Create BuildSteps race-free

Dependencies

• [2] KBZHIGLG Record the machine used for a build step
• [3] 5AIYUMTB Basic remote building
• [4] 24BMQDZA Start of single-process hydra-queue-runner
• [5] ENXUSMSV Make concurrency more robust

Change contents

• edit in src/hydra-queue-runner/hydra-queue-runner.cc at line 361
  [3.8256]

  [3.8256]
  + /* Acquire an exclusive lock on BuildSteps to ensure that we don't

  + race with other threads creating a step of the same build. */

  + txn.exec("lock table BuildSteps in exclusive mode");

  +
• edit in src/hydra-queue-runner/hydra-queue-runner.cc at line 899
  [3.16702]→[2.1232:1366](∅→∅),[2.1366]→[3.16786:16787](∅→∅),[3.14169]→[3.16786:16787](∅→∅),[3.16786]→[3.16786:16787](∅→∅)
  − finishBuildStep(txn, result.startTime, result.stopTime, build->id, stepNr, machine->sshName, bssFailed, result.errorMsg);

  −
• edit in src/hydra-queue-runner/hydra-queue-runner.cc at line 903
  [3.16983]

  [3.16983]
  +

  + finishBuildStep(txn, result.startTime, result.stopTime, build->id, stepNr, machine->sshName, bssFailed, result.errorMsg);