Merge pull request #10570 from layus/shared_caches

Share evaluation caches across installables

Before:

$ rm -rf ~/.cache/nix && time -f '%E' nix build --dry-run \
  'nixpkgs#hello' \
  'nixpkgs#clang' \
  'nixpkgs#cargo' \
  'nixpkgs#rustup' \
  'nixpkgs#bear' \
  'nixpkgs#firefox' \
  'nixpkgs#git-revise' \
  'nixpkgs#hyperfine' \
  'nixpkgs#curlie' \
  'nixpkgs#xz' \
  'nixpkgs#ripgrep'
0:03.61

After:

$ rm -rf ~/.cache/nix && time -f '%E' nix build --dry-run \
  'nixpkgs#hello' \
  'nixpkgs#clang' \
  'nixpkgs#cargo' \
  'nixpkgs#rustup' \
  'nixpkgs#bear' \
  'nixpkgs#firefox' \
  'nixpkgs#git-revise' \
  'nixpkgs#hyperfine' \
  'nixpkgs#curlie' \
  'nixpkgs#xz' \
  'nixpkgs#ripgrep'
0:01.46

This could probably use a more proper benchmark...

Fixes #313

(cherry picked from commit de51e5c335865e3e0a8cccd283fec1a52cce243f)
Change-Id: I9350bebd462b6af12c51db5bf432321abfe84a16

src/libexpr/eval.hh

@@ -33,6 +33,10 @@ class EvalState;
 class StorePath;
 struct SingleDerivedPath;
 enum RepairFlag : bool;
+struct MemoryInputAccessor;
+namespace eval_cache {
+    class EvalCache;
+}
 
 
 /**
@@ -234,6 +238,11 @@ public:
         return *new EvalErrorBuilder<T>(*this, args...);
     }
 
+    /**
+     * A cache for evaluation caches, so as to reuse the same root value if possible
+     */
+    std::map<const Hash, ref<eval_cache::EvalCache>> evalCaches;
+
 private:
 
     /* Cache for calls to addToStore(); maps source paths to the store