[foozzie] Make comparison before crash sensitive to single characters

The assumtion that V8 has no output differences within a single line
before a stack overflow, didn't hold. The prefix of e.g. console.info
can lead to a difference in a recursive call.

This change makes foozzie's output capping before a crash work on the
level of characters instead of lines to fix this.

No-Try: true
Bug: chromium:1050942
Change-Id: I13f747caf4f5848d40c31bd4232811285bab3c17
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2049844Reviewed-by: Liviu Rau <liviurau@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66217}

tools/clusterfuzz/v8_foozzie_test.py

@@ -142,8 +142,8 @@ v8-foozzie source: foo
       output1 = output(stdout1, is_crash1)
       output2 = output(stdout2, is_crash2)
       self.assertEquals(
-          (capped_lines1.splitlines(), capped_lines2.splitlines()),
-          v8_suppressions.get_lines_capped(output1, output2))
+          (capped_lines1, capped_lines2),
+          v8_suppressions.get_output_capped(output1, output2))
 
     # No capping, already equal.
     check('1\n2', '1\n2', True, True, '1\n2', '1\n2')
@@ -153,15 +153,23 @@ v8-foozzie source: foo
     # Cap smallest if all runs crash.
     check('1\n2', '1\n2\n3', True, True, '1\n2', '1\n2')
     check('1\n2\n3', '1\n2', True, True, '1\n2', '1\n2')
+    check('1\n2', '1\n23', True, True, '1\n2', '1\n2')
+    check('1\n23', '1\n2', True, True, '1\n2', '1\n2')
     # Cap the non-crashy run.
     check('1\n2\n3', '1\n2', False, True, '1\n2', '1\n2')
     check('1\n2', '1\n2\n3', True, False, '1\n2', '1\n2')
+    check('1\n23', '1\n2', False, True, '1\n2', '1\n2')
+    check('1\n2', '1\n23', True, False, '1\n2', '1\n2')
     # The crashy run has more output.
     check('1\n2\n3', '1\n2', True, False, '1\n2\n3', '1\n2')
     check('1\n2', '1\n2\n3', False, True, '1\n2', '1\n2\n3')
+    check('1\n23', '1\n2', True, False, '1\n23', '1\n2')
+    check('1\n2', '1\n23', False, True, '1\n2', '1\n23')
     # Keep output difference when capping.
     check('1\n2', '3\n4\n5', True, True, '1\n2', '3\n4')
     check('1\n2\n3', '4\n5', True, True, '1\n2', '4\n5')
+    check('12', '345', True, True, '12', '34')
+    check('123', '45', True, True, '12', '45')
 
 
 def cut_verbose_output(stdout):

tools/clusterfuzz/v8_suppressions.py

@@ -136,32 +136,28 @@ IGNORE_LINES = [re.compile(exp) for exp in IGNORE_LINES]
 ORIGINAL_SOURCE_PREFIX = 'v8-foozzie source: '
 
 
-def get_lines_capped(output1, output2):
-  """Returns a pair of stdout line lists.
+def get_output_capped(output1, output2):
+  """Returns a pair of stdout strings.
 
-  The lists are safely capped if at least one run has crashed. We assume
-  that output can never break off within the last line. If this assumption
-  turns out wrong, we need to add capping of the last line, too.
+  The strings are safely capped if at least one run has crashed.
   """
-  output1_lines = output1.stdout.splitlines()
-  output2_lines = output2.stdout.splitlines()
 
   # No length difference or no crash -> no capping.
-  if (len(output1_lines) == len(output2_lines) or
+  if (len(output1.stdout) == len(output2.stdout) or
       (not output1.HasCrashed() and not output2.HasCrashed())):
-    return output1_lines, output2_lines
+    return output1.stdout, output2.stdout
 
   # Both runs have crashed, cap by the shorter output.
   if output1.HasCrashed() and output2.HasCrashed():
-    cap = min(len(output1_lines), len(output2_lines))
+    cap = min(len(output1.stdout), len(output2.stdout))
   # Only the first run has crashed, cap by its output length.
   elif output1.HasCrashed():
-    cap = len(output1_lines)
+    cap = len(output1.stdout)
   # Similar if only the second run has crashed.
   else:
-    cap = len(output2_lines)
+    cap = len(output2.stdout)
 
-  return output1_lines[0:cap], output2_lines[0:cap]
+  return output1.stdout[0:cap], output2.stdout[0:cap]
 
 
 def line_pairs(lines):
@@ -297,7 +293,8 @@ class V8Suppression(Suppression):
 
   def diff(self, output1, output2):
     # Diff capped lines in the presence of crashes.
-    return self.diff_lines(*get_lines_capped(output1, output2))
+    return self.diff_lines(
+        *map(str.splitlines, get_output_capped(output1, output2)))
 
   def diff_lines(self, output1_lines, output2_lines):
     return diff_output(