include the filename in erroneous diff output where possible (#1452)

Failing test cases often start out with:

```
incorrect output, diff:

--- expected
+++ actual
```

which makes it difficult to figure out where the expected output might
live.  That information can be derived from examining the tests, of
course, but it'd be much nicer if it were provided in the diff to see
straightaway.

We do this by introducing a new check, one which takes a filename of
expected output, which then enables us to display the failing file,
e.g.:

```
incorrect output, diff:

--- /home/froydnj/src/binaryen.git/test/passes/code-folding.txt
+++ actual
```

which is arguably nicer.  Having this new check also enables reducing
some boilerplate `open(...).read()` calls in various places.

There are still a few places using `fail_if_not_identical`, usually
because `.strip()` is used on the expected output.

5 files changed, 25 insertions, 31 deletions

diff --git a/scripts/test/asm2wasm.py b/scripts/test/asm2wasm.py
index abcd41bb4..3fb94fd1d 100644..100755
--- a/scripts/test/asm2wasm.py
+++ b/scripts/test/asm2wasm.py
@@ -20,7 +20,7 @@ import subprocess
 from support import run_command
 from shared import (
     ASM2WASM, WASM_OPT, binary_format_check, delete_from_orbit,
-    fail, fail_with_error, fail_if_not_identical, options, tests
+    fail_with_error, options, tests, fail_if_not_identical_to_file
 )
 
 
@@ -69,9 +69,7 @@ def test_asm2wasm():
           # verify output
           if not os.path.exists(wasm):
             fail_with_error('output .wast file %s does not exist' % wasm)
-          expected = open(wasm, 'rb').read()
-          if actual != expected:
-            fail(actual, expected)
+          fail_if_not_identical_to_file(actual, wasm)
 
           binary_format_check(wasm, verify_final_result=False)
 
@@ -128,9 +126,8 @@ def test_asm2wasm():
           run_command(cmd)
           if not os.path.isfile(jsmap):
             fail_with_error('Debug info map not created: %s' % jsmap)
-          with open(wasm + '.map', 'rb') as expected:
-            with open(jsmap, 'rb') as actual:
-              fail_if_not_identical(actual.read(), expected.read())
+          with open(jsmap, 'rb') as actual:
+            fail_if_not_identical_to_file(actual.read(), wasm + '.map')
           with open('a.wasm', 'rb') as binary:
             url_section_name = bytearray([16]) + bytearray('sourceMappingURL')
             url = 'http://example.org/' + jsmap
diff --git a/scripts/test/lld.py b/scripts/test/lld.py
index f6aa91e6f..01ed51a4a 100755
--- a/scripts/test/lld.py
+++ b/scripts/test/lld.py
@@ -17,8 +17,8 @@
 import os
 from support import run_command
 from shared import (
-    fail, fail_with_error, files_with_pattern, options,
-    WASM_EMSCRIPTEN_FINALIZE
+    fail_with_error, files_with_pattern, options,
+    WASM_EMSCRIPTEN_FINALIZE, fail_if_not_identical_to_file
 )
 
 
@@ -44,9 +44,7 @@ def test_wasm_emscripten_finalize():
       if not os.path.exists(expected_file):
         print actual
         fail_with_error('output ' + expected_file + ' does not exist')
-      expected = open(expected_file, 'rb').read()
-      if actual != expected:
-        fail(actual, expected)
+      fail_if_not_identical_to_file(actual, expected_file)
 
 
 if __name__ == '__main__':
diff --git a/scripts/test/s2wasm.py b/scripts/test/s2wasm.py
index a3345f87b..0e0bec621 100755
--- a/scripts/test/s2wasm.py
+++ b/scripts/test/s2wasm.py
@@ -17,8 +17,8 @@
 import os
 from support import run_command
 from shared import (
-    fail, fail_with_error, fail_if_not_contained,
-    options, S2WASM, WASM_SHELL
+    fail_with_error, fail_if_not_contained,
+    options, S2WASM, WASM_SHELL, fail_if_not_identical_to_file
 )
 
