Fuzz combinations of two modules (#7144)

The new Two fuzz testcase handler generates two wasm files and
then runs them in V8, linking them using JS. It then optimizes at least one of
the two and runs it again, and checks for differences.

This is similar to the Split fuzzer for wasm-split, but the two wasm files
are arbitrary and not the result of splitting.

Also lower CtorEval priority a little. It is not very important.

scripts/fuzz_opt.py

@@ -1360,7 +1360,7 @@ def can_run_on_wasm(self, wasm):
 
 # Tests wasm-ctor-eval
 class CtorEval(TestCaseHandler):
-    frequency = 0.2
+    frequency = 0.1
 
     def handle(self, wasm):
         # get the expected execution results.
@@ -1477,7 +1477,9 @@ def can_run_on_wasm(self, wasm):
 FUNC_NAMES_REGEX = re.compile(r'\n [(]func [$](\S+)')
 
 
-# Tests wasm-split
+# Tests wasm-split. This also tests that fuzz_shell.js properly executes 2 wasm
+# files, which adds coverage for ClusterFuzz (which sometimes runs two wasm
+# files in that way).
 class Split(TestCaseHandler):
     frequency = 1  # TODO: adjust lower when we actually enable this
 
@@ -1670,6 +1672,78 @@ def ensure(self):
         tar.close()
 
 
+# Tests linking two wasm files at runtime, and that optimizations do not break
+# anything. This is similar to Split(), but rather than split a wasm file into
+# two and link them at runtime, this starts with two separate wasm files.
+class Two(TestCaseHandler):
+    frequency = 0.2
+
+    def handle(self, wasm):
+        # Generate a second wasm file, unless we were given one (useful during
+        # reduction).
+        second_wasm = abspath('second.wasm')
+        given = os.environ.get('BINARYEN_SECOND_WASM')
+        if given:
+            # TODO: should we de-nan this etc. as with the primary?
+            shutil.copyfile(given, second_wasm)
+        else:
+            second_input = abspath('second_input.dat')
+            make_random_input(random_size(), second_input)
+            args = [second_input, '-ttf', '-o', second_wasm]
+            run([in_bin('wasm-opt')] + args + GEN_ARGS + FEATURE_OPTS)
+
+        # The binaryen interpreter only supports a single file, so we run them
+        # from JS using fuzz_shell.js's support for two files.
+        #
+        # Note that we *cannot* run each wasm file separately and compare those
+        # to the combined output, as fuzz_shell.js intentionally allows calls
+        # *between* the wasm files, through JS APIs like call-export*. So all we
+        # do here is see the combined, linked behavior, and then later below we
+        # see that that behavior remains even after optimizations.
+        output = run_d8_wasm(wasm, args=[second_wasm])
+
+        if output == IGNORE:
+            # There is no point to continue since we can't compare this output
+            # to anything.
+            return
+
+        if output.strip() == 'exception thrown: failed to instantiate module':
+            # We may fail to instantiate the modules for valid reasons, such as
+            # an active segment being out of bounds. There is no point to
+            # continue in such cases, as no exports are called.
+            return
+
+        # Make sure that fuzz_shell.js actually executed all exports from both
+        # wasm files.
+        exports = get_exports(wasm, ['func']) + get_exports(second_wasm, ['func'])
+        assert output.count(FUZZ_EXEC_CALL_PREFIX) == len(exports)
+
+        output = fix_output(output)
+
+        # Optimize at least one of the two.
+        wasms = [wasm, second_wasm]
+        for i in range(random.randint(1, 2)):
+            wasm_index = random.randint(0, 1)
+            name = wasms[wasm_index]
+            new_name = name + f'.opt{i}.wasm'
+            opts = get_random_opts()
+            run([in_bin('wasm-opt'), name, '-o', new_name] + opts + FEATURE_OPTS)
+            wasms[wasm_index] = new_name
+
+        # Run again, and compare the output
+        optimized_output = run_d8_wasm(wasms[0], args=[wasms[1]])
+        optimized_output = fix_output(optimized_output)
+
+        compare(output, optimized_output, 'Two')
+
+    def can_run_on_wasm(self, wasm):
+        # We cannot optimize wasm files we are going to link in closed world
+        # mode. We also cannot run shared-everything code in d8 yet. We also
+        # cannot compare if there are NaNs (as optimizations can lead to
+        # different outputs).
+        return not CLOSED_WORLD and all_disallowed(['shared-everything']) and not NANS
+
+
 # The global list of all test case handlers
 testcase_handlers = [
     FuzzExec(),
@@ -1683,6 +1757,7 @@ def ensure(self):
     # Split(),
     RoundtripText(),
     ClusterFuzz(),
+    Two(),
 ]
 
 
@@ -2136,6 +2211,9 @@ def get_random_opts():
 #   bash %(reduce_sh)s
 #
 # You may also need to add  --timeout 5  or such if the testcase is a slow one.
+#
+# If the testcase handler uses a second wasm file, you may be able to reduce it
+# using BINARYEN_SECOND_WASM.
 #
                   ''' % {'wasm_opt': in_bin('wasm-opt'),
                          'bin': shared.options.binaryen_bin,