Provide as much diagnostic information as possible in "panic: ..." messages.

The convention is that when the interpreter dies with an internal error, the
message starts "panic: ". Historically, many panic messages had been terse
fixed strings, which means that the out-of-range values that triggered the
panic are lost. Now we try to report these values, as such panics may not be
repeatable, and the original error message may be the only diagnostic we get
when we try to find the cause.

We can't report diagnostics when the panic message is generated by something
other than croak(), as we don't have *printf-style format strings. Don't
attempt to report values in panics related to *printf buffer overflows, as
attempting to format the values to strings may repeat or compound the
original error.

1 files changed, 6 insertions, 3 deletions

diff --git a/sv.c b/sv.c
index 1fc5459891..dff16078b4 100644
--- a/sv.c
+++ b/sv.c
@@ -4478,7 +4478,8 @@ Perl_sv_setpvn(pTHX_ register SV *const sv, register const char *const ptr, regi
         /* len is STRLEN which is unsigned, need to copy to signed */
 	const IV iv = len;
 	if (iv < 0)
-	    Perl_croak(aTHX_ "panic: sv_setpvn called with negative strlen");
+	    Perl_croak(aTHX_ "panic: sv_setpvn called with negative strlen %"
+		       IVdf, iv);
     }
     SvUPGRADE(sv, SVt_PV);
 
@@ -5793,7 +5794,8 @@ Perl_sv_insert_flags(pTHX_ SV *const bigstr, const STRLEN offset, const STRLEN l
     bigend = big + SvCUR(bigstr);
 
     if (midend > bigend)
-	Perl_croak(aTHX_ "panic: sv_insert");
+	Perl_croak(aTHX_ "panic: sv_insert, midend=%p, bigend=%p",
+		   midend, bigend);
 
     if (mid - big > bigend - midend) {	/* faster to shorten from end */
 	if (littlelen) {
@@ -7076,7 +7078,8 @@ Perl_sv_pos_b2u(pTHX_ register SV *const sv, I32 *const offsetp)
     s = (const U8*)SvPV_const(sv, blen);
 
     if (blen < byte)
-	Perl_croak(aTHX_ "panic: sv_pos_b2u: bad byte offset");
+	Perl_croak(aTHX_ "panic: sv_pos_b2u: bad byte offset, blen=%"UVuf
+		   ", byte=%"UVuf, (UV)blen, (UV)byte);
 
     send = s + byte;