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| -rw-r--r-- | CHANGELOG.md | 3 | ||||
| -rw-r--r-- | doc/future-designs/hardlink.md | 74 |
2 files changed, 3 insertions, 74 deletions
diff --git a/CHANGELOG.md b/CHANGELOG.md index 5d67d6d6..4c16fbb5 100644 --- a/CHANGELOG.md +++ b/CHANGELOG.md @@ -20,6 +20,9 @@ A feature release on top of `1.3.0`, backwards compatible. - Dependencies have been updated to also build with gcc 14. - Portability improvements of the code by not relying on implementation details of the compiler. +- Local execution no longer has the requirement that there exist no + more files with identical content than the hardlink limit of the + underlying file system. - Various improvements of the documentation. ## Release `1.3.0` (2024-05-08) diff --git a/doc/future-designs/hardlink.md b/doc/future-designs/hardlink.md deleted file mode 100644 index e415c3ea..00000000 --- a/doc/future-designs/hardlink.md +++ /dev/null @@ -1,74 +0,0 @@ -# Living with hardlink limit - -## Current state and shortcomings - -The build tool stores all files in a content-addressable store. -For each action, a fresh action directory is created in input files -are hard-linked from the content-addressable store to the action -directory. Output files are linked back into the content-addressable -store (after fixing permissions and time stamp) before the action -directory is removed. - -This has the consequence that the link count of a file in CAS -is (essentially) the number of times a file with that content is -in use in currently running actions. Now, on some file systems -the maximal link count can be as low as 2**16, i.e., 64k. However, -it is not an uncommon request to rerun certain test cases 100k -times (especially when using remote execution) to look for rare -race conditions. In this case, the action summarizing the various -test runs (which is especially important for that large number of -tests) has all the test output and results as input, including a -large number of test runs with the same output state (like `PASS`), -thus hitting the maximal link count. - -## Proposed solution - -### Changes to the setup of local actions - -So far, when creating a link as part of setting up a local action -fails, the whole setup up aborted (see `LocalAction::StageInput`) -and the `LocalAction::Run` method returns `std::nullopt` indicating -that it failed to run the action at all. This will be changed in -the following way. - - - The `FileManager::CreateHardLink` function distinguishes in - its return value between failure due to reaching the hardlink - limit (indicated by `errno` being set to `EMLINK` in the underlying - `link`(2) call) and other forms of failure. - - - When creation of a hard link fails in `LocalAction::StageInput` - due to the hardlink limit, a copy of the file is created with - correct permission and timestamp (and, if executable, in a dedicated - subprocess to avoid file descriptors to the file outliving the - copying); this copy is created in a temporary directory with life - time being the creation of the action directory (or reaching of - the hardlink limit again, whatever is earlier, see next item). - This copy is then hard linked into the action directory. - - - To avoid copying the same file over and over again, a reference to - mutable map in the scope of `LocalAction::CreateDirectoryStructure` - is passed to all calls of `LocalAction::StageInput`. This map maps - `Artifact::ObjectInfo` to the copy of the file to be used, if it - was copied already; it also keeps the ephemeral directories for - those copies alive. When creating a hardlink fails, regardless - of whether the file to be hardlinked is in CAS or already a - temporary copy, a copy is created and the entry in that map - updated accordingly. This might get some ephemeral directories - out of scope (and hence being cleaned up) that host a file where - the hardlink limit is already reached; the exisitng hard links - will keep the file alive on disk as long as needed. As creating - the action directory is a single-threaded operation, this inplace - update of the map to file copies is a safe operation. - -### Rules changes - -The changes to the way justbuild handles the setup of action -directories works around the hardlink limit for actions executed -by justbuild itself, i.e., for local execution and actions executed -by a `just execute` instance. It does not change, however, the -way other remote-execution services work. So, in order to allow -delegating action that are expected to require large link counts (like -the test-summary action) to a suitable execution endpoint, rules -will honor appropriate configuration variables to add additional -remote-execution properties to those actions, in the same way, our -typesetting rules do this already. |