I have observed this on a linux box running fedora29

$ python3 --version
Python 3.7.5
$ uname -a
Linux faraday.inria.fr 5.3.11-100.fc29.x86_64 #1 SMP Tue Nov 12 20:41:25 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
$ cat /etc/fedora-release
Fedora release 29 (Twenty Nine)

============ steps to reproduce:

This assumes that /root is not readable by lambda users

----- as root:

# mkdir /tmp/foo
# cd /tmp/foo
# touch a b d e
# ln -s /root/anywhere c

# ls -l
total 0
-rw-r--r-- 1 root root  0 Nov 22 14:51 a
-rw-r--r-- 1 root root  0 Nov 22 14:51 b
lrwxrwxrwx 1 root root 14 Nov 22 14:53 c -> /root/anywhere
-rw-r--r-- 1 root root  0 Nov 22 14:51 d
-rw-r--r-- 1 root root  0 Nov 22 14:51 e


----- as a lambda user:

we can see all files

$ ls -l /tmp/foo
total 0
-rw-r--r-- 1 root root  0 Nov 22 14:51 a
-rw-r--r-- 1 root root  0 Nov 22 14:51 b
lrwxrwxrwx 1 root root 14 Nov 22 14:53 c -> /root/anywhere
-rw-r--r-- 1 root root  0 Nov 22 14:51 d
-rw-r--r-- 1 root root  0 Nov 22 14:51 e

and with glob.glob() too

In [1]: import glob

In [2]: for filename in glob.glob("/tmp/foo/*"):
   ...:     print(filename)
   ...:
/tmp/foo/c
/tmp/foo/e
/tmp/foo/d
/tmp/foo/b
/tmp/foo/a


BUT Path.glob() is not working as expected

In [3]: from pathlib import Path

In [4]: for filename in Path("/tmp/foo/").glob("*"):
   ...:     print(filename)
   ...:



----- If I now I go back as root and remove the problematic file in /tmp/foo

# rm /tmp/foo/c


----- and try again as a lambda user

In [5]: for filename in Path("/tmp/foo/").glob("*"):
   ...:     print(filename)
   ...:
/tmp/foo/e
/tmp/foo/d
/tmp/foo/b
/tmp/foo/a


============ discussion

in my case in a real application I was getting *some* files - not an empty list like here. 

I ran strace on that real application
it's fairly clear from that output that the odd symlink is causing the scanning of all files to break instead of continuing (see snip below)
of course the order in which files are read from the disk will impact the behaviour, that's why I created the symlink last, that might need to be changed to reproduce successfully in another setup



============ strace extract

<snip>
getdents64(3, /* 189 entries */, 32768) = 8640
getdents64(3, /* 0 entries */, 32768)   = 0
close(3)                                = 0
stat("/var/lib/rhubarbe-images/centos.ndz", {st_mode=S_IFREG|0644, st_size=1002438656, ...}) = 0
stat("/var/lib/rhubarbe-images/oai-enb.ndz", {st_mode=S_IFREG|0644, st_size=2840592384, ...}) = 0
<snip>
stat("/var/lib/rhubarbe-images/ubuntu-floodlight.ndz", {st_mode=S_IFREG|0644, st_size=2559574016, ...}) = 0
stat("/var/lib/rhubarbe-images/ndnsim.ndz", {st_mode=S_IFREG|0644, st_size=4153409536, ...}) = 0

==> that's the line about the broken symlink in my real app
stat("/var/lib/rhubarbe-images/push-to-preplab.sh", 0x7ffd3ac4a140) = -1 EACCES (Permission denied)
==> and here it stops scanning files while there are still quite a lot to be dealt with

write(1, "/var/lib/rhubarbe-images/fedora-"..., 82/var/lib/rhubarbe-images/fedora-31.ndz
/var/lib/rhubarbe-images/fedora-31-ssh.ndz
) = 82
rt_sigaction(SIGINT, {sa_handler=SIG_DFL, sa_mask=[], sa_flags=SA_RESTORER, sa_restorer=0x7fc583705e70}, {sa_handler=0x7fc583936f10, sa_mask=[], \
sa_flags=SA_RESTORER, sa_restorer=0x7fc583705e70}, 8) = 0
sigaltstack(NULL, {ss_sp=0x560a7dac3330, ss_flags=0, ss_size=16384}) = 0
sigaltstack({ss_sp=NULL, ss_flags=SS_DISABLE, ss_size=0}, NULL) = 0
exit_group(0)                           = ?
+++ exited with 0 +++