Ensure the expiry thread does not deadlock the server

If, for some reason, an fcore lock could not get aquired, the
expiry thread would keep the server locked until that file was
available again.

Avoid this situation with a trivial trylock and retry.

src/tus_file_exp.c

@@ -30,6 +30,8 @@
 
 #include <pthread.h>
 #include <math.h>
+#include <unistd.h>
+#include <stdio.h>
 
 #include "vdef.h"
 #include "vrt.h"
@@ -41,6 +43,8 @@
 #include "tus_file_exp.h"
 #include "tus_file_exp_setup.h"
 
+#define RETRY_SECONDS	10
+
 struct tus_exp {
 	unsigned	magic;
 #define TUS_EXP_MAGIC	0x105e8900
@@ -96,11 +100,24 @@ tus_exp_thread(void *p)
 
 		AZ(pthread_mutex_unlock(&e->mtx));
 
+		AN(fcore);
 		srv = fcore->server;
-		tus_server_lock(srv);
-		AZ(pthread_mutex_lock(&fcore->mtx));
-		tus_file_del(&fcore);
-		tus_server_unlock(srv);
+
+		while (fcore != NULL) {
+			struct tus_file_core *has_lock = fcore;
+
+			tus_server_lock(srv);
+			(void) tus_file_trylock(&has_lock);
+			if (has_lock != NULL)
+				tus_file_del(&fcore);
+			tus_server_unlock(srv);
+
+			if (has_lock == NULL) {
+				fprintf(stderr, "tus expiry: fcore %p locked\n",
+				    fcore);
+				sleep(RETRY_SECONDS);
+			}
+		}
 
 		AZ(pthread_mutex_lock(&e->mtx));
 	}