Manage Devel::Cover coverage reporting errors.

Devel::Cover errors in _report() would end the process.  That wouldn't
normally matter since Devel::Cover tries very hard to be the last thing to be
run anyway.

But it is important just before an exec().  When Devel::Cover sees an exec()
call it first calls report() then calls the exec().  In this case it is
important that any errors in report() do not prevent the exec() from running.

We go belt and braces here: put an eval {} around the _report() and try to
nicely report the problem, and add G_EVAL to call_pv().

Also add a test for this.

This problem was found by Daisuke Maki.  He also diagnosed it, provided the
test case and suggested the solution.

Cover.xs

@@ -412,7 +412,7 @@ static void call_report(pTHX)
 {
     dSP;
     PUSHMARK(SP);
-    call_pv("Devel::Cover::report", G_VOID|G_DISCARD);
+    call_pv("Devel::Cover::report", G_VOID|G_DISCARD|G_EVAL);
     SPAGAIN;
 }
 

lib/Devel/Cover.pm

@@ -661,7 +661,24 @@ my %Seen;
 
 sub report
 {
-    _report();
+    local $@;
+    eval
+    {
+        _report();
+    };
+    if ($@)
+    {
+        print STDERR <<"EOM" unless $Silent;
+Devel::Cover: Oops, it looks like something went wrong writing the coverage.
+              It's possible that more bad things may happen but we'll try to
+              carry on anyway as if nothing happened.  At a minimum you'll
+              probably find that you are missing coverage.  If you're
+              interrested, the problem was:
+
+$@
+
+EOM
+    }
     return unless $Self_cover;
     $Self_cover_run = 1;
     _report();

test_output/cover/exec_die.5.006001

@@ -0,0 +1,59 @@
+Reading database from ...
+
+
+------------------------------------------ ------ ------ ------ ------ ------
+File                                         stmt   bran   cond    sub  total
+------------------------------------------ ------ ------ ------ ------ ------
+tests/exec_die                               50.0   50.0    n/a    0.0   44.4
+Total                                        50.0   50.0    n/a    0.0   44.4
+------------------------------------------ ------ ------ ------ ------ ------
+
+
+Run: ...
+Perl version: ...
+OS: ...
+Start: ...
+Finish: ...
+
+tests/exec_die
+
+line  err   stmt   bran   cond    sub   code
+1                                       #!/usr/bin/perl
+2                                       
+3                                       # Copyright 2007-2011, Paul Johnson (pjcj@cpan.org)
+4                                       
+5                                       # This software is free.  It is licensed under the same terms as Perl itself.
+6                                       
+7                                       # The latest version of this software should be available from my homepage:
+8                                       # http://www.pjcj.net
+9                                       
+10             1                        my $pid = fork;
+11                                      
+12    ***      1     50                 if ($pid)
+13                                      {
+14             1                            wait;
+15                                      }
+16                                      else
+17                                      {
+18    ***      0                    0       local *Devel::Cover::_report = sub { die "Badness happened!" };
+      ***      0                        
+19    ***      0                            exec "echo We want to be able to see this.";
+20                                      }
+
+
+Branches
+--------
+
+line  err      %   true  false   branch
+----- --- ------ ------ ------   ------
+12    ***     50      1      0   if ($pid) { }
+
+
+Uncovered Subroutines
+---------------------
+
+Subroutine Count Location         
+---------- ----- -----------------
+__ANON__       0 tests/exec_die:18
+
+

tests/exec_die

@@ -0,0 +1,20 @@
+#!/usr/bin/perl
+
+# Copyright 2007-2011, Paul Johnson (pjcj@cpan.org)
+
+# This software is free.  It is licensed under the same terms as Perl itself.
+
+# The latest version of this software should be available from my homepage:
+# http://www.pjcj.net
+
+my $pid = fork;
+
+if ($pid)
+{
+    wait;
+}
+else
+{
+    local *Devel::Cover::_report = sub { die "Badness happened!" };
+    exec "echo We want to be able to see this.";
+}