Memoise checkSourcePath()

This prevents hydra-eval-jobs from statting the same files over and
over again.

src/libexpr/eval.cc

@@ -352,6 +352,10 @@ Path EvalState::checkSourcePath(const Path & path_)
 {
     if (!allowedPaths) return path_;
 
+    auto i = resolvedPaths.find(path_);
+    if (i != resolvedPaths.end())
+        return i->second;
+
     bool found = false;
 
     for (auto & i : *allowedPaths) {
@@ -369,8 +373,10 @@ Path EvalState::checkSourcePath(const Path & path_)
     Path path = canonPath(path_, true);
 
     for (auto & i : *allowedPaths) {
-        if (isDirOrInDir(path, i))
+        if (isDirOrInDir(path, i)) {
+            resolvedPaths[path_] = path;
             return path;
+        }
     }
 
     throw RestrictedPathError("access to path '%1%' is forbidden in restricted mode", path);

src/libexpr/eval.hh

@@ -7,6 +7,7 @@
 #include "hash.hh"
 
 #include <map>
+#include <unordered_map>
 
 
 namespace nix {
@@ -100,6 +101,9 @@ private:
 
     std::map<std::string, std::pair<bool, std::string>> searchPathResolved;
 
+    /* Cache used by checkSourcePath(). */
+    std::unordered_map<Path, Path> resolvedPaths;
+
 public:
 
     EvalState(const Strings & _searchPath, ref<Store> store);