readline: clear the error flag if the error happens to be EAGAIN

(or the equivalent EWOULDBLOCK)

This allows questionable code that tries to combine select and the
readline flavour of buffered I/O to limp along.  Such code is still
risky due to select() checking the underlying OS handle and not
the perl handle.

Fixes Perl#22883

MANIFEST

@@ -6404,6 +6404,7 @@ t/op/range.t				See if .. works
 t/op/read.t				See if read() works
 t/op/readdir.t				See if readdir() works
 t/op/readline.t				See if <> / readline / rcatline work
+t/op/readline_nb.t			Test <> error handling on non-blocking handles
 t/op/recurse.t				See if deep recursion works
 t/op/ref.t				See if refs and objects work
 t/op/refstack.t				See if a ref counted stack fixes things

pp_hot.c

@@ -3994,6 +3994,21 @@ PP(pp_match)
     return NORMAL;
 }
 
+/* errno can be either EAGAIN or EWOULDBLOCK for a socket() read that
+   is non-blocking but would have blocked if blocking
+*/
+PERL_STATIC_INLINE bool
+error_is_would_block(int err) {
+#ifdef EAGAIN
+    if (err == EAGAIN)
+        return true;
+#endif
+#ifdef EWOULDBLOCK
+    if (err == EWOULDBLOCK)
+        return true;
+#endif
+    return false;
+}
 
 /* Perl_do_readline(): implement <$fh>, readline($fh) and glob('*.h')
  *
@@ -4236,6 +4251,9 @@ Perl_do_readline(pTHX)
                                    (STATUS_CURRENT & 0x80) ? ", core dumped" : "");
                 }
             }
+            else if (error_is_would_block(errno)) {
+                PerlIO_clearerr(fp);
+            }
 
             if (gimme == G_SCALAR) {
                 if (type != OP_RCATLINE) {

t/op/readline_nb.t

@@ -0,0 +1,36 @@
+#!./perl
+
+BEGIN {
+    chdir 't' if -d 't';
+    require './test.pl';
+    set_up_inc('../lib');
+    require Config; Config->import;
+
+    skip_all_if_miniperl();
+}
+
+use strict;
+use IO::Select;
+
+$Config{d_pipe}
+  or skip_all("No pipe");
+
+my ($in, $out);
+pipe($in, $out)
+  or skip_all("Cannot pipe: $!");
+
+$in->blocking(0)
+  or skip_all("Cannot make pipe non-blocking");
+
+my $line = <$in>;
+is($line, undef, "error reading");
+ok(!$in->error, "but did not set error flag");
+close $out;
+$line = <$in>;
+is($line, undef, "nothing to read, but eof");
+ok(!$in->error, "still did not set error flag");
+ok($in->eof, "did set eof");
+ok(close($in), "close success");
+
+
+done_testing();