Allow dlopen of plugins to fail

It happens with some frequency that plugins that might be unimportant to
the evaluation at hand mismatch with the nix version, leading to
spurious load failures. Let's make these non fatal.

Change-Id: Iba10e951d171725ccf1a121bcd9be1e1d6ad69eb

src/libstore/globals.cc

@@ -342,7 +342,7 @@ void initPlugins()
             void *handle =
                 dlopen(file.c_str(), RTLD_LAZY | RTLD_LOCAL);
             if (!handle)
-                throw Error("could not dynamically open plugin file '%s': %s", file, dlerror());
+                warn("could not dynamically open plugin file '%s': %s", file, dlerror());
         }
     }
 

tests/functional/local.mk

@@ -141,7 +141,8 @@ endif
 $(d)/test-libstoreconsumer.sh.test $(d)/test-libstoreconsumer.sh.test-debug: \
   $(buildprefix)$(d)/test-libstoreconsumer/test-libstoreconsumer
 $(d)/plugins.sh.test $(d)/plugins.sh.test-debug: \
-  $(buildprefix)$(d)/plugins/libplugintest.$(SO_EXT)
+  $(buildprefix)$(d)/plugins/libplugintest.$(SO_EXT) \
+  $(buildprefix)$(d)/plugins/libplugintestfail.$(SO_EXT)
 
 install-tests += $(foreach x, $(nix_tests), $(d)/$(x))
 

tests/functional/plugins.sh

@@ -4,6 +4,20 @@ if [[ $BUILD_SHARED_LIBS != 1 ]]; then
     skipTest "Plugins are not supported"
 fi
 
-res=$(nix --option setting-set true --option plugin-files $PWD/plugins/libplugintest* eval --expr builtins.anotherNull)
+res=$(nix --option setting-set true --option plugin-files $PWD/plugins/libplugintest.* eval --expr builtins.anotherNull)
 
 [ "$res"x = "nullx" ]
+
+# Plugin load failing due to missing symbols
+res=$(nix --option plugin-files $PWD/plugins/libplugintestfail.* eval --expr '1234 + 5' 2>&1)
+# We expect this to succeed evaluating
+echo "$res" | grep 1239 >/dev/null
+# On Linux, we expect this to print some failure of dlopen.
+# Only on Linux do we expect for sure that -z now is set on the .so file, so it
+# will definitely fail to load instead of lazy loading (and thus not hitting
+# the missing symbol).
+# FIXME(jade): does there exist an equivalent of -z now on macOS that eluded us
+# in search?
+if [[ "$(uname -s)" == Linux ]]; then
+    echo "$res" | grep "could not dynamically open plugin file" >/dev/null
+fi

tests/functional/plugins/local.mk

@@ -1,4 +1,4 @@
-libraries += libplugintest
+libraries += libplugintest libplugintestfail
 
 libplugintest_DIR := $(d)
 
@@ -9,3 +9,19 @@ libplugintest_ALLOW_UNDEFINED := 1
 libplugintest_EXCLUDE_FROM_LIBRARY_LIST := 1
 
 libplugintest_CXXFLAGS := -I src/libutil -I src/libstore -I src/libexpr -I src/libfetchers
+
+libplugintestfail_DIR := $(d)
+
+libplugintestfail_SOURCES := $(d)/plugintestfail.cc
+
+libplugintestfail_ALLOW_UNDEFINED := 1
+
+libplugintestfail_EXCLUDE_FROM_LIBRARY_LIST := 1
+
+libplugintestfail_CXXFLAGS := -I src/libutil -I src/libstore -I src/libexpr -I src/libfetchers -DMISSING_REFERENCE
+
+# Make sure that the linker strictly evaluates all symbols on .so load on Linux
+# so it will definitely fail to load as expected.
+ifdef HOST_LINUX
+  libplugintestfail_LDFLAGS += -z now
+endif

tests/functional/plugins/plugintest.cc

@@ -1,8 +1,15 @@
 #include "config.hh"
 #include "primops.hh"
+#include <stdlib.h>
 
 using namespace nix;
 
+#ifdef MISSING_REFERENCE
+extern void meow();
+#else
+#define meow() {}
+#endif
+
 struct MySettings : Config
 {
     Setting<bool> settingSet{this, false, "setting-set",
@@ -13,6 +20,11 @@ MySettings mySettings;
 
 static GlobalConfig::Register rs(&mySettings);
 
+[[gnu::used, gnu::unused, gnu::retain]]
+static void maybeRequireMeowForDlopen() {
+    meow();
+}
+
 static void prim_anotherNull (EvalState & state, const PosIdx pos, Value ** args, Value & v)
 {
     if (mySettings.settingSet)

tests/functional/plugins/plugintestfail.cc

@@ -0,0 +1 @@
+plugintest.cc