You don’t symlink those folders, you add them to $PATH and $LD_LIBRARY_PATH environment variables respectively, whether through a wrapper script or using the environment: functionality in the snapcraft.yaml (My memory might be incorrect, but I’m under the impression this is default behaviour anyway? If it isn’t, consider using extensions or Snapcraft Desktop Helpers to help set up the environment)
Strict snaps make use of mount namespaces, which means the filesystem they see isn’t the same as the filesystem the host sees, this is why they don’t match up. You can influence the folders outside of $SNAP to some degree by using layouts, but by design, the host doesn’t have influence here.
Effectively, every strict snap is guaranteed to have the same root filesystem on every system they’re installed on.
My question is more from the point of view of Snap creation. What filesystem hierarchy should I expect? I’m building a snap with core20, but I have $SNAP/bin and $SNAP/usr/bin as different directories (no symlinks). I was expecting the same structure as core20, but it is not the case.
whatever your source (or your stage packages) defines … snapcraft will not explicitly create any symlinks on its own inside your snap, if you want/need them you need to do that yourself … this is all a bit esoterical though since both $SNAP/bin and $SNAP/usr/bin will both be automatically in your PATH anyway (unless you override PATH in an environment stanza) so it doesnt really matter where in these two paths your binaries end up …
the filesystems differ because the setup as a whole changed between the two releases when ubuntu moved to the /usr merge (i think arch did that earlier already) …
in 18.04 (which core18 is based on) /bin, /lib and /sbin are actual dirs like in all releases before, from 20.04 on ubuntu did the switch to the new systemd induced setup …
you just use the ln command in your snapcraft.yaml from an ```override-build: |````scriptlet …
here is an ugly hack where i use some links to cheat around the fact that my libs are newer than the binary wants:
I made a dummy snap based on core20 and installed a libcurl package, just to have something with lib/ in it:
$ cat snap/snapcraft.yaml
name: my-snap-name
base: core20
version: '0.1'
summary: Single-line elevator pitch for your amazing snap
description: This is my-snap's description.
grade: devel
confinement: devmode
parts:
my-part:
plugin: nil
stage-packages:
- libcurl4-gnutls-dev
The final snap filesystem is then:
$ ls -la /snap/my-snap-name/current/
drwxr-xr-x 7 root root 85 Mar 3 12:55 ./
drwxr-xr-x 3 root root 4.0K Mar 3 12:55 ../
drwxr-xr-x 5 root root 58 Mar 3 12:55 etc/
drwxr-xr-x 3 root root 39 Mar 3 12:55 lib/
drwxr-xr-x 2 root root 43 Mar 3 12:55 lib64/
drwxr-xr-x 2 root root 32 Mar 3 12:52 meta/
drwxr-xr-x 6 root root 77 Mar 3 12:55 usr/
$ ls -la /snap/my-snap-name/current/usr/
drwxr-xr-x 6 root root 77 Mar 3 12:55 ./
drwxr-xr-x 7 root root 85 Mar 3 12:55 ../
drwxr-xr-x 2 root root 34 Mar 3 12:55 bin/
drwxr-xr-x 3 root root 39 Mar 3 12:55 include/
drwxr-xr-x 4 root root 64 Mar 3 12:55 lib/
drwxr-xr-x 6 root root 67 Mar 3 12:55 share/
I was expecting lib/ to be a symlink to usr/lib, because it is in Ubuntu 20.04, but that is not the case here. I know those paths are in PATH and LD_LIBRARY_PATH, but when we need to compile a package to snap it, it becomes a burden to find if a library is in bin or usr/bin when we ./configure the source.
Is it safe to create links in / within override-build like:
this links to a place outside of your build environment and essentially mangles the host … i wouldnt recommend doing that … i also dont really get the issue with the whole linking you seem to have … if you use configure the pkg-config files of the dependencies should properly resolve to the places the build-packages: were unpacked (including the $SNAP_STAGE bits that snapcraft might add to them for staged parts) … all these symlinks dont really matter, they are just a convenience so the old paths from before the systemd induced change still work but they have absolutely no meaning when building snaps
