Unexplained pattern match fail on OTP 19 only on Ubuntu

I’m wrestling with a pattern match failure that I can not explain. It happens only on OTP 19 (not OTP 18) and only on the Ubuntu VMs as configured on travis-ci.org. (I can not reproduce on my Mac, which is also using OTP 19.)

The relevant code in question is in https://github.com/scouten/sqlite_ecto. (I’m trying to update sqlite_ecto so that it runs in OTP 19. This is a step along the way to seeing if I can make it work with Ecto 2.x.)

I’ve tried to narrow it down to a simpler case, but that case works as expected.

https://travis-ci.org/scouten/sqlite_ecto/builds/186266623 contains an example of this failure pattern.

Look for the string “HEY, WATCH THIS” in the debug build (look for the extended logs, and then scan down about a page). In the OTP 18 builds, you’ll see the following pattern:

translate_value value = 1 is_integer = true type = "decimal(2,1)"
yup, matches
tv integer clause result = 1.0 is_integer = false
tv float clause float = 1.0 precision = 2 scale = 1
result = #Decimal<1.0>

which means that the pattern match at https://github.com/scouten/sqlitex/blob/0367a06b9308093e1d7fe564278650031c44f18b/lib/sqlitex/row.ex#L59 is matching correctly and causing the raw SQL result value to be parsed into a Decimal object.

In the OTP 19 builds, you’ll see the following different output:

translate_value value = 1 is_integer = true type = "decimal(2,1)"
yup, matches
tv default clause val = 1 is_integer true type = "decimal(2,1)"
result = 1

which means the same pattern match has somehow failed and it’s falling through (incorrectly) to a default clause later.

I’m stuck. No idea why OTP 18 and 19 behave differently (and only on Ubuntu OS).


Update: I’ve hacked around this as shown in https://github.com/scouten/sqlitex/pull/1.

But … ugh. That seems like it should not be necessary.

What are your elixir versions ?

The Travis job is set to try 1.2, 1.3, and 1.3.4. Fails on all three versions but only when running on OTP 19.

Succeeds on all three versions when running on OTP 18.

I’ve nailed this down further. Looks like OTP 19.0.x had a bug in pattern matching in some platforms.

See https://github.com/scouten/otp-pattern-match and its associated Travis CI build log for details.

Closing the loop: Filed an Elixir bug against this.