Use a single Mill server process #5066
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Fixes #5031
We now only have a single server process running at a time, and no longer keep up to 5 separate
serverDir-1toserverDir-5folders. Essentially we are migrating from the style of concurrency used bygradleandmvndto the style of concurrency used bybazelWe now only hold the
clientLockuntil the server process is initialized (and has taken it'sprocessLock) to avoid redundant initialization, and after that we release it to allow multiple clients to connectSince there is only one server and one
serverDir, we have to remove several per-server files since there is no way to ensure they are unique in the presence of multiple clientsrunArgscan no longer be a file, and instead we send its contents over the socket before starting theInputPumperWe can no longer rely on
clientLockprobing to check if the client has disconnected, and so we instead have the server sendProxyStream.HEARTBEATbytes (127) and look for errorsServer#runcan no longer accept socket connections and process them in a single-threaded loop, and instead needs to spawn off threads to handle each connected socket--watchmode and idle we want to allow another client to proceed with execution. So locking needs to be done at a finer-grained granularity within the serverThe original
millLockfile (renamedmillOutLock) has been combined with aMemoryLockviaDoubleLock, which lets us check the memory-lock first to avoid taking the file-lock twice on the same JVM and causing aOverlappingFileLockException. We still need to keep the file-lock to provide mutex between server and no-server processes, or between multiple no-server processesNow that things are in one process, that opens up the possibility of loosening the concurrency constraints in future, e.g. allowing multiple concurrent evaluations that do not interfere with each other.