Don't lock the BuildSteps table when inserting

Dependencies

• [2] LVQXQIYA Kill active build steps when builds are cancelled
• [3] NTEDD7T4 Provide a plugin hook for when build steps finish
• [4] KBZHIGLG Record the machine used for a build step
• [5] 62MQPRXC Pass null values to libpqxx properly
• [6] LJILHOJ7 Create BuildSteps race-free
• [7] 24BMQDZA Start of single-process hydra-queue-runner
• [8] MSIHMO45 Tweak build steps
• [9] HUUZFPPK Fix race between the queue monitor and the builder threads
• [10] VQISTKOP hydra-queue-runner: Use substitutes
• [*] UQQ4IL55 Add a error type for "unsupported system type"

Change contents

• edit in src/hydra-queue-runner/hydra-queue-runner.cc at line 283
  [3.8256]→[3.0:198](∅→∅)
  − /* 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 291
  [3.339]

  [2.2914]
  + restart:
• replacement in src/hydra-queue-runner/hydra-queue-runner.cc at line 294
  [3.8432]→[3.93:115](∅→∅),[3.115]→[3.375:578](∅→∅)
  − txn.parameterized

  − ("insert into BuildSteps (build, stepnr, type, drvPath, busy, startTime, system, status, propagatedFrom, errorMsg, stopTime, machine) values ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12)")
  [3.8432]

  [2.2963]
  + auto r = txn.parameterized

  + ("insert into BuildSteps (build, stepnr, type, drvPath, busy, startTime, system, status, propagatedFrom, errorMsg, stopTime, machine) values ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12) on conflict do nothing")
• edit in src/hydra-queue-runner/hydra-queue-runner.cc at line 308
  [12.75]

  [3.8996]
  +

  + if (r.affected_rows() == 0) goto restart;
• edit in src/hydra-queue-runner/hydra-queue-runner.cc at line 341
  [3.691]

  [2.3055]
  + restart:
• replacement in src/hydra-queue-runner/hydra-queue-runner.cc at line 344
  [3.737]→[3.737:900](∅→∅)
  − txn.parameterized

  − ("insert into BuildSteps (build, stepnr, type, drvPath, busy, status, startTime, stopTime) values ($1, $2, $3, $4, $5, $6, $7, $8)")
  [3.737]

  [3.900]
  + auto r = txn.parameterized

  + ("insert into BuildSteps (build, stepnr, type, drvPath, busy, status, startTime, stopTime) values ($1, $2, $3, $4, $5, $6, $7, $8) on conflict do nothing")
• edit in src/hydra-queue-runner/hydra-queue-runner.cc at line 354
  [3.1057]

  [3.1057]
  +

  + if (r.affected_rows() == 0) goto restart;