Show a clear error on -o pointing to an invalid directory. fixes emsc…

…ripten-core#8549 (emscripten-core#8551)

Without this, Python will give an error "no such file" when we try to write to that file, which can be confusing. Worse, this will happen on the first file we try to write to, which may be one of the helper files (.wasm, .mem, etc.), which is even more confusing.

emcc.py

@@ -712,6 +712,13 @@ def uniquename(name):
   # specified_target is the user-specified one, target is what we will generate
   if specified_target:
     target = specified_target
+    # check for the existence of the output directory now, to avoid having
+    # to do so repeatedly when each of the various output files (.mem, .wasm,
+    # etc) are written. This gives a more useful error message than the
+    # IOError and python backtrace that users would otherwise see.
+    dirname = os.path.dirname(target)
+    if dirname and not os.path.isdir(dirname):
+      exit_with_error("specified output file (%s) is in a directory that does not exist" % target)
   else:
     target = 'a.out.js'
 

tests/test_other.py

@@ -7847,6 +7847,10 @@ def test_invalid_mem(self):
     ret = self.expect_fail([PYTHON, EMCC, path_from_root('tests', 'hello_world.c'), '-s', 'WASM_MEM_MAX=33MB+1'])
     self.assertContained('WASM_MEM_MAX must be a multiple of 64KB', ret)
 
+  def test_invalid_output_dir(self):
+    ret = self.expect_fail([PYTHON, EMCC, path_from_root('tests', 'hello_world.c'), '-o', os.path.join('NONEXISTING_DIRECTORY', 'out.js')])
+    self.assertContained('specified output file (NONEXISTING_DIRECTORY%sout.js) is in a directory that does not exist' % os.path.sep, ret)
+
   @unittest.skipIf(SPIDERMONKEY_ENGINE not in JS_ENGINES, 'cannot run without spidermonkey')
   def test_binaryen_ctors(self):
     # ctor order must be identical to js builds, deterministically