PartitionTestSuite.TestHandleAssetsNoHardwareYaml fails: "open /writable/cache/hardware.yaml: permission denied

[Leo Arias]

One one of my machines I started getting one failure from the unit tests:

FAIL: bootloader_uboot_test.go:230: PartitionTestSuite.TestHandleAssetsNoHardwareYaml

FAIL: bootloader_uboot_test.go:230: PartitionTestSuite.TestHandleAssetsNoHardwareYaml

bootloader_uboot_test.go:238:
    c.Assert(bootloader.HandleAssets(), IsNil)
... value *os.PathError = &os.PathError{Op:"open", Path:"/writable/cache/hardware.yaml", Err:0xd} ("open /writable/cache/hardware.yaml: permission denied")

On my other machine it doesn't fail, so I went and looked into the permissions of that dir:

$ sudo ls -lah /writable
totalo 12K
drwxr-x--- 3 root root 4,0K Jun 2 04:53 .
drwxr-xr-x 24 root root 4,0K Jun 2 04:53 ..
drwxr-x--- 3 root root 4,0K Jun 2 04:53 cache

On the other machine, the directory is readable by my user.

Now, the problem is that we are relying in a /writable directory, that I'm not sure if will always exist. Looking at the test, I see that the directory is being patched with /tmp, but too late.

[Leo Arias (elopio)]

I think that as a result of a bad merge here: https://code.launchpad.net/~mvo/snappy/snappy-improved-developer-mode2/+merge/261692
the patch of the cache dir was removed. So this test bug is back.