[perl #128951] Fix ASan error with @{\327

By \327 I mean character number 327 in octal.

Without memory tools like ASan, it produces garbled output.  The added
test fails like this:

# Failed test 18 - @ { \327 \n - used to garble output (or fail asan) [perl \#128951] at ./test.pl line 1058
#      got "Unrecognized character \\xD7; marked by <-- HERE after \x{a0}\x{f6}@3\x{a8}\x{7f}\000\000@{<-- HERE near column -1 at - line 1."
# expected "Unrecognized character \\xD7; marked by <-- HERE after @{<-- HERE near column 3 at - line 1."

Dave Mitchell’s explanation from the RT ticket:
> The src code contains the bytes:
>
>     @ { \327 \n
>
> after seeing "@{" the lexer calls scan_ident(), which sees the \327 as an
> ident, then calls S_skipspace_flags() to skip the spaces following the
> ident.  This moves the current cursor position to the \n, and since that's
> a line boundary, its updates PL_linestart and PL_bufptr to point to \n
> too.
>
> When it finds that the next char isn't a '}', it does this:
>
>             /* Didn't find the closing } at the point we expected, so restore
>                state such that the next thing to process is the opening { and */
> 	    s = SvPVX(PL_linestr) + bracket; /* let the parser handle it */
>
> i.e. it moves s back to the "{\317" then continues.
>
> However, PL_linestart doesn't get reset, so later when the parser
> encounters the \327 and tries to croak with "Unrecognized character %s ...",
> when it prints out the section of src code in error, since s < PL_linestr,
> negative string lengths and ASAN errors ensue.

This commit fixes it by passing the LEX_NO_INCLINE flag (added by
2179133), which specifies that we are not trying to read past the
newline but simply peek ahead.  In that case lex_read_space does not
reset PL_linestart.

But that does cause problems with code like:

${;
#line 3

}

because we end up jumping ahead via skipspace without updating the
line number.  So we need to do a skipspace_flags(..., LEX_NO_INCLINE)
first (i.e., peek ahead), and then when we know we don’t need to go
back again we can skipspace(...) for real.

Author: Father Chrysostomos

t/op/lex.t

@@ -7,7 +7,7 @@ use warnings;
 
 BEGIN { chdir 't' if -d 't'; require './test.pl'; }
 
-plan(tests => 27);
+plan(tests => 28);
 
 {
     no warnings 'deprecated';
@@ -129,7 +129,7 @@ fresh_perl_is(
   '* <null> ident'
 );
 SKIP: {
-    skip "Different output on EBCDIC (presumably)", 2 if $::IS_EBCDIC;
+    skip "Different output on EBCDIC (presumably)", 3 if $::IS_EBCDIC;
     fresh_perl_is(
       qq'"ab}"ax;&\0z\x8Ao}\x82x;', <<gibberish,
 Bareword found where operator expected at - line 1, near ""ab}"ax"
@@ -150,6 +150,13 @@ gibberish
        { stderr => 1 },
       'gibberish containing &{+z} - used to crash [perl #123753]'
     );
+    fresh_perl_is(
+      "\@{\327\n", <<\gibberisi,
+Unrecognized character \xD7; marked by <-- HERE after @{<-- HERE near column 3 at - line 1.
+gibberisi
+       { stderr => 1 },
+      '@ { \327 \n - used to garble output (or fail asan) [perl #128951]'
+    );
 }
 
 fresh_perl_is(

toke.c

@@ -9074,6 +9074,8 @@ S_scan_ident(pTHX_ char *s, char *dest, STRLEN destlen, I32 ck_uni)
     else if (ck_uni && bracket == -1)
 	check_uni();
     if (bracket != -1) {
+        bool skip;
+        char *s2;
         /* If we were processing {...} notation then...  */
 	if (isIDFIRST_lazy_if(d,is_utf8)) {
             /* if it starts as a valid identifier, assume that it is one.
@@ -9122,13 +9124,19 @@ S_scan_ident(pTHX_ char *s, char *dest, STRLEN destlen, I32 ck_uni)
 
         if ( !tmp_copline )
             tmp_copline = CopLINE(PL_curcop);
-        if (s < PL_bufend && isSPACE(*s)) {
-            s = skipspace(s);
-        }
+        if ((skip = s < PL_bufend && isSPACE(*s)))
+            /* Avoid incrementing line numbers or resetting PL_linestart,
+               in case we have to back up.  */
+            s2 = skipspace_flags(s, LEX_NO_INCLINE);
+        else
+            s2 = s;
 
         /* Expect to find a closing } after consuming any trailing whitespace.
          */
-	if (*s == '}') {
+        if (*s2 == '}') {
+            /* Now increment line numbers if applicable.  */
+            if (skip)
+                s = skipspace(s);
 	    s++;
 	    if (PL_lex_state == LEX_INTERPNORMAL && !PL_lex_brackets) {
 		PL_lex_state = LEX_INTERPEND;