Add a crude tracing mechansim for the build results

Add a `_NIX_TRACE_BUILT_OUTPUTS` environment variable that can be set to
a filename in which the result of each build will be logged.

This is intentionally crude and undocumented as it’s only meant to be a
temporary thing to assess the usefulness of CA derivations.
Any other use would need a cleaner re-implementation first.

1 files changed, 8 insertions, 0 deletions

diff --git a/src/libstore/build/derivation-goal.cc b/src/libstore/build/derivation-goal.cc
index a713d7222..c6ab5c3d1 100644
--- a/src/libstore/build/derivation-goal.cc
+++ b/src/libstore/build/derivation-goal.cc
@@ -17,6 +17,7 @@
 #include <regex>
 #include <queue>
 
+#include <fstream>
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <sys/un.h>
@@ -1337,6 +1338,13 @@ void DerivationGoal::done(BuildResult::Status status, std::optional<Error> ex)
     }
 
     worker.updateProgress();
+
+    auto traceBuiltOutputsFile = getEnv("_NIX_TRACE_BUILT_OUTPUTS").value_or("");
+    if (traceBuiltOutputsFile != "") {
+        std::fstream fs;
+        fs.open(traceBuiltOutputsFile, std::fstream::out);
+        fs << worker.store.printStorePath(drvPath) << "\t" << result.toString() << std::endl;
+    }
 }