Fix root relocation regression

When relocating the root directory, make sure we insert the new path's
dirname to dirNames[] even if the root itself is owned by the package.

This appears to have been the intention from the first version (largely
untouched since) of this code as we allow the root to pass through the
first checks (by setting len to 0 in that case) as well as the second
for loop where we do the relocations.

This allows fsm to properly create and remove the relocated directory
since we're now using fd-based calls (#1919) and the parent directory
needs to be opened first.

No need to do string comparison here, the empty basename signals that
we're processing the root directory, so just use that.

Building a relocatable package that owns the root directory seems to be
a handy way to create user-installable packages (see RHEL-28967) and it
happened to work before with the path-based calls so this technically
was a regression.  Add a test that emulates this use case.

Fixes: #3173

lib/relocation.c

@@ -181,8 +181,9 @@ void rpmRelocateFileList(rpmRelocation *relocations, int numRelocations,
 	rpmFileTypes ft;
 	int fnlen;
 
+	size_t baselen = strlen(baseNames[i]);
 	size_t len = maxlen +
-		strlen(dirNames[dirIndexes[i]]) + strlen(baseNames[i]) + 1;
+		strlen(dirNames[dirIndexes[i]]) + baselen + 1;
 	if (len >= fileAlloced) {
 	    fileAlloced = len * 2;
 	    fn = xrealloc(fn, fileAlloced);
@@ -244,8 +245,9 @@ assert(fn != NULL);		/* XXX can't happen */
 	    continue;
 	}
 
-	/* Relocation on full paths only, please. */
-	if (fnlen != len) continue;
+	/* Relocation on '/' and full paths only, please. */
+	if (baselen && fnlen != len)
+	    continue;
 
 	rpmlog(RPMLOG_DEBUG, "relocating %s to %s\n",
 	       fn, relocations[j].newPath);

tests/data/SPECS/rootfs.spec

@@ -0,0 +1,17 @@
+Name:           rootfs
+Version:        1.0
+Release:        1
+Summary:        Package owning top-level root directory
+License:        GPL
+BuildArch:      noarch
+Prefix:         /
+
+%description
+%{summary}.
+
+%install
+mkdir -p $RPM_BUILD_ROOT/foo
+touch $RPM_BUILD_ROOT/foo/bar
+
+%files
+/

tests/rpmi.at

@@ -1170,6 +1170,31 @@ runroot rpm -ql reloc
 [])
 RPMTEST_CLEANUP
 
+AT_SETUP([rpm -i relocatable package 3])
+AT_KEYWORDS([install relocate])
+RPMTEST_USER
+RPMDB_INIT
+
+runroot rpmbuild --quiet -bb /data/SPECS/rootfs.spec
+
+RPMTEST_CHECK([
+runroot_user \
+  rpm -U --prefix \$PWD/root --dbpath \$PWD/rpmdb \
+  /build/RPMS/noarch/rootfs-1.0-1.noarch.rpm
+],
+[0],
+[],
+[])
+
+RPMTEST_CHECK([
+runroot_user rpm -e --dbpath \$PWD/rpmdb rootfs
+runroot_user test ! -d \$PWD/root
+],
+[0],
+[],
+[])
+RPMTEST_CLEANUP
+
 AT_SETUP([rpm -i with/without --excludedocs])
 AT_KEYWORDS([install excludedocs])
 RPMTEST_CHECK([