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

src/libutil/util.hh

@@ -401,6 +401,7 @@ void ignoreException();
 /* Some ANSI escape sequences. */
 #define ANSI_NORMAL "\e[0m"
 #define ANSI_BOLD "\e[1m"
+#define ANSI_FAINT "\e[2m"
 #define ANSI_RED "\e[31;1m"
 #define ANSI_GREEN "\e[32;1m"
 #define ANSI_BLUE "\e[34;1m"

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()
@@ -41,6 +43,11 @@ struct NixArgs : virtual MultiCommand, virtual MixCommonArgs
                 throw Exit();
             });
 
+        mkFlag()
+            .longName("print-build-logs")
+            .description("print full build logs on stderr")
+            .set(&printBuildLogs, true);
+
         mkFlag()
             .longName("version")
             .description("show version information")
@@ -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();

src/nix/progress-bar.cc

@@ -2,6 +2,7 @@
 #include "util.hh"
 #include "sync.hh"
 #include "store-api.hh"
+#include "names.hh"
 
 #include <atomic>
 #include <map>
@@ -38,6 +39,7 @@ private:
         std::map<ActivityType, uint64_t> expectedByType;
         bool visible = true;
         ActivityId parent;
+        std::optional<std::string> name;
     };
 
     struct ActivitiesByType
@@ -68,10 +70,16 @@ private:
 
     std::condition_variable quitCV, updateCV;
 
+    bool printBuildLogs;
+    bool isTTY;
+
 public:
 
-    ProgressBar()
+    ProgressBar(bool printBuildLogs, bool isTTY)
+        : printBuildLogs(printBuildLogs)
+        , isTTY(isTTY)
     {
+        state_.lock()->active = isTTY;
         updateThread = std::thread([&]() {
             auto state(state_.lock());
             while (state->active) {
@@ -109,8 +117,14 @@ public:
 
     void log(State & state, Verbosity lvl, const std::string & s)
     {
+        if (state.active) {
             writeToStderr("\r\e[K" + s + ANSI_NORMAL "\n");
             draw(state);
+        } else {
+            auto s2 = s + ANSI_NORMAL "\n";
+            if (!isTTY) s2 = filterANSIEscapes(s2, true);
+            writeToStderr(s2);
+        }
     }
 
     void startActivity(ActivityId act, Verbosity lvl, ActivityType type,
@@ -141,6 +155,7 @@ public:
             auto nrRounds = getI(fields, 3);
             if (nrRounds != 1)
                 i->s += fmt(" (round %d/%d)", curRound, nrRounds);
+            i->name = DrvName(name).name;
         }
 
         if (type == actSubstitute) {
@@ -217,6 +232,9 @@ public:
                 auto i = state->its.find(act);
                 assert(i != state->its.end());
                 ActInfo info = *i->second;
+                if (printBuildLogs) {
+                    log(*state, lvlInfo, ANSI_FAINT + info.name.value_or("unnamed") + "> " + ANSI_NORMAL + lastLine);
+                } else {
                     state->activities.erase(i->second);
                     info.lastLine = lastLine;
                     state->activities.emplace_back(info);
@@ -224,6 +242,7 @@ public:
                     update();
                 }
             }
+        }
 
         else if (type == resUntrustedPath) {
             state->untrustedPaths++;
@@ -395,9 +414,9 @@ public:
     }
 };
 
-void startProgressBar()
+void startProgressBar(bool printBuildLogs)
 {
-    logger = new ProgressBar();
+    logger = new ProgressBar(printBuildLogs, isatty(STDERR_FILENO));
 }
 
 void stopProgressBar()

src/nix/progress-bar.hh

@@ -4,7 +4,7 @@
 
 namespace nix {
 
-void startProgressBar();
+void startProgressBar(bool printBuildLogs = false);
 
 void stopProgressBar();