file storage backend: Spuriously deletes storage keys ending with .temp

[maxb]

I happened to browse the diff of #15946 when it was mentioned in another issue.

I immediately spotted a defect in the code.

It uses ".temp" as a suffix for temporary files without caring whether that temporary file name will accidentally collide with a legitimate other storage entry...

$ vault secrets enable kv
Success! Enabled the kv secrets engine at: kv/
$ vault write kv/example.temp foo=bar
Success! Data written to: kv/example.temp
$ vault list kv
Keys
----
example.temp
$ vault write kv/example foo=bar
Success! Data written to: kv/example
$ vault list kv
Keys
----
example

kv/example.temp was unexpectedly overwritten by writing to kv/example !!!