Fix ASCII-only conversion logic (bug#40407)

To sidestep conversion altogether when EOL conversion applies, we must
either be encoding a string without NL, or decoding without CR.

* src/coding.c (string_ascii_p): Revert to a pure predicate.
(code_convert_string): Fix logic.  Don't use uninitialised
ascii_p (removed).  Use memchr to detect CR or LF in string when needed.
* test/src/coding-tests.el (coding-nocopy-ascii):
Update tests to include encodings with explicit EOL conversions.

1 files changed, 17 insertions, 29 deletions

diff --git a/src/coding.c b/src/coding.c
index ffcb9cf0a1a..450c498f1e8 100644
--- a/src/coding.c
+++ b/src/coding.c
@@ -9474,22 +9474,15 @@ not fully specified.)  */)
   return code_convert_region (start, end, coding_system, destination, 1, 0);
 }
 
-/* Non-zero if STR contains only characters 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)
+/* Whether STRING only contains chars in the 0..127 range.  */
+static bool
+string_ascii_p (Lisp_Object string)
 {
-  ptrdiff_t nbytes = SBYTES (str);
-  bool CR_Seen = false;
+  ptrdiff_t nbytes = SBYTES (string);
   for (ptrdiff_t i = 0; i < nbytes; i++)
-    {
-      if (SREF (str, i) > 127)
-        return 0;
-      if (SREF (str, i) == '\r')
-        CR_Seen = true;
-    }
-  return CR_Seen ? -1 : 1;
+    if (SREF (string, i) > 127)
+      return false;
+  return true;
 }
 
 Lisp_Object
@@ -9526,24 +9519,19 @@ code_convert_string (Lisp_Object string, Lisp_Object coding_system,
   if (EQ (dst_object, Qt))
     {
       /* Fast path for ASCII-only input and an ASCII-compatible coding:
-         act as identity if no EOL conversion is neede.  */
-      int ascii_p;
+         act as identity if no EOL conversion is needed.  */
       Lisp_Object attrs = CODING_ID_ATTRS (coding.id);
       if (! NILP (CODING_ATTR_ASCII_COMPAT (attrs))
           && (STRING_MULTIBYTE (string)
-              ? (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)));
-        }
+              ? (chars == bytes) : string_ascii_p (string))
+          && (EQ (CODING_ID_EOL_TYPE (coding.id), Qunix)
+              || inhibit_eol_conversion
+              || ! memchr (SDATA (string), encodep ? '\n' : '\r', bytes)))
+        return (nocopy
+                ? string
+                : (encodep
+                   ? make_unibyte_string (SSDATA (string), bytes)
+                   : make_multibyte_string (SSDATA (string), bytes, bytes)));
     }
   else if (BUFFERP (dst_object))
     {