reader: fix/unbreak reading of here strings

The repair in 55a27e01a6 wasn't right.

pkgs/racket-test-core/tests/racket/read.rktl

@@ -214,6 +214,9 @@
 (err/rt-test (readstr "\"\\uD800\\uDD1\"") exn:fail:read?)
 
 (test "abc" readstr "#<<EOS\nabc\nEOS\n")
+(test "abc\ndef" readstr "#<<EOS\nabc\ndef\nEOS\n")
+(test "\n" readstr "#<<EOS\n\n\nEOS\n")
+(test "abc" readstr "#<<EOS\nabc\nEOS\n   ")
 (test "abc" readstr "#<<EOS\nabc\nEOS")
 (test "ok\r\nmore\r" readstr "#<<EOS\r\nok\r\nmore\r\nEOS\r")
 (err/rt-test (readstr "#<<EOS\nabc\nEO") exn:fail:read?)

racket/src/expander/read/string.rkt

@@ -172,7 +172,7 @@
   ;; Parse terminator
   (define full-terminator
     (cons
-     #\newline
+     #\newline ;; assumption below that this character is first
      (let loop ()
        (define c (read-char/special in config source))
        (cond
@@ -193,7 +193,7 @@
       (unless (null? terminator)
         (reader-error in config #:due-to c
                       "found end-of-file before terminating `~a`"
-                      (list->string full-terminator)))]
+                      (list->string (cdr full-terminator))))]
      [(not (char? c))
       (reader-error in config #:due-to c
                     "found non-character while reading `#<<`")]
@@ -207,8 +207,13 @@
       (unless (null? terminator-accum)
         (for ([c (in-list (reverse terminator-accum))])
           (accum-string-add! accum-str c)))
-      (accum-string-add! accum-str c)
-      (loop full-terminator null)]))
+      (cond
+        [(char=? c #\newline)
+         ;; assume `full-terminator` starts with #\newline
+         (loop (cdr full-terminator) (list #\newline))]
+        [else
+         (accum-string-add! accum-str c)
+         (loop full-terminator null)])]))
 
   ;; Done
   (define str (accum-string-get! accum-str config))

racket/src/racket/src/startup.inc

@@ -52069,7 +52069,8 @@ static const char *startup_source =
 "(let-values(((c102_0) c_88)"
 "((temp103_2)"
 "                                                                          \"found end-of-file before terminating `~a`\")"
-"((temp104_2)(list->string full-terminator_0)))"
+"((temp104_2)"
+"(list->string(cdr full-terminator_0))))"
 "(reader-error10.1"
 " #f"
 " #f"
@@ -52149,8 +52150,15 @@ static const char *startup_source =
 " for-loop_6)"
 " lst_96)))"
 "(void))))"
+"(if(char=? c_88 '#\\newline)"
+"(let-values()"
+"(loop_113"
+"(cdr full-terminator_0)"
+"(list '#\\newline)))"
+"(let-values()"
+"(begin"
 "(accum-string-add! accum-str_5 c_88)"
-"(loop_113 full-terminator_0 null))))))))))))"
+"(loop_113 full-terminator_0 null)))))))))))))))"
 " loop_113)"
 " full-terminator_0"
 " null)"