysis of golang/dep#913 harder than necessary. This imprecise error handling is fixed by #21 .

4. Stat tmplock by name (should still be dir+"."+name). It should still exist, error if not.
5. Stat the real lock dir+name. Should exist. Error if doesn't.
6. Make sure that both the tmplock and the new lock are the same file.
7. Get the owner of the lock.
   etc

correct.