install(1) confusing error message

[Miod Vallat]

This is what happens when install(1) is used to install files on a
read-only filesystem:

# mount -u -o ro /usr
# cd /usr/src
# make build
cd /usr/src/share/mk && exec  make install
install -c -o root -g bin -m 444 bsd.README bsd.dep.mk bsd.lib.mk bsd.man.mk 
bsd.nls.mk  bsd.obj.mk bsd.own.mk bsd.port.arch.mk bsd.port.mk  
bsd.port.subdir.mk bsd.prog.mk  bsd.regress.mk bsd.subdir.mk bsd.sys.mk sys.mk 
bsd.lkm.mk  bsd.xconf.mk bsd.xorg.mk /usr/share/mk
install: /usr/share/mk/bsd.README: File exists
*** Error 71 in share/mk (Makefile:13 'install')
*** Error 1 in /usr/src (Makefile:78 'build')
#

Not really helpful. With the diff below, I will now get:

install: /usr/share/mk/bsd.README: Read-only file system

which helps figuring out what is wrong.

Miod

Index: xinstall.c
===================================================================
RCS file: /cvs/src/usr.bin/xinstall/xinstall.c,v
retrieving revision 1.52
diff -u -p -r1.52 xinstall.c
--- xinstall.c  14 Sep 2012 00:00:29 -0000      1.52
+++ xinstall.c  14 Feb 2013 20:32:21 -0000
@@ -639,8 +639,10 @@ create_newfile(char *path, struct stat *
                /* It is ok for the target file not to exist. */
                if (rename(path, backup) < 0 && errno != ENOENT)
                        err(EX_OSERR, "rename: %s to %s (errno %d)", path, 
backup, errno);
-       } else
-               (void)unlink(path);
+       } else {
+               if (unlink(path) < 0 && errno != ENOENT)
+                       err(EX_OSERR, "%s", path);
+       }
 
        return(open(path, O_CREAT | O_RDWR | O_EXCL, S_IRUSR | S_IWUSR));
 }