fix deadlock in shmget

`shmget` deadlocks when called more than once in the same process. It
locks the mutex, then calls `ashv_read_remote_segment`, which asks the
background thread in the same process to find a shmid, which in turn
tries to lock the mutex which is already locked.

shmem.c

@@ -347,7 +347,12 @@ int shmget(key_t key, size_t size, int flags)
 				path_buffer[path_length] = '\0';
 				int shmid = atoi(path_buffer);
 				if (shmid != 0) {
-					int idx = ashv_read_remote_segment(shmid);
+					int idx = ashv_find_local_index(shmid);
+
+					if (idx == -1) {
+						idx = ashv_read_remote_segment(shmid);
+					}
+
 					if (idx != -1) {
 						pthread_mutex_unlock(&mutex);
 						return shmem[idx].id;

test/deadlock.c

@@ -0,0 +1,11 @@
+#include "utils.h"
+
+int main() {
+    key_t SHMEM_KEY = ftok(".", 23);
+
+    int shmid_from_shmget;
+    if ((shmid_from_shmget = shmget(SHMEM_KEY, 30, IPC_CREAT | 0666)) < 0) error_exit("shmget");
+    if ((shmid_from_shmget = shmget(SHMEM_KEY, 30, IPC_CREAT | 0666)) < 0) error_exit("shmget");
+
+    return 0;
+}