If DeleteFile fails on Windows due to another process (anti-virus,
probably) having our file open, instead leave a marker file that the
file is logically deleted, and remove it from API calls and clean it
up lazily later.
Updates tailscale/corp#1626
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
It used to just store received files URL-escaped on disk, but that was
a half done lazy implementation, and pushed the burden to callers to
validate and write things to disk in an unescaped way.
Instead, do all the validation in the receive handler and only
accept filenames that are UTF-8 and in the intersection of valid
names that all platforms support.
Fixestailscale/corp#1594
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
