chhsiao/overwrite repository overview

The second layer contains the following filesystem structure:

• /merged/: m1
• /replaced1/: r1
• /replaced2/: r2
• /replaced3
• /replaced4@ -> /merged
• /foo@ -> baz
• /bar@ -> bar (dangling link)
• /baz
• /xyz@ -> ../../../../../../../abc The symbolic link /xyz should link to /abc after chroot, and thus a dangling link. However, during image provisioning, it would link to the abc file under the agent's work directory on the host filesystem, according to the provisioner filesystem layout⁠. In test ProvisionerDockerBackendTest.ROOT_INTERNET_CURL_DTYPE_Overwrite⁠, a non-empty file abc is created under the agent's work directory, which should not be overwritten during provisioning.

The third layer will overwrite directories /replaced1/ and /replaced2/ with regular file /replaced1 and symbolic link /replaced2@ -> /merged, and overwrite files /replaced3 and /replaced4 with directories of the same names. Also, link /bar is replaced by a file and file /baz is replaced by a link. No whiteout files will be generated. The final filesystem structure will be:

• /merged/: m1 m2
• /replaced1
• /replaced2@ -> /merged
• /replaced3/: (empty)
• /replaced4/: (empty)
• /foo@ -> bar
• /bar
• /baz@ -> baz (dangling link)
• /xyz

Test ProvisionerDockerBackendTest.ROOT_INTERNET_CURL_DTYPE_Overwrite⁠ verifies the following statements after provisioning:

• /replaced1 is a regular file.
• /replaced2 is a symbolic link.
• /replaced2/ contains both m1 and m2 but not r2.
• /replaced3 and /replaced4 are directories.
• /replaced4/ does not contain m1
• /foo links to a regular file, meaning that the old link is replaced with the new link.
• /bar is a not a symbolic link.
• /baz is a symbolic lick.
• abc on the host filesystem is not overwritten by xyz and remains non-empty.