posix_fallocate() failed: Operation not permitted while opening .realm file - realm

Question

I get the below error when i try to open and download .realm file in /tmp directory of serverless framework.
{"errorType":"Runtime.UnhandledPromiseRejection","errorMessage":"Error: posix_fallocate() failed: Operation not permitted" }
Below is the code:
let realm = new Realm({path: '/tmp/custom.realm', schema: [schema1, schema2]});
realm.write(() => {
console.log('completed==');
});

Answer 1

EDIT: this might soon be finally fixed in Realm-Core: see issue 4957.
In case you'll run into this problem elsewhere, here's a workaround.
This caused by AWS Lambda not supporting the fallocate and fallocate64 system calls. Instead of returning the correct error code in this case, which would be EINVAL for not supported on this file system, Amazon has blocked the system call so that it returns EPERM. Realm-Core has code that handles EINVAL return value correctly but will be bewildered by the unexpected EPERM returned from the system call.
The solution is to add a small shared library as a layer to the lambda: compile the following C file on Linux machine or inside lambda-ci Docker image:
#include <errno.h>
#include <fcntl.h>
int posix_fallocate(int __fd, off_t __offset, off_t __len) {
return EINVAL;
}
int posix_fallocate64(int __fd, off_t __offset, off_t __len) {
return EINVAL;
}
Now, compile this to a shared object with something like
gcc -shared fix.c -o fix.so
Then add it to a root of a ZIP file:
zip layer.zip fix.so
Create a new lambda layer from this zip
Add the lambda layer to your lambda function
Finally make the shared object be loaded by configuring the environment value LD_PRELOAD with value /opt/fix.so to your Lambda.
Enjoy.