Fix decoding ASCII strings with embedded CR characters

* src/coding.c (string_ascii_p): Return a negative value if an
all-ASCII string STR includes the CR character, otherwise a
positive value.
(code_convert_string): If the string is ASCII, but includes CR
characters, use the fast path only if EOL doesn't need to be
decoded.  (Bug#40519)

* test/src/coding-tests.el (coding-nocopy-ascii): Add tests for
bug#40519.

2 files changed, 43 insertions, 11 deletions

diff --git a/src/coding.c b/src/coding.c
index 49c1e625d57..24a832ff3ee 100644
--- a/src/coding.c
+++ b/src/coding.c
@@ -9471,15 +9471,22 @@ not fully specified.)  */)
   return code_convert_region (start, end, coding_system, destination, 1, 0);
 }
 
-/* Whether a string only contains chars in the 0..127 range.  */
-static bool
+/* Non-zero if STR contains only characterss in the 0..127 range.
+   Positive if STR includes characters that don't need EOL conversion
+   on decoding, negative otherwise.  */
+static int
 string_ascii_p (Lisp_Object str)
 {
   ptrdiff_t nbytes = SBYTES (str);
+  bool CR_Seen = false;
   for (ptrdiff_t i = 0; i < nbytes; i++)
-    if (SREF (str, i) > 127)
-      return false;
-  return true;
+    {
+      if (SREF (str, i) > 127)
+        return 0;
+      if (SREF (str, i) == '\r')
+        CR_Seen = true;
+    }
+  return CR_Seen ? -1 : 1;
 }
 
 Lisp_Object
@@ -9517,15 +9524,23 @@ code_convert_string (Lisp_Object string, Lisp_Object coding_system,
     {
       /* Fast path for ASCII-only input and an ASCII-compatible coding:
          act as identity.  */
+      int ascii_p;
       Lisp_Object attrs = CODING_ID_ATTRS (coding.id);
       if (! NILP (CODING_ATTR_ASCII_COMPAT (attrs))
           && (STRING_MULTIBYTE (string)
-              ? (chars == bytes) : string_ascii_p (string)))
-        return (nocopy
-                ? string
-                : (encodep
-                   ? make_unibyte_string (SSDATA (string), bytes)
-                   : make_multibyte_string (SSDATA (string), bytes, bytes)));
+              ? (chars == bytes) : ((ascii_p = string_ascii_p (string)) != 0)))
+        {
+          if (ascii_p > 0
+              || (ascii_p < 0
+                  && (EQ (CODING_ID_EOL_TYPE (coding.id), Qunix)
+                      || inhibit_eol_conversion)))
+            return (nocopy
+                    ? string
+                    : (encodep
+                       ? make_unibyte_string (SSDATA (string), bytes)
+                       : make_multibyte_string (SSDATA (string),
+                                                bytes, bytes)));
+        }
     }
   else if (BUFFERP (dst_object))
     {
diff --git a/test/src/coding-tests.el b/test/src/coding-tests.el
index 93e6709d442..83a06b8179e 100644
--- a/test/src/coding-tests.el
+++ b/test/src/coding-tests.el
@@ -388,6 +388,23 @@
   (let* ((uni (apply #'string (number-sequence 0 127)))
          (multi (string-to-multibyte uni)))
     (dolist (s (list uni multi))
+      (dolist (coding '(us-ascii-unix iso-latin-1-unix utf-8-unix))
+        (should-not (eq (decode-coding-string s coding nil) s))
+        (should-not (eq (encode-coding-string s coding nil) s))
+        (should (eq (decode-coding-string s coding t) s))
+        (should (eq (encode-coding-string s coding t) s)))))
+  (let* ((uni (apply #'string (number-sequence 15 127)))
+         (multi (string-to-multibyte uni)))
+    (dolist (s (list uni multi))
+      (dolist (coding '(us-ascii iso-latin-1 utf-8))
+        (should-not (eq (decode-coding-string s coding nil) s))
+        (should-not (eq (encode-coding-string s coding nil) s))
+        (should (eq (decode-coding-string s coding t) s))
+        (should (eq (encode-coding-string s coding t) s)))))
+  (let* ((uni (apply #'string (number-sequence 0 127)))
+         (multi (string-to-multibyte uni))
+         (inhibit-eol-conversion t))
+    (dolist (s (list uni multi))
       (dolist (coding '(us-ascii iso-latin-1 utf-8))
         (should-not (eq (decode-coding-string s coding nil) s))
         (should-not (eq (encode-coding-string s coding nil) s))