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, 14 insertions, 1 deletions

diff --git a/guix/store.scm b/guix/store.scm
index 8b35fc8d7a..b1bdbf3813 100644
--- a/guix/store.scm
+++ b/guix/store.scm
@@ -155,7 +155,7 @@
             derivation-log-file
             log-file))
 
-(define %protocol-version #x162)
+(define %protocol-version #x163)
 
 (define %worker-magic-1 #x6e697863)               ; "nixc"
 (define %worker-magic-2 #x6478696f)               ; "dxio"
@@ -709,6 +709,15 @@ encoding conversion errors."
                             ;; disabled by default.
                             print-extended-build-trace?
 
+                            ;; When true, the daemon prefixes builder output
+                            ;; with "@ build-log" traces so we can
+                            ;; distinguish it from daemon output, and we can
+                            ;; distinguish each builder's output
+                            ;; (PRINT-BUILD-TRACE must be true as well.)  The
+                            ;; latter is particularly useful when
+                            ;; MAX-BUILD-JOBS > 1.
+                            multiplexed-build-output?
+
                             build-cores
                             (use-substitutes? #t)
 
@@ -757,6 +766,10 @@ encoding conversion errors."
                            `(("print-extended-build-trace"
                               . ,(if print-extended-build-trace? "1" "0")))
                            '())
+                     ,@(if multiplexed-build-output?
+                           `(("multiplexed-build-output"
+                              . ,(if multiplexed-build-output? "true" "false")))
+                           '())
                      ,@(if timeout
                            `(("build-timeout" . ,(number->string timeout)))
                            '())