 
@@ -64,9 +64,7 @@ def test_s2wasm():
         if not expected_exists:
           print actual
           fail_with_error('output ' + expected_file + ' does not exist')
-        expected = open(expected_file, 'rb').read()
-        if actual != expected:
-          fail(actual, expected)
+        fail_if_not_identical_to_file(actual, expected_file)
 
         # verify with options
         cmd = S2WASM + [full, '--global-base=1024'] + stack_alloc
diff --git a/scripts/test/shared.py b/scripts/test/shared.py
index 23f10ca2c..9921d0ada 100644
--- a/scripts/test/shared.py
+++ b/scripts/test/shared.py
@@ -344,17 +344,17 @@ def fail_with_error(msg):
       raise
 
 
-def fail(actual, expected):
+def fail(actual, expected, fromfile='expected'):
   diff_lines = difflib.unified_diff(
       expected.split('\n'), actual.split('\n'),
-      fromfile='expected', tofile='actual')
+      fromfile=fromfile, tofile='actual')
   diff_str = ''.join([a.rstrip() + '\n' for a in diff_lines])[:]
   fail_with_error("incorrect output, diff:\n\n%s" % diff_str)
 
 
-def fail_if_not_identical(actual, expected):
+def fail_if_not_identical(actual, expected, fromfile='expected'):
   if expected != actual:
-    fail(actual, expected)
+    fail(actual, expected, fromfile=fromfile)
 
 
 def fail_if_not_contained(actual, expected):
@@ -362,6 +362,11 @@ def fail_if_not_contained(actual, expected):
     fail(actual, expected)
 
 
+def fail_if_not_identical_to_file(actual, expected_file):
+  with open(expected_file, 'rb') as f:
+    fail_if_not_identical(actual, f.read(), fromfile=expected_file)
+
+
 if len(requested) == 0:
   tests = sorted(os.listdir(os.path.join(options.binaryen_test)))
 else:
@@ -401,10 +406,8 @@ def binary_format_check(wast, verify_final_result=True, wasm_as_args=['-g'],
   subprocess.check_call(cmd, stdout=subprocess.PIPE)
 
   if verify_final_result:
-    expected = open(wast + binary_suffix).read()
     actual = open('ab.wast').read()
-    if actual != expected:
-      fail(actual, expected)
+    fail_if_not_identical_to_file(actual, wast + binary_suffix)
 
   return 'ab.wast'
 
diff --git a/scripts/test/wasm2asm.py b/scripts/test/wasm2asm.py
index 6e6b32c81..81cb9b5c5 100755
--- a/scripts/test/wasm2asm.py
+++ b/scripts/test/wasm2asm.py
@@ -4,7 +4,8 @@ import os
 
 from support import run_command
 from shared import (
-    WASM2ASM, MOZJS, NODEJS, fail_if_not_identical, options, tests
+    WASM2ASM, MOZJS, NODEJS, fail_if_not_identical, options, tests,
+    fail_if_not_identical_to_file
 )
 
 # tests with i64s, invokes, etc.
@@ -33,8 +34,7 @@ def test_wasm2asm_output():
 
     cmd = WASM2ASM + [os.path.join(options.binaryen_test, wasm)]
     out = run_command(cmd)
-    expected = open(expected_file).read()
-    fail_if_not_identical(out, expected)
+    fail_if_not_identical_to_file(out, expected_file)
 
     if not NODEJS and not MOZJS:
       print 'No JS interpreters. Skipping spec tests.'
@@ -79,13 +79,11 @@ def test_asserts_output():
     wasm = os.path.join(options.binaryen_test, wasm)
     cmd = WASM2ASM + [wasm, '--allow-asserts']
     out = run_command(cmd)
-    expected = open(asserts_expected_file).read()
-    fail_if_not_identical(out, expected)
+    fail_if_not_identical_to_file(out, asserts_expected_file)
 
     cmd += ['--pedantic']
     out = run_command(cmd)
-    expected = open(traps_expected_file).read()
-    fail_if_not_identical(out, expected)
+    fail_if_not_identical_to_file(out, traps_expected_file)
 
 
 def test_wasm2asm():