[CMake] FetchContent/ExternalProject and URL_HASH

[Craig Scott]

On Mon, Jul 22, 2019 at 10:37 AM Dustyn Blasig <dustyn at blasig.us> wrote:

> Thanks for the info, Craig.
>
> I'm not very familiar with the intricacies of network downloads. If the
> download itself guarantees the file was transferred correctly and the
> checksum is only be used to verify its authenticity, then we probably don't
> need it as we're only downloading these artifacts from trusted internal
> sources. However, if the checksums need to be used in some cases to verify
> the download was actually received and is in one correct piece then pulling
> the checksum file and basically failing if either is corrupted is fine for
> our use case. Can we assume the former, that CMake (and the underlying
> tools) will guarantee the file is downloaded successfully even in the event
> of a CTRL-C interruption or other signals?
>

I don't think it is realistic to expect CMake or the underlying tools to
still give you a successful file download if you interrupt it. ;)

One of the things the file(DOWNLOAD) command uses the checksum for is to
check if it can avoid having to download the file already exists. Without
the checksum, if there is already a file at the destination, CMake can't
tell if it is the right file and will download it again each time.
Something I hadn't considered in my previous reply was if the file to be
downloaded is very big, then it may still end up being more efficient to
download the separate checksum file each time and read the big file's
checksum from it to avoid re-downloading the big file if you've already got
it from a previous run.

Another reason you might want to explicitly specify the checksum rather
than download a checksum file is that after you've done a configure once,
you won't need to do any network communication to get it for subsequent
runs because the checksum can be used to confirm you already have the right
file. This allows you to run configure while connected to the network, then
disconnect and work offline thereafter (handy if you're on a laptop and
travelling!).

Another use for the checksum file is to ensure you are receiving the file
you expect from the source. This can help catch things like
man-in-the-middle attacks or other malicious acts where the download is
intercepted and some other file substituted. If you have a trustworthy
connection to the source, this is less likely to be a concern for you, but
I'll leave that to your own judgement.



>
>
> On Sun, Jul 21, 2019 at 3:49 AM Craig Scott <craig.scott at crascit.com>
> wrote:
>
>>
>>
>> On Wed, Jul 17, 2019 at 12:59 PM Dustyn Blasig <dustyn at blasig.us> wrote:
>>
>>> Hi All,
>>>
>>> We are pulling some artifacts from Artifactory which provides a checksum
>>> file along with the artifacts at <artifact-url>.md5 or .sha256. If I do not
>>> include URL_HASH, does CMake automatically check to see if such a checksum
>>> file exists and use it's value for the hash check? Or is there a way to
>>> provide a URL for the checksum file rather than having to do file(DOWNLOAD
>>> <checksum>), file(STRING <checksum-file>), URL_HASH=<checksum-var>?
>>>
>>
>> The point of the checksum file is to verify the file downloaded. It
>> doesn't make a whole lot of sense to then download another file to provide
>> that checksum, you'd just be moving the problem along one level of
>> indirection. The assumption is when you provide the URL to be downloaded,
>> if you want to use a checksum then you should also be able to provide that
>> along with the URL. When the URL is being constructed on-the-fly though,
>> this isn't typically true. In that case, you can't typically provide a
>> checksum that isn't itself downloaded and therefore needs to be verified
>> itself.
>>
>> To more directly answer your question, CMake doesn't offer any feature to
>> automatically download a checksum file (that I'm aware of). The file
>> command expects that actual checksum, not a location for where to retrieve
>> it from for the reasons mentioned above.
>>
>
-- 
Craig Scott
Melbourne, Australia
https://crascit.com

Get the hand-book for every CMake user: Professional CMake: A Practical
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