Handle completely undecoded input in term (Bug#29918)

* lisp/term.el (term-emulate-terminal): Avoid errors if the whole
decoded string is eight-bit characters.  Don't attempt to save the
string for next iteration in that case.
* test/lisp/term-tests.el (term-decode-partial)
(term-undecodable-input): New tests.

2 files changed, 24 insertions, 4 deletions

diff --git a/lisp/term.el b/lisp/term.el
index cbef68dc0ac..9785ce30249 100644
--- a/lisp/term.el
+++ b/lisp/term.el
@@ -2900,11 +2900,12 @@ See `term-prompt-regexp'."
                           ;; next time.
                           (when (= funny str-length)
                             (let ((partial 0))
-                              (while (eq (char-charset (aref decoded-substring
-                                                             (- count 1 partial)))
-                                         'eight-bit)
+                              (while (and (< partial count)
+                                          (eq (char-charset (aref decoded-substring
+                                                                  (- count 1 partial)))
+                                              'eight-bit))
                                 (cl-incf partial))
-                              (when (> partial 0)
+                              (when (> count partial 0)
                                 (setq term-terminal-undecoded-bytes
                                       (substring decoded-substring (- partial)))
                                 (setq decoded-substring
diff --git a/test/lisp/term-tests.el b/test/lisp/term-tests.el
index c2b90dea604..237e19d6b15 100644
--- a/test/lisp/term-tests.el
+++ b/test/lisp/term-tests.el
@@ -144,6 +144,25 @@ This is a reduced example from GNU nano's initial screen."
                     `("\e[1;3r" "\e[2;1H" ,x "\r\e[1A" ,y))
                    (concat y "\n" x)))))
 
+(ert-deftest term-decode-partial () ;; Bug#25288.
+  "Test multibyte characters sent into multiple chunks."
+  ;; Set `locale-coding-system' so test will be deterministic.
+  (let* ((locale-coding-system 'utf-8-unix)
+         (string (make-string 7 ?ш))
+         (bytes (encode-coding-string string locale-coding-system)))
+    (should (equal string
+                   (term-test-screen-from-input
+                    40 1 `(,(substring bytes 0 (/ (length bytes) 2))
+                           ,(substring bytes (/ (length bytes) 2))))))))
+
+(ert-deftest term-undecodable-input () ;; Bug#29918.
+  "Undecodable bytes should be passed through without error."
+  (let* ((locale-coding-system 'utf-8-unix) ; As above.
+         (bytes "\376\340\360\370")
+         (string (decode-coding-string bytes locale-coding-system)))
+    (should (equal string
+                   (term-test-screen-from-input
+                    40 1 bytes)))))
 
 (provide 'term-tests)