./mach jstests 
[   17|    0|    0|    0]   0% >                                       |  13.9sTraceback (most recent call last):
  File "/home/matthew/unified/js/src/tests/jstests.py", line 676, in <module>
    sys.exit(main())
  File "/home/matthew/unified/js/src/tests/jstests.py", line 624, in main
    results.push(out)
  File "/home/matthew/unified/js/src/tests/lib/results.py", line 247, in push
    result = TestResult.from_output(output)
  File "/home/matthew/unified/js/src/tests/lib/results.py", line 176, in from_output
    if test.error not in err:
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 222: ordinal not in range(128)

Doing a little tweaking, and printing err, and we see the error triggers here:

[   18|    0|    0|    0]   0% >                                       |  14.0s/home/matthew/unified/js/src/tests/test262/language/identifiers/vertical-tilde-start.js:17:4 SyntaxError: illegal character:
/home/matthew/unified/js/src/tests/test262/language/identifiers/vertical-tilde-start.js:17:4 var ⸯ; // U+2E2F
/home/matthew/unified/js/src/tests/test262/language/identifiers/vertical-tilde-start.js:17:4 ....^

Where presumably U+2E2F is what's making python fall over.

For further debugging, I put

        print(err)
        print(type(err))
        print(type(test.error))
        sys.stdout.flush()

just above the

        if test.error is not None:

That prints out pretty much the same thing in the good and bad cases:

[   17|    0|    0|    0]   0% >                                       |  14.0s/home/matthew/unified/js/src/tests/test262/language/identifiers/val-with-via-escape-hex.js:17:4 SyntaxError: with is an invalid identifier:
/home/matthew/unified/js/src/tests/test262/language/identifiers/val-with-via-escape-hex.js:17:4 var \u{77}ith = 123;
/home/matthew/unified/js/src/tests/test262/language/identifiers/val-with-via-escape-hex.js:17:4 ....^

<type 'str'>
<type 'unicode'>
[   18|    0|    0|    0]   0% >                                       |  14.0s/home/matthew/unified/js/src/tests/test262/language/identifiers/vertical-tilde-start.js:17:4 SyntaxError: illegal character:
/home/matthew/unified/js/src/tests/test262/language/identifiers/vertical-tilde-start.js:17:4 var ⸯ; // U+2E2F
/home/matthew/unified/js/src/tests/test262/language/identifiers/vertical-tilde-start.js:17:4 ....^

<type 'str'>
<type 'unicode'>

Here's where my python foo runs aground.

Ran hg bisect to find the regressing changeset:

The first bad revision is:
changeset:   593977:b79ab69854b0
user:        Ricky Stewart <rstewart@mozilla.com>
date:        Tue May 05 20:32:12 2020 +0000
summary:     Bug 1632916 - Run JS/web-platform/ipdl build machinery in Python 3 r=jgraham,nika,glandium

This patch gets me back in action, but definitely seems wrong

diff --git a/js/src/tests/lib/results.py b/js/src/tests/lib/results.py
--- a/js/src/tests/lib/results.py
+++ b/js/src/tests/lib/results.py
@@ -173,7 +173,7 @@ class TestResult:
 
         if test.error is not None:
             expected_rcs.append(3)
-            if test.error not in err:
+            if test.error not in unicode(err, 'utf-8'):
                 failures += 1
                 results.append((cls.FAIL, "Expected uncaught error: {}".format(test.error)))

Ricky, not sure what to do about this.

For me this reproduces easily enough on a clobber build with the following mozconfig:

ac_add_options --enable-application=js

ac_add_options --enable-optimize
ac_add_options --enable-debug
#ac_add_options --disable-debug

# ac_add_options --enable-smoosh
ac_add_options --enable-tests
ac_add_options --enable-binast

# Dump opt builds into another dir.
mk_add_options MOZ_OBJDIR=@TOPSRCDIR@/obj-opt-shell-@CONFIG_GUESS@

Bugbug thinks this bug is a defect, but please change it back in case of error.

Set release status flags based on info from the regressing bug 1632916

Bug 1632916 regressed mach jstests, which still run under Python 2 (unlike the in-build jstests, which use Python 3). Here we call into different logic for Python 2 and 3 to make sure there aren't type mismatches down the line.

https://hg.mozilla.org/mozilla-central/rev/ae53dbf57eeb