Truncate the log just before starting the remote build

Dependencies

• [2] XCDTFZUY hydra-queue-runner: Fix build
• [3] 5AIYUMTB Basic remote building
• [4] DIEY5USN Keep better bytesReceived/bytesSent stats
• [5] BYVRA54Q Temporarily disable machines on any exception, not just connection failures
• [*] GS4BE6TB Asynchronously compress build logs

Change contents

• edit in src/hydra-queue-runner/build-remote.cc at line 138
  [3.87]→[3.3335:3336](∅→∅),[2.553]→[3.3335:3336](∅→∅),[3.3335]→[3.3335:3336](∅→∅),[3.3336]→[2.554:574](∅→∅)
  −

  − logFD = -1;
• edit in src/hydra-queue-runner/build-remote.cc at line 223
  [3.3310]

  [7.511]
  +

  + /* Truncate the log to get rid of messages about substitutions

  + etc. on the remote system. */

  + if (lseek(logFD.get(), SEEK_SET, 0) != 0)

  + throw SysError("seeking to the start of log file ‘%s’", result.logFile);

  +

  + if (ftruncate(logFD.get(), 0) == -1)

  + throw SysError("truncating log file ‘%s’", result.logFile);
• edit in src/hydra-queue-runner/build-remote.cc at line 232
  [7.512]

  [3.3311]
  + logFD = -1;

  +