fix(eat): guard against a nil charset wedging the terminal

EAT 0.9.4's parser accepts more charset-designation final bytes than its store step maps. A designation like ESC ( A (UK) isn't one of the two it handles ("0" and "B"), so it stores nil as that slot's charset. The next character written then fails (cl-assert charset) in eat--t-write. Since writes run off the output-queue timer, it repeats once per output chunk. An agent terminal that emits one of these bytes throws "cl-assertion-failed (charset)" hundreds of times and stops rendering.

I added filter-args advice on eat--t-set-charset that coerces a nil charset to us-ascii before it's stored, so an unmapped designation falls back to plain ASCII instead of wedging. Patching the vendored pcase would be cleaner, but a package update reverts it. The advice loads with eat, since the target is an internal function.

2 files changed, 70 insertions, 0 deletions

diff --git a/modules/eat-config.el b/modules/eat-config.el
index 1de24dc4..d66507c4 100644
--- a/modules/eat-config.el
+++ b/modules/eat-config.el
@@ -83,6 +83,33 @@ ARGS is (TERMINAL OUTPUT)."
 
 (advice-add 'eat-term-process-output :filter-args #'cj/--eat-reset-sgr-at-newline)
 
+;; EAT 0.9.4 charset-designation bug.  Its parser (eat--t-handle-output) accepts
+;; a wide set of final bytes for an `ESC ( x' charset designation, but the store
+;; step (eat--t-set-charset) only maps two of them: "0" -> dec-line-drawing and
+;; "B" -> us-ascii.  Any other accepted byte (e.g. ESC ( A, the UK set) falls
+;; through the `pcase' to nil, and nil gets stored as that slot's charset.  The
+;; next character written then trips (cl-assert charset) in eat--t-write, and
+;; because writes are driven by the eat--process-output-queue timer it errors
+;; once per output chunk -- the "cl-assertion-failed (charset) [N times]" storm.
+;; We can't patch the vendored pcase without a fork the next package update would
+;; revert, so guard the one function that injects the nil: coerce a nil charset
+;; to us-ascii (the safe default the byte would have mapped to) before it lands.
+
+(declare-function eat--t-set-charset "eat")
+
+(defun cj/--eat-charset-never-nil (args)
+  "`:filter-args' advice for `eat--t-set-charset'.
+ARGS is (SLOT CHARSET).  Return it with a nil CHARSET replaced by
+`us-ascii', so EAT never stores nil for a charset designation it does
+not recognize (which would later trip (cl-assert charset) on write)."
+  (list (car args) (or (cadr args) 'us-ascii)))
+
+;; eat--t-set-charset is an internal function defined only once eat.el loads
+;; (the package is deferred), so add the advice after load rather than at top
+;; level.
+(with-eval-after-load 'eat
+  (advice-add 'eat--t-set-charset :filter-args #'cj/--eat-charset-never-nil))
+
 ;; ------------------------------- eat package ---------------------------------
 
 (use-package eat
diff --git a/tests/test-eat-config--charset-never-nil.el b/tests/test-eat-config--charset-never-nil.el
new file mode 100644
index 00000000..dfd38cb5
--- /dev/null
+++ b/tests/test-eat-config--charset-never-nil.el
@@ -0,0 +1,43 @@
+;;; test-eat-config--charset-never-nil.el --- Tests for the EAT charset nil-guard -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Unit tests for `cj/--eat-charset-never-nil', the `:filter-args' guard on
+;; `eat--t-set-charset'.  eat 0.9.4 accepts more charset-designation final
+;; bytes in its parser than its store step maps, so an unmapped designation
+;; (e.g. ESC ( A) stores nil as that slot's charset; the next character write
+;; then trips (cl-assert charset) in `eat--t-write', repeatedly, via the
+;; output-queue timer.  The guard coerces a nil charset to `us-ascii' before
+;; it is ever stored, so the assertion can't fire.
+
+;;; Code:
+
+(require 'ert)
+
+;; Stub keymap dep before loading the module (matches the other module tests).
+(defvar cj/custom-keymap (make-sparse-keymap)
+  "Stub keymap for testing.")
+
+(require 'eat-config)
+
+(ert-deftest test-eat-config-charset-never-nil-normal-real-charset-unchanged ()
+  "Normal: a real charset value passes through unchanged."
+  (should (equal (cj/--eat-charset-never-nil '(g0 us-ascii))
+                 '(g0 us-ascii)))
+  (should (equal (cj/--eat-charset-never-nil '(g1 dec-line-drawing))
+                 '(g1 dec-line-drawing))))
+
+(ert-deftest test-eat-config-charset-never-nil-error-nil-becomes-us-ascii ()
+  "Error: a nil charset (unmapped designation) is coerced to `us-ascii'."
+  (should (equal (cj/--eat-charset-never-nil '(g0 nil))
+                 '(g0 us-ascii)))
+  (should (equal (cj/--eat-charset-never-nil '(g3 nil))
+                 '(g3 us-ascii))))
+
+(ert-deftest test-eat-config-charset-never-nil-boundary-preserves-slot ()
+  "Boundary: the slot symbol is preserved for every slot, coerced or not."
+  (dolist (slot '(g0 g1 g2 g3))
+    (should (eq (car (cj/--eat-charset-never-nil (list slot nil))) slot))
+    (should (eq (cadr (cj/--eat-charset-never-nil (list slot nil))) 'us-ascii))))
+
+(provide 'test-eat-config--charset-never-nil)
+;;; test-eat-config--charset-never-nil.el ends here