tools/jtag: libftdi from $T27_LIBFTDI or the platform default, so B2 reads on Linux (Closes #7927)

[dmitrii-f-t27]

Closes #7927

Description

tools/jtag/mpsse_jtag.py loaded libftdi from a fixed Homebrew path, so stage B2 of t27c silicon (read_verdict.py) failed on Linux with OSError: /opt/homebrew/lib/libftdi1.2.dylib: cannot open shared object file. Die C of R3-3 passed only with a symlink at that path inside the container (#7906, "Caveats").

This is step 2 of #7927. Step 1 was #7928 (merged): the owner-approved exception for this file in tools/policy/foreign-exceptions.txt.

Changes

tools/jtag/mpsse_jtag.py, 3 lines in, 3 lines out:

import ctypes, ctypes.util, os, sys, time
# libftdi1: $T27_LIBFTDI if set; else Homebrew on macOS; else the loader search, so B2 reads on Linux (#7927).
LIB = os.environ.get("T27_LIBFTDI") or ("/opt/homebrew/lib/libftdi1.2.dylib" if sys.platform == "darwin" else ctypes.util.find_library("ftdi1") or "libftdi1.so.2")

• macOS keeps the existing path, so the current benches do not change.
• The issue's version spans 4 lines. It is folded to replace exactly the 3 lines it removes (the blank line becomes the comment), because the foreign-line rule (check_budget, owner rule 2026-10-09) does not allow a foreign file to grow.

No .t27 for this change. There is no Python back end, so no spec can generate this file. The port specs/port/tools/jtag/mpsse_jtag.t27 is a stub. The route is the owner-approved exception from #7928. No spec, seal or conformance vector changes.

Testing

On the head commit (macOS 26, Python 3.14.3, libftdi 1.5_2 from Homebrew):

• Only-t27 gate (gen/c/policy/own_language.c, --ci, exceptions read from the base): exit 0.
• Foreign-line budget (check_budget from the base): exit 0 (3 3 tools/jtag/mpsse_jtag.py).
• python3 -c "import sys; sys.path.insert(0,'tools/jtag'); import mpsse_jtag": exit 0, LIB = /opt/homebrew/lib/libftdi1.2.dylib, the same as on master.
• T27_LIBFTDI=/opt/homebrew/Cellar/libftdi/1.5_2/lib/libftdi1.2.dylib: the module loads that exact path.
• T27_LIBFTDI=/nonexistent/...: OSError that names the given path, so a bad override is reported, not hidden.
• py_compile on mpsse_jtag.py, read_verdict.py, read_user1.py, link_relay.py: OK.
• Path-resolution check with ctypes.CDLL stubbed (sys.platform, T27_LIBFTDI and find_library varied over 8 cases: darwin, linux, env set, env empty, find_library None): 8/8 PASS on this branch. Negative control on master's file: 2/8 (only the two darwin defaults pass).

Not verified

• A real import on Linux without the symlink. Docker on this machine could not start a container (its storage is read-only), so the Linux cases above are covered only by the stubbed resolution check.
• t27c silicon B2 on a Linux host with a board.

Review Notes

tools/policy/foreign-exceptions.txt:449 still says "owner approval pending". That is out of date since #7928 got owner-approved-foreign. It is not changed here: any edit to that file needs the label again. It can be updated in the next PR that has the label.

Dmitrii Fedorov (@dmitrii-f-t27)

[dmitrii-f-t27]

Updated 2026-10-11. Head 0375bb7ee55dd62370a5383ebb90e397a79a91f6 merges master 5e5493cab into this branch, so CI now runs against the current master. The PR diff is still one file, tools/jtag/mpsse_jtag.py.

The required contexts (validate, check-linked-issue, parse-ratchet, own-language) pass. No conflicts with master.

Three checks are red. Each one fails the same way on master:

Check	This PR	Master, same failure
Corpus ratchet	run 38147806957: types ratchet, Entry, Inputs, SkillMeta NEW conflict	5e5493cab run 38146939229: the same 3 names
coverage	run 38147806832: queen_QueenHolds.json gen-drift (gen_hash_rust)	5e5493cab run 38146939200: the same seal
cli-tri	run 38147806776: tri census pin --gate, quiet 185 to 188, shell 305 to 306	push to master a31427373 run 38145387223: the same two census moves

The earlier reds on head 868a3ffb5 (duplicate-bodies, Verilog widths) are green on this head.

Nothing to fix in this PR. These failures come from master.

Dmitrii Fedorov (@dmitrii-f-t27)