Fix publishing deadlock

Requirements

All new code should be covered with tests, documentation should be updated. CI should pass.

Description of the Change

A race condition for publishing packages and mirrors at the same time was introduced in commit 77d7c38.

The problem is that when opening a leveldb transaction and performing another 'put' to the db the system freezes.

See this sample code for explanation:

package main

import (
    "fmt"

    "github.com/syndtr/goleveldb/leveldb"
)

func main() {
    db, err := leveldb.OpenFile("whatever", nil)
    if err != nil {
        panic(err)
    }
    defer db.Close()

    transaction, err := db.OpenTransaction()
    if err != nil {
        panic(err)
    }
    defer transaction.Discard()

    fmt.Println("Putting A1")
    transaction.Put([]byte("A1"), []byte("value"), nil)

    fmt.Println("Putting A2 (non transaction)")
    db.Put([]byte("A2"), []byte("value"), nil) // <-- will freeze here!
    fmt.Println("Finished putting A2")

    fmt.Println("Committing transaction")
    err = transaction.Commit()
    if err != nil {
        panic(err)
    }
}

The problem is actually not in goleveldb, but rather in the aptly pool lock: https://github.com/aptly-dev/aptly/blob/master/files/package_pool.go#L234

1) [THREAD-1] Creates transaction 2) [THREAD-2] Mirror publishes (pool gets locked) -> publish is waiting on db.Put(...) for transaction to finish. 3) [THREAD-1] Tries to lock the pool (wich is already locked by [THREAD-2] that is waiting for the current transaciton to finish). dead lock

Checklist

[ ] unit-test added (if change is algorithm)
[ ] functional test added/updated (if change is functional)
[ ] man page updated (if applicable)
[ ] bash completion updated (if applicable)
[ ] documentation updated
[ ] author name in AUTHORS

aptly-dev / aptly