import, builtins.readFile: Handle diverted stores

Fixes #1791

src/libexpr/eval.cc

@@ -375,6 +375,16 @@ void EvalState::checkURI(const std::string & uri)
 }
 
 
+Path EvalState::toRealPath(const Path & path, const PathSet & context)
+{
+    // FIXME: check whether 'path' is in 'context'.
+    return
+        !context.empty() && store->isInStore(path)
+        ? store->toRealPath(path)
+        : path;
+};
+
+
 void EvalState::addConstant(const string & name, Value & v)
 {
     Value * v2 = allocValue();

src/libexpr/eval.hh

@@ -112,6 +112,15 @@ public:
 
     void checkURI(const std::string & uri);
 
+    /* When using a diverted store and 'path' is in the Nix store, map
+       'path' to the diverted location (e.g. /nix/store/foo is mapped
+       to /home/alice/my-nix/nix/store/foo). However, this is only
+       done if the context is not empty, since otherwise we're
+       probably trying to read from the actual /nix/store. This is
+       intended to distinguish between import-from-derivation and
+       sources stored in the actual /nix/store. */
+    Path toRealPath(const Path & path, const PathSet & context);
+
     /* Parse a Nix expression from the specified file. */
     Expr * parseExprFromFile(const Path & path);
     Expr * parseExprFromFile(const Path & path, StaticEnv & staticEnv);

src/libexpr/primops.cc

@@ -84,10 +84,10 @@ static void prim_scopedImport(EvalState & state, const Pos & pos, Value * * args
             % path % e.path % pos);
     }
 
-    path = state.checkSourcePath(path);
+    Path realPath = state.checkSourcePath(state.toRealPath(path, context));
 
     if (state.store->isStorePath(path) && state.store->isValidPath(path) && isDerivation(path)) {
-        Derivation drv = readDerivation(path);
+        Derivation drv = readDerivation(realPath);
         Value & w = *state.allocValue();
         state.mkAttrs(w, 3 + drv.outputs.size());
         Value * v2 = state.allocAttr(w, state.sDrvPath);
@@ -114,7 +114,7 @@ static void prim_scopedImport(EvalState & state, const Pos & pos, Value * * args
     } else {
         state.forceAttrs(*args[0]);
         if (args[0]->attrs->empty())
-            state.evalFile(path, v);
+            state.evalFile(realPath, v);
         else {
             Env * env = &state.allocEnv(args[0]->attrs->size());
             env->up = &state.baseEnv;
@@ -127,8 +127,8 @@ static void prim_scopedImport(EvalState & state, const Pos & pos, Value * * args
                 env->values[displ++] = attr.value;
             }
 
-            printTalkative("evaluating file '%1%'", path);
+            printTalkative("evaluating file '%1%'", realPath);
-            Expr * e = state.parseExprFromFile(resolveExprPath(path), staticEnv);
+            Expr * e = state.parseExprFromFile(resolveExprPath(realPath), staticEnv);
 
             e->eval(state, *env, v);
         }
@@ -863,7 +863,7 @@ static void prim_readFile(EvalState & state, const Pos & pos, Value * * args, Va
         throw EvalError(format("cannot read '%1%', since path '%2%' is not valid, at %3%")
             % path % e.path % pos);
     }
-    string s = readFile(state.checkSourcePath(path));
+    string s = readFile(state.checkSourcePath(state.toRealPath(path, context)));
     if (s.find((char) 0) != string::npos)
         throw Error(format("the contents of the file '%1%' cannot be represented as a Nix string") % path);
     mkString(v, s.c_str());

src/libstore/store-api.hh

@@ -597,6 +597,11 @@ public:
        "nix-cache-info" file. Lower value means higher priority. */
     virtual int getPriority() { return 0; }
 
+    virtual Path toRealPath(const Path & storePath)
+    {
+        return storePath;
+    }
+
 protected:
 
     Stats stats;
@@ -639,9 +644,10 @@ public:
 
     virtual Path getRealStoreDir() { return storeDir; }
 
-    Path toRealPath(const Path & storePath)
+    Path toRealPath(const Path & storePath) override
     {
-        return getRealStoreDir() + "/" + baseNameOf(storePath);
+        assert(isInStore(storePath));
+        return getRealStoreDir() + "/" + std::string(storePath, storeDir.size() + 1);
     }
 
     std::shared_ptr<std::string> getBuildLog(const Path & path) override;