Show mounted btrfs subvolumes

I looked at the Gnu core-utils source code, particularly at the df command.

It recursively descends the hierarchy until the device IDs change. At the point where the IDs change is the mount point.

I just tried to find the mount point of the filesystem that ~/home/me/a-dir/another-dir is in. I did:

stat . #noting device IDs
while id not changes and root not hit
do
  cd ..
  stat .
done
if root not hit
then
  cd -
fi

(this code is pseudo bash, all the conditional and loops where done manually. It just to prove the concept. I will leave the programming and translation to python as on exercise for the reader.)

btrfs subvolume show /directory/in/question

Most likely you need sudo. The command will succeed if it's a subvolume; it will fail otherwise. You can redirect output to /dev/null and rely solely on the exit status.

This will test every directory available in the directory tree:

sudo find / -type d -exec sh -c '
   for d do
      btrfs subvolume show "$d" >/dev/null 2>&1 && printf "%s\n" "$d"
   done' find-sh {} +

You may want to exclude paths that are beyond suspicion. The following code excludes /proc, /sys and /dev:

sudo find / -type d \( \
     \( -path /proc -prune \) -o \
     \( -path /sys -prune \) -o \
     \( -path /dev -prune \) -o \
     \( -exec sh -c '
        for d do
           btrfs subvolume show "$d" >/dev/null 2>&1 && printf "%s\n" "$d"
        done
     ' find-sh {} + \) \)

Related: How to determine which subvolume a directory/file is on?