nix: Add --print-build-logs flag

This causes 'nix' to print build log output to stderr rather than
showing the last log line in the progress bar. Log lines are prefixed
by the name of the derivation (minus the version string), e.g.

  binutils> make[1]: Leaving directory '/build/binutils-2.31.1'
  binutils-wrapper> unpacking sources
  binutils-wrapper> patching sources
  ...
  binutils-wrapper> Using dynamic linker: '/nix/store/kr51dlsj9v5cr4n8700jliyz8v5b2q7q-bootstrap-stage0-glibc/lib/ld-linux-x86-64.so.2'
  bootstrap-stage2-gcc-wrapper> unpacking sources
  ...
  linux-headers> unpacking sources
  linux-headers> unpacking source archive /nix/store/8javli69jhj3bkql2c35gsj5vl91p382-linux-4.19.16.tar.xz

1 files changed, 8 insertions, 2 deletions

diff --git a/src/nix/main.cc b/src/nix/main.cc
index 64c1dc35787c..4f87ad72b65c 100644
--- a/src/nix/main.cc
+++ b/src/nix/main.cc
@@ -20,6 +20,8 @@ std::string programPath;
 
 struct NixArgs : virtual MultiCommand, virtual MixCommonArgs
 {
+    bool printBuildLogs = false;
+
     NixArgs() : MultiCommand(*RegisterCommand::commands), MixCommonArgs("nix")
     {
         mkFlag()
@@ -42,6 +44,11 @@ struct NixArgs : virtual MultiCommand, virtual MixCommonArgs
             });
 
         mkFlag()
+            .longName("print-build-logs")
+            .description("print full build logs on stderr")
+            .set(&printBuildLogs, true);
+
+        mkFlag()
             .longName("version")
             .description("show version information")
             .handler([&]() { printVersion(programName); });
@@ -98,8 +105,7 @@ void mainWrapped(int argc, char * * argv)
 
     Finally f([]() { stopProgressBar(); });
 
-    if (isatty(STDERR_FILENO))
-        startProgressBar();
+    startProgressBar(args.printBuildLogs);
 
     args.command->prepare();
     args.command->run();