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.

2 files changed, 31 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;
+    }
 }
 
 
diff --git a/src/libstore/store-api.hh b/src/libstore/store-api.hh
index 3a385b293..3dd446f23 100644
--- a/src/libstore/store-api.hh
+++ b/src/libstore/store-api.hh
@@ -155,6 +155,29 @@ struct BuildResult
     } status = MiscFailure;
     std::string errorMsg;
 
+    std::string toString() const {
+        auto strStatus = [&]() {
+            switch (status) {
+                case Built: return "Built";
+                case Substituted: return "Substituted";
+                case AlreadyValid: return "AlreadyValid";
+                case PermanentFailure: return "PermanentFailure";
+                case InputRejected: return "InputRejected";
+                case OutputRejected: return "OutputRejected";
+                case TransientFailure: return "TransientFailure";
+                case CachedFailure: return "CachedFailure";
+                case TimedOut: return "TimedOut";
+                case MiscFailure: return "MiscFailure";
+                case DependencyFailed: return "DependencyFailed";
+                case LogLimitExceeded: return "LogLimitExceeded";
+                case NotDeterministic: return "NotDeterministic";
+                case ResolvesToAlreadyValid: return "ResolvesToAlreadyValid";
+                default: return "Unknown";
+            };
+        }();
+        return strStatus + ((errorMsg == "") ? "" : " : " + errorMsg);
+    }
+
     /* How many times this build was performed. */
     unsigned int timesBuilt = 0;