Git rev: master/f19b4e995ea47f9243f152b39337330307453c9f

I found the bitbake doesn't check the m5d and sha256 checksum for source tarball if the tarball and .done file already exist in download directory in some case.

Here is a simple example for connman:
1. bitbake connman
2. Set invalid md5 and sha256 checksums in connman bb file.
3. bitbake connman -c cleansstate
4. bitbake connman

Looks like the bitbake doesn't check the checksum and it can be built without errors.

Another example is a little complex. The current connman version is 1.25 in master and I want to downgrade to 1.24:
Steps:
1. Make sure the source tarball and .done file are in the download directory
   $ ls downloads/connman-1.24.tar.xz* -l
-rw-r--r-- 1 builder builder 629616 Jun  8 02:59 downloads/connman-1.24.tar.xz
-rw-r--r-- 1 builder builder      0 Nov  3 16:21 downloads/connman-1.24.tar.xz.done

2. mv meta/recipes-connectivity/connman/connman_1.25.bb to meta/recipes-connectivity/connman/connman_1.24.bb
   Do not change any lines in the bb file, that means the SRC_URI[md5sum] and SRC_URI[sha256sum] are for 1.25, not for 1.24

3. bitbake connman

The connman 1.24 can be built without any errors.

I need to clean the source tarball by using:
bitbake connman -c cleanall

And run:
bitbake connman

Then the checksum error occurs: 
#############
WARNING: Checksum failure encountered with download of http://kernel.org/pub/linux/network/connman/connman-1.24.tar.xz - will attempt other sources if available
WARNING: Renaming /buildarea3/yzhao1/poky-build/build-qemux86-64/downloads/connman-1.24.tar.xz to /buildarea3/yzhao1/poky-build/build-qemux86-64/downloads/connman-1.24.tar.xz_bad-checksum_dd6e1b4d9b9a28d127edb9f9b58bdec1
ERROR: Checksum failure fetching http://kernel.org/pub/linux/network/connman/connman-1.24.tar.xz
###############

If I change the SRC_URI[md5sum] and SRC_URI[sha256sum] to 1.24 in bb file and move the bb file to connman_1.25.bb, the connman 1.25 also can be built.(the 1.25 source tarball and .done file already exist in download directory).