Loop mount an S3 or Ceph object

This is a fun, small nbdkit Python plugin using the Boto3 AWS SDK:

#!/usr/sbin/nbdkit python

import nbdkit
import boto3
from contextlib import closing

API_VERSION = 2

def thread_model():
    return nbdkit.THREAD_MODEL_PARALLEL

def config(key, value):
    global access_key, secret_key, endpoint_url, bucket_name, key_name

    if key == "access-key" or key == "access_key":
        access_key = value
    elif key == "secret-key" or key == "secret_key":
        secret_key = value
    elif key == "endpoint-url" or key == "endpoint_url":
        endpoint_url = value
    elif key == "bucket":
        bucket_name = value
    elif key == "key":
        key_name = value
    else:
        raise Exception("unknown parameter %s" % key)

def open(readonly):
    global access_key, secret_key, endpoint_url

    s3 = boto3.client("s3",
                      aws_access_key_id = access_key,
                      aws_secret_access_key = secret_key,
                      endpoint_url = endpoint_url)
    if s3 is None:
        raise Exception("could not connect to S3")
    return s3

def get_size(s3):
    global bucket_name, key_name

    resp = s3.get_object(Bucket = bucket_name, Key = key_name)
    size = resp['ResponseMetadata']['HTTPHeaders']['content-length']
    return int(size)

def pread(s3, buf, offset, flags):
    global bucket_name, key_name

    size = len(buf)
    rnge = 'bytes=%d-%d' % (offset, offset+size-1)
    resp = s3.get_object(Bucket = bucket_name, Key = key_name, Range = rnge)
    body = resp['Body']
    with closing(body):
        buf[:] = body.read(size)

This lets you loop mount a single object (file):

$ ./nbdkit-S3-plugin -f -v -U /tmp/sock \
  access_key="XYZ" secret_key="XYZ" \
  bucket="my_files" key="fedora-28.iso"

$ sudo nbd-client -b 2048 -unix /tmp/sock /dev/nbd0
Negotiation: ..size = 583MB
$ ls /dev/nbd0
 nbd0    nbd0p1  nbd0p2  
$ sudo mount -o ro /dev/nbd0p1 /tmp/mnt
$ ls -l /tmp/mnt
 total 11
 dr-xr-xr-x. 3 root root 2048 Apr 25  2018 EFI
 -rw-r--r--. 1 root root 2532 Apr 23  2018 Fedora-Legal-README.txt
 dr-xr-xr-x. 3 root root 2048 Apr 25  2018 images
 drwxrwxr-x. 2 root root 2048 Apr 25  2018 isolinux
 -rw-r--r--. 1 root root 1063 Apr 21  2018 LICENSE
 -r--r--r--. 1 root root  454 Apr 25  2018 TRANS.TBL

I should note this is a bit different from s3fs which is a FUSE driver that mounts all the files in a bucket.