daemon: Support multiplexed build output.

This allows clients to tell whether output comes from the daemon or, if
it comes from a builder, from which builder it comes.  The latter is
particularly useful when MAX-BUILD-JOBS > 1.

* nix/libstore/build.cc (DerivationGoal::tryBuildHook)
(DerivationGoal::startBuilder): Print the child's PID in "@ build-started"
traces.
(DerivationGoal::handleChildOutput): Define 'prefix', pass it to
'writeToStderr'.
* nix/libstore/globals.cc (Settings:Settings): Initialize
'multiplexedBuildOutput'.
(Settings::update): Likewise.
* nix/libstore/globals.hh (Settings)[multiplexedBuildOutput]: New field.
Update 'printBuildTrace' documentation.
* nix/libstore/worker-protocol.hh (PROTOCOL_VERSION): Bump to 0.163.
* nix/nix-daemon/nix-daemon.cc (performOp) <wopSetOptions>: Special-case
"multiplexed-build-output" and remove "use-ssh-substituter".
* guix/store.scm (set-build-options): Add #:multiplexed-build-output?
and honor it.
(%protocol-version): Bump to #x163.
* tests/store.scm ("multiplexed-build-output"): New test.

fixlet

1 files changed, 2 insertions, 0 deletions

diff --git a/nix/libstore/globals.cc b/nix/libstore/globals.cc
index 94c2e516f8..4b5b485e65 100644
--- a/nix/libstore/globals.cc
+++ b/nix/libstore/globals.cc
@@ -36,6 +36,7 @@ Settings::Settings()
     buildTimeout = 0;
     useBuildHook = true;
     printBuildTrace = false;
+    multiplexedBuildOutput = false;
     reservedSize = 8 * 1024 * 1024;
     fsyncMetadata = true;
     useSQLiteWAL = true;
@@ -120,6 +121,7 @@ void Settings::update()
     _get(maxBuildJobs, "build-max-jobs");
     _get(buildCores, "build-cores");
     _get(thisSystem, "system");
+    _get(multiplexedBuildOutput, "multiplexed-build-output");
     _get(maxSilentTime, "build-max-silent-time");
     _get(buildTimeout, "build-timeout");
     _get(reservedSize, "gc-reserved-space");