add stack sampling profiler for flamegraphs

This is code is a bit rough in the sense that I have taken shortcuts to
quickly iterate.
This is not ready for merging yet, but I think it's already useful for
people that want to generate flamegraphs for things as large as NixOS.

Usage:

NIX_PROFILE_FILE=/tmp/nixos-trace nix eval -v --no-eval-cache
.#nixosConfigurations.turingmachine.config.system.build.toplevel

The result can be imported in tools that support folded stacks i.e.
https://www.speedscope.app/ or the original flamegraph script
(https://github.com/brendangregg/FlameGraph)

The profiler records stack trace of the nix evaluation every 10ms (100Hz).
The resulting file is 1.9GB uncompressed and 5.4MB compressed with zstd.

src/libexpr/eval.cc

@@ -364,6 +364,36 @@ EvalState::EvalState(
 
 EvalState::~EvalState()
 {
+  auto profileFile = getEnv("NIX_PROFILE_FILE");
+
+  std::map<PosIdx, std::string> cachedPositions;
+
+  if (!profileFile.has_value()) {
+        return;
+  }
+  std::ofstream profileStream(profileFile.value());
+  if (!profileStream) {
+      return;
+  }
+  for (auto & [stack, count] : callCount) {
+      auto first = true;
+      for (auto & pos : stack) {
+          if (first) {
+              first = false;
+          } else {
+              profileStream << ";";
+          }
+          if (auto it = cachedPositions.find(pos); it != cachedPositions.end()) {
+              profileStream << it->second;
+          } else {
+              std::stringstream posStr;
+              posStr << positions[pos];
+              cachedPositions[pos] = posStr.str();
+              profileStream << posStr.str();
+          }
+      }
+      profileStream << " " << count << std::endl;
+  }
 }
 
 
@@ -1484,15 +1514,36 @@ void ExprOpHasAttr::eval(EvalState & state, Env & env, Value & v)
     v.mkBool(true);
 }
 
-
 void ExprLambda::eval(EvalState & state, Env & env, Value & v)
 {
     v.mkLambda(&env, this);
 }
 
+static const std::chrono::duration SAMPLE_INTERVAL = std::chrono::microseconds(10);
+namespace {
+class SampleStack {
+    EvalState & state;
+public:
+    SampleStack(EvalState & state, const PosIdx pos) : state(state) {
+        state.stack.push_back(pos);
+    }
+    ~SampleStack() {
+        auto now = std::chrono::high_resolution_clock::now();
+        if (now - state.lastStackSample > SAMPLE_INTERVAL) {
+            state.callCount[state.stack] += 1;
+            state.lastStackSample = now;
+        }
+        if (state.stack.size() > 0) {
+            state.stack.pop_back();
+        }
+    }
+};
+};
+
 void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value & vRes, const PosIdx pos)
 {
     auto _level = addCallDepth(pos);
+    SampleStack _sample(*this, pos);
 
     auto trace = settings.traceFunctionCalls
         ? std::make_unique<FunctionCallTrace>(positions[pos])

src/libexpr/eval.hh

@@ -15,6 +15,7 @@
 #include "search-path.hh"
 #include "repl-exit-status.hh"
 #include "ref.hh"
+#include "pos-idx.hh"
 
 #include <map>
 #include <optional>
@@ -375,6 +376,10 @@ private:
 
 public:
 
+    std::vector<PosIdx> stack = {};
+    std::map<std::vector<PosIdx>, int> callCount = {};
+    std::chrono::system_clock::time_point lastStackSample;
+
     EvalState(
         const LookupPath & _lookupPath,
         ref<Store> store,
@@ -850,6 +855,7 @@ private:
 
     bool countCalls;
 
+
     typedef std::map<std::string, size_t> PrimOpCalls;
     PrimOpCalls primOpCalls;