Fix rpmdb --exportdb from database on read-only media

Read-only media doesn't need any locking by definition. But if the
file we're trying to open doesn't exist there, we can't very well open
it for even reading, causing the unnecessary locking to fail
unnecessarily. Feed it a dup of our STDIN_FILENO to keep the beast happy.

Couldn't figure how to get a read-only mount inside the test-root
so we need to add a pre-generated rpmdb and then run it without
having the test-image read-write mounted. The rpmdb file is ridiculously
big just for testing this corner-case purpose, but maybe it could be
used to speed up some other tests later on.

Fixes: rpm-software-management#3371 rpm-software-management#1266

lib/rpmlock.cc

@@ -34,6 +34,10 @@ static rpmlock rpmlock_new(const char *lock_path, const char *descr)
 	if (lock->fd == -1) {
 	    if (errno == EACCES)
 		lock->fd = open(lock_path, O_RDONLY);
+	    else if (errno == EROFS) {
+		/* read-only media doesn't need locks, just feed it something */
+		lock->fd = dup(STDIN_FILENO);
+	    }
 	    if (lock->fd == -1) {
 		delete lock;
 		lock = NULL;

tests/rpmdb.at

@@ -81,6 +81,15 @@ RPMTEST_CLEANUP
 AT_SETUP([rpmdb --export and --import])
 AT_KEYWORDS([rpmdb])
 
+# This needs to run *without* RPMDB_INIT to test behavior on read-only mount
+RPMTEST_CHECK([
+run rpmdb --exportdb --dbpath ${RPMTEST}/data/misc/ > rdonly.list
+test -s rdonly.list
+],
+[0],
+[],
+[])
+
 RPMTEST_CHECK([
 RPMDB_INIT
 runroot rpm -U /data/RPMS/hlinktest-1.0-1.noarch.rpm