3 files changed, 37 insertions, 0 deletions
diff --git a/third_party/nix/src/libexpr/eval.cc b/third_party/nix/src/libexpr/eval.cc
index 5f272b62b8b8..f068b6ba6a48 100644
--- a/third_party/nix/src/libexpr/eval.cc
+++ b/third_party/nix/src/libexpr/eval.cc
@@ -379,6 +379,7 @@ EvalState::EvalState(const Strings& _searchPath, const ref<Store>& store)
 EvalState::~EvalState() = default;
 
 Path EvalState::checkSourcePath(const Path& path_) {
+  TraceFileAccess(path_);
   if (!allowedPaths) {
     return path_;
   }
@@ -1819,6 +1820,20 @@ void EvalState::printStats() {
   }
 }
 
+void EvalState::TraceFileAccess(const Path& realPath) {
+  if (file_access_trace_fn.has_value()) {
+    if (last_traced_file != realPath) {
+      (*file_access_trace_fn)(realPath);
+      // Basic deduplication.
+      last_traced_file = std::string(realPath);
+    }
+  }
+}
+
+void EvalState::EnableFileAccessTracing(std::function<void(const Path&)> fn) {
+  file_access_trace_fn = fn;
+}
+
 size_t valueSize(const Value& v) {
   traceable_flat_hash_set<const Bindings*> seenBindings;
   traceable_flat_hash_set<const Env*> seenEnvs;
diff --git a/third_party/nix/src/libexpr/eval.hh b/third_party/nix/src/libexpr/eval.hh
index e08ec04ef598..c244b6eecc73 100644
--- a/third_party/nix/src/libexpr/eval.hh
+++ b/third_party/nix/src/libexpr/eval.hh
@@ -284,6 +284,10 @@ class EvalState : public gc {
 
   void realiseContext(const PathSet& context);
 
+  /* File access tracing. */
+  void TraceFileAccess(const Path& path);
+  void EnableFileAccessTracing(std::function<void(const Path&)> fn);
+
  private:
   unsigned long nrEnvs = 0;
   unsigned long nrValuesInEnvs = 0;
@@ -299,6 +303,10 @@ class EvalState : public gc {
 
   bool countCalls;
 
+  std::optional<std::function<void(const Path&)>> file_access_trace_fn =
+      std::nullopt;
+  Path last_traced_file = "";
+
   typedef std::map<Symbol, size_t> PrimOpCalls;
   PrimOpCalls primOpCalls;
 
diff --git a/third_party/nix/src/nix-instantiate/nix-instantiate.cc b/third_party/nix/src/nix-instantiate/nix-instantiate.cc
index be96943b1234..7b8d6dd3d61f 100644
--- a/third_party/nix/src/nix-instantiate/nix-instantiate.cc
+++ b/third_party/nix/src/nix-instantiate/nix-instantiate.cc
@@ -96,6 +96,7 @@ static int _main(int argc, char** argv) {
     bool findFile = false;
     bool evalOnly = false;
     bool parseOnly = false;
+    bool traceFileAccess = false;
     OutputKind outputKind = okPlain;
     bool xmlOutputSourceLocation = true;
     bool strict = false;
@@ -143,6 +144,14 @@ static int _main(int argc, char** argv) {
                       repair = Repair;
                     } else if (*arg == "--dry-run") {
                       settings.readOnlyMode = true;
+                    } else if (*arg == "--trace-file-access") {
+                      traceFileAccess = true;
+                    } else if (*arg == "--trace-file-access=true") {
+                      traceFileAccess = true;
+                    } else if (*arg == "--trace-file-access=false") {
+                      traceFileAccess = false;
+                    } else if (*arg == "--notrace-file-access") {
+                      traceFileAccess = false;
                     } else if (*arg != "" && arg->at(0) == '-') {
                       return false;
                     } else {
@@ -161,6 +170,11 @@ static int _main(int argc, char** argv) {
 
     auto state = std::make_unique<EvalState>(myArgs.searchPath, store);
     state->repair = repair;
+    if (traceFileAccess) {
+      state->EnableFileAccessTracing([](const Path& path) {
+        std::cerr << "trace: depot-scan: " << path << "\n";
+      });
+    }
 
     Bindings& autoArgs = *myArgs.getAutoArgs(*state);