{"action":"opened","number":3509,"pull_request":{"url":"https://api.github.com/repos/haskell/cabal/pulls/3509","id":75218983,"html_url":"https://github.com/haskell/cabal/pull/3509","diff_url":"https://github.com/haskell/cabal/pull/3509.diff","patch_url":"https://github.com/haskell/cabal/pull/3509.patch","issue_url":"https://api.github.com/repos/haskell/cabal/issues/3509","number":3509,"state":"open","locked":false,"title":"Fix issue 3460: avoid concurrent package db creation","user":{"login":"dcoutts","id":1339423,"avatar_url":"https://avatars.githubusercontent.com/u/1339423?v=3","gravatar_id":"","url":"https://api.github.com/users/dcoutts","html_url":"https://github.com/dcoutts","followers_url":"https://api.github.com/users/dcoutts/followers","following_url":"https://api.github.com/users/dcoutts/following{/other_user}","gists_url":"https://api.github.com/users/dcoutts/gists{/gist_id}","starred_url":"https://api.github.com/users/dcoutts/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/dcoutts/subscriptions","organizations_url":"https://api.github.com/users/dcoutts/orgs","repos_url":"https://api.github.com/users/dcoutts/repos","events_url":"https://api.github.com/users/dcoutts/events{/privacy}","received_events_url":"https://api.github.com/users/dcoutts/received_events","type":"User","site_admin":false},"body":"See issue #3460. Avoids things like\r\n```\r\nghc-pkg: cannot create: /path/to/main-package/dist-newstyle/packagedb/ghc-7.10.3 already exists\r\n```\r\n\r\nPreviously, the package dbs were initialised as part of building individual packages, and these actions can execute concurrently for parallel builds. This lead to clashes because the package dbs to create were typically shared between several packages the creation was not safe to run concurrently.\r\n\r\nSo instead we initialise all the ones we'll need in advance of building any packages. The alternative would have been to use a lock, but there's no particular advantage in trying to delay initialisation and avoiding locks keeps things simpler.\r\n\r\nThis should fix #3460\r\n\r\nAnother similar issue was fixed by 1acc00f which serialised the registration of packages in the local inplace package db, avoiding lost updates (which could previously be observed in test case T3460)\r\n\r\nThanks to @ezyang for the test 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