Support restoring workspace archives from another OS

lib/travis/build/script/shared/workspace.rb

@@ -40,7 +40,26 @@ def fetch
         end
 
         def expand
-          sh.cmd "tar -xPzf ${CASHER_DIR}/#{name}-fetch.tgz"
+          archive = "${CASHER_DIR}/#{name}-fetch.tgz"
+          # For any root directories that `tar` would be extracting stuff to,
+          # create missing ones and give the user permissions for existing ones.
+          # Needed when restoring an archive from another OS with a different filesystem hierarchy.
+          sh.raw "tar -tPf \"#{archive}\" | " +
+                     # BSD(OSX) xargs doesn't support `-d` so -0 is the only way to handle paths with spaces.
+                     # BSD awk doesn't support \0 in variables so have to use printf.
+                     # BSD dirname doesn't support multiple arguments; -P 2 reduces time from 30s to 20s on a
+                     # sample archive with ~20k entries, further increase doesn't reduce time.
+                     "awk '{printf(\"%s%c\",$0,0)}' | " +
+                     "xargs -0 $([[ $TRAVIS_OS_NAME =~ (osx|freebsd) ]] && echo '-n 1 -P 2') dirname | " +
+                     "sort | uniq | " +
+                     # Print only lines that don't have the same prefix as a previous one -- to get only root dirs.
+                     # Initial `a` must be an impossible prefix. Since BSD awk doesn't support \0,
+                     # \n is acceptable since `tar -t` delimits entries with \n with no option to change that
+                     "awk 'index($0,a)!=1{a=$0;printf(\"%s%c\",$0,0)} BEGIN{a=\"\\n\"}' | " +
+                     # `install` is more convenient than `mkdir -p` since it can set UID/GID as the same time.
+                     # It only sets UID/GID on the leaf entry
+                     "xargs -0 echo $([[ $TRAVIS_OS_NAME != 'windows' ]] && echo 'sudo') install -o \"${USER}\" -g \"$(id -g)\" -d"
+          sh.cmd "tar -xPf #{archive}"
         end
 
         # for creating workspace