Track function start and ends for flame graphs

With this patch, and this file I called `log.py`:

    #!/usr/bin/env nix-shell
    #!nix-shell -i python3 -p python3 --pure

    import sys
    from pprint import pprint

    stack = []
    timestack = []

    for line in open(sys.argv[1]):
        components = line.strip().split(" ", 2)
        if components[0] != "function-trace":
            continue

        direction = components[1]
        components = components[2].rsplit(" ", 2)

        loc = components[0]
        _at = components[1]
        time = int(components[2])

        if direction == "entered":
            stack.append(loc)
            timestack.append(time)
        elif direction == "exited":
            dur = time - timestack.pop()
            vst = ";".join(stack)
            print(f"{vst} {dur}")
            stack.pop()

and:

    nix-instantiate --trace-function-calls -vvvv ../nixpkgs/pkgs/top-level/release.nix -A unstable > log.matthewbauer 2>&1
    ./log.py ./log.matthewbauer > log.matthewbauer.folded
    flamegraph.pl --title matthewbauer-post-pr log.matthewbauer.folded > log.matthewbauer.folded.svg

I can make flame graphs like: http://gsc.io/log.matthewbauer.folded.svg

---

Includes test cases around function call failures and tryEval. Uses
RAII so the finish is always called at the end of the function.

1 files changed, 6 insertions, 2 deletions

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index d8e10d9f2..9f4b6b411 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -9,6 +9,7 @@
 #include "json.hh"
 
 #include <algorithm>
+#include <chrono>
 #include <cstring>
 #include <unistd.h>
 #include <sys/time.h>
@@ -16,7 +17,6 @@
 #include <iostream>
 #include <fstream>
 
-#include <sys/time.h>
 #include <sys/resource.h>
 
 #if HAVE_BOEHMGC
@@ -1094,9 +1094,13 @@ void EvalState::callPrimOp(Value & fun, Value & arg, Value & v, const Pos & pos)
     }
 }
 
-
 void EvalState::callFunction(Value & fun, Value & arg, Value & v, const Pos & pos)
 {
+    std::optional<FunctionCallTrace> trace;
+    if (evalSettings.traceFunctionCalls) {
+        trace.emplace(pos);
+    }
+
     forceValue(fun, pos);
 
     if (fun.type == tPrimOp || fun.type == tPrimOpApp) {