Tweak stderr handling for older BackupPC versions

This makes the default for a protocol-28 server process be --stderr=client
instead of --stderr=errors.  See rsync's github issue #95.

NEWS.md

@@ -130,6 +130,9 @@
 
  - Work around a glibc bug where lchmod() breaks in a chroot w/o /proc mounted.
 
+ - Try to support a client that sent a remote rsync a wacko stderr file handle
+   (such as an older File::RsyncP perl library used by BackupPC).
+
  - Some manpage improvements.
 
 ### PACKAGING RELATED:

compat.c

@@ -52,6 +52,8 @@ extern int need_messages_from_generator;
 extern int delete_mode, delete_before, delete_during, delete_after;
 extern int do_compression;
 extern int do_compression_level;
+extern int saw_stderr_opt;
+extern int msgs2stderr;
 extern char *shell_cmd;
 extern char *partial_dir;
 extern char *files_from;
@@ -622,6 +624,9 @@ void setup_protocol(int f_out,int f_in)
 	if (read_batch)
 		check_batch_flags();
 
+	if (!saw_stderr_opt && protocol_version <= 28 && am_server)
+		msgs2stderr = 0; /* The client side may not have stderr setup for us. */
+
 #ifndef SUPPORT_PREALLOCATION
 	if (preallocate_files && !am_sender) {
 		rprintf(FERROR, "preallocation is not supported on this %s\n",

options.c

@@ -91,6 +91,7 @@ int implied_dirs = 1;
 int missing_args = 0; /* 0 = FERROR_XFER, 1 = ignore, 2 = delete */
 int numeric_ids = 0;
 int msgs2stderr = 2; /* Default: send errors to stderr for local & remote-shell transfers */
+int saw_stderr_opt = 0;
 int allow_8bit_chars = 0;
 int force_delete = 0;
 int io_timeout = 0;
@@ -1882,6 +1883,7 @@ int parse_arguments(int *argc_p, const char ***argv_p)
 					"--stderr mode \"%s\" is not one of errors, all, or client\n", arg);
 				return 0;
 			}
+			saw_stderr_opt = 1;
 			break;
 		}
 
@@ -1900,6 +1902,9 @@ int parse_arguments(int *argc_p, const char ***argv_p)
 		}
 	}
 
+	if (msgs2stderr != 2)
+		saw_stderr_opt = 1;
+
 	if (version_opt_cnt) {
 		print_rsync_version(FINFO);
 		exit_cleanup(0);