[Ada] gnatxref: infinite loop on symbols not found

[Pierre-Marie de Rodat <derodat at adacore dot com>]

This patch fixes a bug in which if a symbol is not found, gnatxref can
sometimes enter an infinite loop. No impact on compilation.

Tested on x86_64-pc-linux-gnu, committed on trunk

2019-09-19  Bob Duff  <duff@adacore.com>

gcc/ada/

	* xref_lib.adb (Get_Symbol_Name): If we reach EOF in the first
	loop without finding the symbol, return "???". Otherwise, it's
	an infinite loop.
	(Parse_EOL): Assert that we're not already at EOF.  Remove
	processing of LF/CR -- there are no operating systems that use
	that.

--- gcc/ada/xref_lib.adb
+++ gcc/ada/xref_lib.adb
@@ -723,6 +723,8 @@ package body Xref_Lib is
    is
    begin
       loop
+         pragma Assert (Source (Ptr) /= EOF);
+
          --  Skip to end of line
 
          while Source (Ptr) /= ASCII.CR and then Source (Ptr) /= ASCII.LF
@@ -737,11 +739,9 @@ package body Xref_Lib is
             Ptr := Ptr + 1;
          end if;
 
-         --  Skip past CR/LF or LF/CR combination
+         --  Skip past CR/LF
 
-         if (Source (Ptr) = ASCII.CR or else Source (Ptr) = ASCII.LF)
-           and then Source (Ptr) /= Source (Ptr - 1)
-         then
+         if Source (Ptr - 1) = ASCII.CR and then Source (Ptr) = ASCII.LF then
             Ptr := Ptr + 1;
          end if;
 
@@ -783,6 +783,7 @@ package body Xref_Lib is
       --  line and column in the dependent unit number Eun. For this we need
       --  to parse the ali file again because the parent entity is not in
       --  the declaration table if it did not match the search pattern.
+      --  If the symbol is not found, we return "???".
 
       procedure Skip_To_Matching_Closing_Bracket;
       --  When Ptr points to an opening square bracket, moves it to the
@@ -803,6 +804,10 @@ package body Xref_Lib is
          --  Look for the X lines corresponding to unit Eun
 
          loop
+            if Ali (Ptr) = EOF then
+               return "???";
+            end if;
+
             if Ali (Ptr) = 'X' then
                Ptr := Ptr + 1;
                Parse_Number (Ali, Ptr, E_Eun);
@@ -832,10 +837,6 @@ package body Xref_Lib is
             exit when Ali (Ptr) = EOF;
          end loop;
 
-         --  We were not able to find the symbol, this should not happen but
-         --  since we don't want to stop here we return a string of three
-         --  question marks as the symbol name.
-
          return "???";
       end Get_Symbol_Name;