options: add --nice process-priority controls

Makefile.in

@@ -49,7 +49,7 @@ OBJS1_NO_MAIN=flist.o rsync.o generator.o receiver.o cleanup.o sender.o exclude.
 OBJS1=$(OBJS1_NO_MAIN) main.o
 OBJS2=options.o io.o compat.o hlink.o token.o uidlist.o socket.o hashtable.o \
 	usage.o fileio.o batch.o clientname.o chmod.o acls.o xattrs.o
-OBJS3=progress.o pipe.o @MD5_ASM@ @ROLL_SIMD@ @ROLL_ASM@
+OBJS3=progress.o pipe.o niceness.o @MD5_ASM@ @ROLL_SIMD@ @ROLL_ASM@
 DAEMON_OBJ = params.o loadparm.o clientserver.o access.o connection.o authenticate.o
 popt_OBJS= popt/popt.o  popt/poptconfig.o \
 	popt/popthelp.o popt/poptparse.o popt/poptint.o

NEWS.md

@@ -12,6 +12,9 @@
   excludes.
 
 ### ENHANCEMENTS:
+- Added `--nice` to adjust CPU and I/O scheduling priorities for local and
+  remote rsync processes. Daemon clients and rrsync cannot raise server-side
+  priorities.
 - Human-readable output now uses dynamic precision to show three significant
   digits and appends `i` to binary unit suffixes. `--progress` uses the same
   formatter for transfer rates and labels byte counts and rates with `B` and
@@ -73,6 +76,7 @@ test improvements.
 - Updated platform tests and fleet-test coverage for the 3.5.0 fixes.
 
 ------------------------------------------------------------------------------
+
 # NEWS for rsync 3.5.0 (13 Aug 2026)
 
 ## Changes in this version:

clientserver.c

@@ -1442,6 +1442,7 @@ int start_daemon(int f_in, int f_out)
 	 * (when rsync is run by init and run by a remote shell). */
 	if (!load_config(0))
 		exit_cleanup(RERR_SYNTAX);
+	niceness_apply();
 
 	/* Bound the handshake before ANY peer input is read -- the PROXY-protocol
 	 * header below is peer-supplied too, and was previously unbounded.  An
@@ -1802,6 +1803,7 @@ int daemon_main(void)
 		exit_cleanup(RERR_SYNTAX);
 	}
 	set_dparams(0);
+	niceness_apply();
 
 	/* "proxy protocol = true" with no trusted-proxy list rejects every
 	 * connection as an untrusted proxy peer (fail-closed).  That is intended,

configure.ac

@@ -13,7 +13,7 @@ AC_CHECK_HEADERS(poll.h sys/fcntl.h sys/select.h fcntl.h sys/time.h sys/unistd.h
     sys/acl.h acl/libacl.h attr/xattr.h sys/xattr.h sys/extattr.h dl.h \
     popt.h popt/popt.h linux/falloc.h netinet/in_systm.h netgroup.h \
     zlib.h xxhash.h openssl/md4.h openssl/md5.h zstd.h lz4.h sys/file.h \
-    sys/resource.h bsd/string.h idn2.h wchar.h)
+    sys/resource.h bsd/string.h idn2.h wchar.h linux/ioprio.h sys/syscall.h)
 AC_CHECK_HEADERS([netinet/ip.h], [], [], [[#include <netinet/in.h>]])
 AC_HEADER_MAJOR_FIXED
 
@@ -978,7 +978,7 @@ AC_CHECK_FUNCS(waitpid wait4 getcwd chown chmod lchmod mknod mkfifo fdopendir \
     seteuid strerror putenv iconv_open locale_charset nl_langinfo getxattr \
     extattr_get_link sigaction sigprocmask setattrlist getgrouplist \
     initgroups utimensat futimens posix_fallocate attropen setvbuf nanosleep usleep \
-    setenv unsetenv mbrtowc)
+    setenv unsetenv mbrtowc setpriority)
 
 dnl dirfd() is a macro or static inline on several systems (the BSDs), so the
 dnl default AC_CHECK_FUNCS link probe -- which declares `char dirfd(void);` and

main.c

@@ -1309,6 +1309,9 @@ void start_server(int f_in, int f_out, int argc, char *argv[])
 
 	if (protocol_version >= 23)
 		io_start_multiplex_out(f_out);
+	/* Priority diagnostics need the negotiated message channel.
+	 * File-list work starts below. */
+	niceness_apply();
 	if (am_daemon && io_timeout && protocol_version >= 31)
 		send_msg_int(MSG_IO_TIMEOUT, io_timeout);
 
@@ -1344,6 +1347,7 @@ int client_run(int f_in, int f_out, pid_t pid, int argc, char *argv[])
 	int exit_code = 0, exit_code2 = 0;
 	char *local_name = NULL;
 
+	niceness_apply();
 	cleanup_child_pid = pid;
 	if (!read_batch) {
 		set_nonblocking(f_in);