Bug #21172: Race condition in `register_fstring`

• Status: Closed
• Assignee: -
• Target version: -
• Backport: 3.1: WONTFIX, 3.2: WONTFIX, 3.3: DONE, 3.4: DONE

[byroot (Jean Boussier)]

This is a rare bug with very low criticality, but it can cause occasional CI failures. I leave it to branch maintainers to decide if it's worth backporting or not.

Timeline

• A "foo" oid=1 string is interned.
• "foo" oid=1 is no longer referenced and will be swept in the future.
• Another "foo" oid=2 string is interned.
• register_fstring finds "foo" oid=1, but since it is about to be swept, removes it from fstring_table and insert "foo" oid=2 instead.
• "foo" oid=1 is swept, since it has the RSTRING_FSTR flag, a st_delete is issued in fstring_table which removes "foo" oid=2.

Unfortunately, I couldn't come up with a minimal reproduction for this, but it did reproduce consistently with a large amount of tests and a very specific seed.

Consequence

This has little to no consequence on real world code. It just breaks a few ruby tests that is attempting to ensure some strings were interned.

Example:

	  1) Failure:
	JSONParserTest#test_freeze [/tmp/ruby/src/trunk/test/json/json_parser_test.rb:361]:
	Expected "foo" (oid=416) to be the same as "foo" (oid=424).
	make: *** [uncommon.mk:953: yes-test-all] Error 1

[byroot (Jean Boussier)]

• Description updated (diff)

[byroot (Jean Boussier)]

• Status changed from Open to Closed

Applied in changeset git|c224ca4feaff20cab03d76439bcbfb35d4e2f6b1.

---

Fix a race condition with interned strings sweeping.

[Bug #21172]

This fixes a rare CI failure.

The timeline of the race condition is:

• A "foo" oid=1 string is interned.
• "foo" oid=1 is no longer referenced and will be swept in the future.
• Another "foo" oid=2 string is interned.
• register_fstring finds "foo" oid=1, but since it is about to be swept,
  removes it from fstring_table and insert "foo" oid=2 instead.
• "foo" oid=1 is swept, since it has the RSTRING_FSTR flag,
  a st_delete is issued in fstring_table which removes "foo" oid=2.

I don't know how to reproduce this bug consistently in a single test
case.

[nagachika (Tomoyuki Chikanaga)]

• Backport changed from 3.1: WONTFIX, 3.2: WONTFIX, 3.3: REQUIRED, 3.4: REQUIRED to 3.1: WONTFIX, 3.2: WONTFIX, 3.3: DONE, 3.4: REQUIRED

ruby_3_3 726bff43b462d2a1b0bc93299cf031202f7fe7a1 merged revision(s) c224ca4feaff20cab03d76439bcbfb35d4e2f6b1.

[byroot (Jean Boussier)]

• Backport changed from 3.1: WONTFIX, 3.2: WONTFIX, 3.3: DONE, 3.4: REQUIRED to 3.1: WONTFIX, 3.2: WONTFIX, 3.3: DONE, 3.4: DONE

https://github.com/ruby/ruby/pull/12873