27/118: Add importNative primop

[Ludovic Courtès]

civodul pushed a commit to branch nix
in repository guix.

commit 5cd022d6c099c583c0494bdacd06f4eb32661135
Author: Shea Levy <address@hidden>
Date:   Sun Jun 1 10:42:56 2014 -0400

    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   |    2 ++
 src/libexpr/primops.cc |   42 ++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 44 insertions(+), 0 deletions(-)

diff --git a/src/libexpr/local.mk b/src/libexpr/local.mk
index b3b4086..75a0e18 100644
--- a/src/libexpr/local.mk
+++ b/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.
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 366911b..d6ac7c9 100644
--- a/src/libexpr/primops.cc
+++ b/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);