Link in objects when they provide commons we need, not just defs (ems…

…cripten-core#4764)

* link in objects when they provide commons we need, not just defs emscripten-core#4763

* test commons linking

tests/test_other.py

@@ -855,6 +855,33 @@ def build(path, args):
 
     assert not os.path.exists('a.out') and not os.path.exists('a.exe'), 'Must not leave unneeded linker stubs'
 
+  def test_commons_link(self):
+    open('a.h', 'w').write(r'''
+#if !defined(A_H)
+#define A_H
+extern int foo[8];
+#endif
+''')
+    open('a.c', 'w').write(r'''
+#include "a.h"
+int foo[8];
+''')
+    open('main.c', 'w').write(r'''
+#include <stdio.h>
+#include "a.h"
+
+int main() {
+    printf("|%d|\n", foo[0]);
+    return 0;
+}
+''')
+
+    subprocess.check_call([PYTHON, EMCC, '-o', 'a.o', 'a.c'])
+    subprocess.check_call([PYTHON, EMAR, 'rv', 'library.a', 'a.o'])
+    subprocess.check_call([PYTHON, EMCC, '-o', 'main.o', 'main.c'])
+    subprocess.check_call([PYTHON, EMCC, '-o', 'a.js', 'main.o', 'library.a', '-s', 'ERROR_ON_UNDEFINED_SYMBOLS=1'])
+    self.assertContained('|0|', run_js('a.js'))
+
   def test_outline(self):
     if WINDOWS and not Building.which('mingw32-make'):
       return self.skip('Skipping other.test_outline: This test requires "mingw32-make" tool in PATH on Windows to drive a Makefile build of zlib')

tools/shared.py

@@ -1456,15 +1456,16 @@ def make_paths_absolute(f):
     # returns True.
     def consider_object(f, force_add=False):
       new_symbols = Building.llvm_nm(f)
-      do_add = force_add or not unresolved_symbols.isdisjoint(new_symbols.defs)
+      provided = new_symbols.defs.union(new_symbols.commons)
+      do_add = force_add or not unresolved_symbols.isdisjoint(provided)
       if do_add:
         logging.debug('adding object %s to link' % (f))
         # Update resolved_symbols table with newly resolved symbols
-        resolved_symbols.update(new_symbols.defs)
+        resolved_symbols.update(provided)
         # Update unresolved_symbols table by adding newly unresolved symbols and
         # removing newly resolved symbols.
         unresolved_symbols.update(new_symbols.undefs.difference(resolved_symbols))
-        unresolved_symbols.difference_update(new_symbols.defs)
+        unresolved_symbols.difference_update(provided)
         actual_files.append(f)
       return do_add