Add importNative primop

This can be used to import a dynamic shared object and return an
arbitrary value, including new primops. This can be used both to test
new primops without having to recompile nix every time, and to build
specialized primops that probably don't belong upstream (e.g. a function
that calls out to gpg to decrypt a nixops secret as-needed).

The imported function should initialize the Value & as needed. A single
import can define multiple values by creating an attrset or list, of
course.

An example initialization function might look like:

extern "C" void initialize(nix::EvalState & state, nix::Value & v)
{
    v.type = nix::tPrimOp;
    v.primOp = NEW nix::PrimOp(myFun, 1, state.symbols.create("myFun"));
}

Then `builtins.importNative ./example.so "initialize"` will evaluate to
the primop defined in the myFun function.

src/libexpr/local.mk

@@ -8,6 +8,8 @@ libexpr_SOURCES := $(wildcard $(d)/*.cc) $(d)/lexer-tab.cc $(d)/parser-tab.cc
 
 libexpr_LIBS = libutil libstore libformat
 
+libexpr_LDFLAGS = -ldl
+
 # The dependency on libgc must be propagated (i.e. meaning that
 # programs/libraries that use libexpr must explicitly pass -lgc),
 # because inline functions in libexpr's header files call libgc.

src/libexpr/primops.cc

@@ -15,6 +15,7 @@
 
 #include <algorithm>
 #include <cstring>
+#include <dlfcn.h>
 
 
 namespace nix {
@@ -129,6 +130,46 @@ static void prim_scopedImport(EvalState & state, const Pos & pos, Value * * args
 }
 
 
+/* Want reasonable symbol names, so extern C */
+/* !!! Should we pass the Pos or the file name too? */
+extern "C" typedef void (*ValueInitializer)(EvalState & state, Value & v);
+
+/* Load a ValueInitializer from a dso and return whatever it initializes */
+static void prim_importNative(EvalState & state, const Pos & pos, Value * * args, Value & v)
+{
+    PathSet context;
+    Path path = state.coerceToPath(pos, *args[0], context);
+
+    try {
+        realiseContext(context);
+    } catch (InvalidPathError & e) {
+        throw EvalError(format("cannot import `%1%', since path `%2%' is not valid, at %3%")
+            % path % e.path % pos);
+    }
+
+    string sym = state.forceStringNoCtx(*args[1], pos);
+
+    void *handle = dlopen(path.c_str(), RTLD_LAZY | RTLD_LOCAL);
+    if (!handle)
+        throw EvalError(format("could not open `%1%': %2%") % path % dlerror());
+
+    dlerror();
+    ValueInitializer func = (ValueInitializer) dlsym(handle, sym.c_str());
+    if(!func) {
+        char *message = dlerror();
+        if (message)
+            throw EvalError(format("could not load symbol `%1%' from `%2%': %3%") % sym % path % message);
+        else
+            throw EvalError(format("symbol `%1%' from `%2%' resolved to NULL when a function pointer was expected")
+                    % sym % path);
+    }
+
+    (func)(state, v);
+
+    /* We don't dlclose because v may be a primop referencing a function in the shared object file */
+}
+
+
 /* Return a string representing the type of the expression. */
 static void prim_typeOf(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
@@ -1327,6 +1368,7 @@ void EvalState::createBaseEnv()
     mkApp(v, *baseEnv.values[baseEnvDispl - 1], *v2);
     forceValue(v);
     addConstant("import", v);
+    addPrimOp("__importNative", 2, prim_importNative);
     addPrimOp("__typeOf", 1, prim_typeOf);
     addPrimOp("isNull", 1, prim_isNull);
     addPrimOp("__isFunction", 1, prim_isFunction);