std.fileread() should not blindly return whatever file it returned

last without checking if the filename changed.

Fixes		#1145
Testcase by:	tobixen

bin/varnishtest/tests/r01145.vtc

+varnishtest "Test fileread for std VMOD"
+
+shell {
+	printf "File One" > "${tmpdir}/one"
+	printf "File Two" > "${tmpdir}/two"
+	printf "File Three" > "${tmpdir}/three"
+}
+
+server s1 {
+	loop 3 {
+		rxreq
+		txresp
+	}
+} -start
+
+
+varnish v1 -vcl+backend {
+	import std from "${topbuild}/lib/libvmod_std/.libs/libvmod_std.so" ;
+
+	sub vcl_deliver {
+            set resp.http.foo = std.fileread("${tmpdir}" + req.url);
+	}
+} -start
+
+
+client c1 {
+	txreq -url "/one"
+	rxresp
+	expect resp.http.foo == "File One"
+
+	txreq -url "/two"
+	rxresp
+	expect resp.http.foo == "File Two"
+
+	txreq -url "/three"
+	rxresp
+	expect resp.http.foo == "File Three"
+} -run
+
+client c1 -run
+client c1 -run
+
+

lib/libvmod_std/vmod_std_fileread.c

@@ -85,17 +85,21 @@ free_frfile(void *ptr)
 const char *
 vmod_fileread(struct sess *sp, struct vmod_priv *priv, const char *file_name)
 {
-	struct frfile *frf;
+	struct frfile *frf = NULL;
 	char *s;
 
 	(void)sp;
 	AN(priv);
+
 	if (priv->priv != NULL) {
 		CAST_OBJ_NOTNULL(frf, priv->priv, CACHED_FILE_MAGIC);
-		return (frf->contents);
+		if (!strcmp(file_name, frf->file_name))
+			return (frf->contents);
 	}
 
 	AZ(pthread_mutex_lock(&frmtx));
+	if (frf != NULL)
+		frf->refcount--;
 	VTAILQ_FOREACH(frf, &frlist, list) {
 		if (!strcmp(file_name, frf->file_name)) {
 			frf->refcount++;