Lin Out_channel shrink cleanup

[jmid]

This PR contains a range of adjustments to address the long shrinking sequences reported in #378:

• bf61729 and d5ff92a disable char element shrinking inside bytes and strings for the test
• 2433827 removes the lift combinator to reduce allocations
• ba20a6f adjusts the cmd frequencies
• 6c8db20 drops flush_all entirely
• Finally e1a1de3 reduces overall Lin allocations during check_seq_cons and should thus benefit other Lin tests too.

Overall, this makes the test behaviour reasonable. Below I include a CI summary table from running 20 focused tests across the 2 CI systems (I note runs taking more than 1000s as "outliers"). Observations:

• There's a trend of more outliers+timeouts after 5.0 - extra GC activity on +5.1.0 has been confirmed with perf and olly gc-stats measurements
• Disabling the test on macOS should be kept for now, an explanation would be nice though
• There was a crazy linux 5.1 debug 12514.3s outlier!

Overall the PR offers improvements, not the final word. They should help reduce false CI alarms though.

The last two commits 460b97c and f7223d6 update the plain Lin Out_channel test to move it closer feature-wise to the Lin DSL version. These are steps towards creating an standalone reproducer for the extra +5.1.0 GC activity.

	Target	Outliers	Timeout	Remarks
✅	Linux 5.1	-
✅	Linux trunk	1973.0s
❌	Linux 5.1 debug	12514.3s	12th run
✅	Linux trunk debug	1462.0s 2350.7s
✅	32bit trunk	-
✅	32bit 5.1	-
✅	Bytecode 5.1	-
✅	Bytecode trunk	-
✅	linux-arm64-5.0	-
✅	linux-arm64-5.1	2367.5s 1150.1s 1360.5s 3558.0s
✅	linux-arm64-5.2	1426.0s 1416.6s 1435.6s 1160.6s
✅	linux-s390x-5.1	1493.5s
✅	linux-s390x-5.2	-
❌	linux-ppc64le-5.2	2497.0s 1434.2s 3345.5s 3099.9s 1852.6s 2503.3s	11th run
❌	macOS 5.1	7728.8s 1371.7s	3rd run
❌	macOS trunk	-	2nd run
✅	macos-arm64-5.0	2090.3s
❌	macOS-arm64-5.1	3017.1s 3278.3s 2001.6s 1961.0s 1866.6s 2803.1s	10th run	6/9 w/no counterexample!
❌	macOS-arm64-5.2	3988.2s 3447.5s 3344.8s 2651.1s 3579.0s	6th run	5/5 w/no counterexample!
✅	Windows 5.1	1030.1s 2523.1s
✅	Windows trunk	2471.4s 1139.8s 2522.6s
✅	Win bytecode 5.1	1391.8s 3730.3s
✅	Win bytecode trunk	-
✅	Cygwin 5.1	1004.0s 1206.1s
✅	Cygwin trunk	1010.6s

[jmid]

Oh wow 😮 linux-arm64 5.2 had a 16443.2s Lin DSL Out_channel test with Domain run, yet somehow still managed to complete within the timeout limit!

[shym]

Looks good to me! :-)

[jmid]

CI summary for f7223d6 from yesterday:

• 3 Windows threadomain failure [ocaml5-issue] Windows failures on threadomain #203
  • bytecode 5.1 - new lock failure variant (already noted in [ocaml5-issue] Windows failures on threadomain #203)
  • bytecode trunk/5.2 crash and trunk/5.2 triggered regular crashes
• 3 Cygwin part2 timeouts [ocaml5-issue] General Cygwin slowness #384 Improve reproducibility of Lin/STM Thread modes #338
  • 5.1 - during a 5961.1s Lin CList int64 test with Thread
  • trunk (PR) during a 8343.3s Lin ref int64 test with Thread
  • trunk (push) during a 4730.3s Lin CList int64 test with Thread
• linux-ppc64le-5.2 failed with SEGV during sequential STM Array and Bytes tests [ocaml5-issue] Crashes and hangs on ppc64 trunk/5.2 #380

For the Cygwin failures, I get suspicious that the slowness is affecting the ability to trigger test failures and thereby cause needlessly long shrink runs 🤔

[jmid]

CI summary for bb94d34:

• multicoretest-ci runs were cancelled
• Cygwin trunk part 1 timeout during Dynlink test [ocaml5-issue] Deadlock in Dynlink test on Cygwin+MinGW+MSVC #307
• MingW bytecode 5.1 and MingW trunk timeouts during threadomain [ocaml5-issue] Windows failures on threadomain #203

So 3/23 failures - but none of them being false positives! 😃

[jmid]

CI summary for b9509bb

• MingW bytecode trunk timeout during threadomain [ocaml5-issue] Windows failures on threadomain #203
• Cygwin 5.1 part 2 timeout during a 3016.7s Lin CList int64 test with Thread, incl. a 5144.7s Lin ref int64 test with Thread [ocaml5-issue] General Cygwin slowness #384 Improve reproducibility of Lin/STM Thread modes #338
• linux-ppc64le-5.2 SEGV'ed during sequential STM Bytes and Float Array tests [ocaml5-issue] Crashes and hangs on ppc64 trunk/5.2 #380
• macos-arm64-5.1 failed with an opam switch 5.1.0~rc2 build failure

So, 4/52 failures with only 1 of them being noise (Cygwin slowness is a genuine issue) and 1 being container related.

I'll merge 😃