Why doesn't my recipe re-build successfully when I'm offline? - yocto

Question

I have a recipe that looks basically like this :
SUMMARY = "SomeLibrary"
LICENSE = "Apache-2.0"
LIC_FILES_CHKSUM = "file://LICENSE;md5=3b83ef96387f14655fc854ddc3c6bd57"
SRC_URI += "git://gitlab.com/some_library/some-library.git;protocol=https;nobranch=1"
SRCREV = "${PV}"
S = "${WORKDIR}/git"
inherit autotools pkgconfig
It builds successfully with bitbake some-library, and I can see there is a git2/gitlab.com.some_library.some-library.git/ directory and a git2/gitlab.com.some_library.some-library.git.done file in my downloads folder (the one DL_DIR point to).
My understanding is that if I then immediately run bitbake -c cleansstate some-library && bitbake some-library, given that there is no change in the recipe, bitbake shouldn't need to download anything (it already has everything it needs). In practice, if I turn off my network connection or add BB_NO_NETWORK="1" to my local.conf, I get the following error :
Initialising tasks: 100% |################################################################| Time: 0:00:01
Sstate summary: Wanted 12 Found 4 Missed 8 Current 251 (33% match, 96% complete)
NOTE: Executing Tasks
ERROR: some-library-v2.3.0-r0 do_fetch: Bitbake Fetcher Error: NetworkAccess('https://gitlab.com/some_library/some-library.git', 'git -c core.fsyncobjectfiles=0 ls-remote "https://gitlab.com/some_library/some-library.git" ')
ERROR: Logfile of failure stored in: /home/myusername/work/builddir/tmp/work/aarch64-poky-linux/some-library/v2.3.0-r0/temp/log.do_fetch.116252
ERROR: Task (/home/myusername/work/builddir/../../layers/meta-mymeta/recipes-core/some-library/some-library_v2.3.0.bb:do_fetch) failed with exit code '1'
NOTE: Tasks Summary: Attempted 806 tasks of which 804 didn't need to be rerun and 1 failed.
Summary: 1 task failed:
/home/myusername/work/builddir/../../layers/meta-mymeta/recipes-core/some-library/some-library_v2.3.0.bb:do_fetch
Summary: There was 1 ERROR message shown, returning a non-zero exit code.
Why is that ? How do other recipes avoid this pitfall ? (when I build my image, this recipe seems to be the only one trying to fetch things from the network, which suggests to me that I'm doing something wrong here)
EDIT :
What really puzzles me is that bitbakes seems to behave differently with recipes other than my own. For example, the recipe for can-utils located at meta-openembedded/meta-oe/recipes-extended/socketcan/can-utils_git.bb looks like this:
SUMMARY = "Linux CAN network development utilities"
LICENSE = "GPLv2 & BSD-3-Clause"
LIC_FILES_CHKSUM = "file://include/linux/can.h;endline=44;md5=a9e1169c6c9a114a61329e99f86fdd31"
DEPENDS = "libsocketcan"
SRC_URI = "git://github.com/linux-can/${BPN}.git;protocol=https;branch=master"
SRCREV = "da65fdfe0d1986625ee00af0b56ae17ec132e700"
PV = "2020.02.04"
S = "${WORKDIR}/git"
inherit autotools pkgconfig
which is very similar, but when I set BB_NO_NETWORK="1" in my local.conf and run bitbake -c cleansstate can-utils && bitbake can-utils I get Tasks Summary: Attempted 842 tasks of which 822 didn't need to be rerun and all succeeded.

Answer 1

This works for me:
After configuring the project, add the following lines to build/conf/site.conf file:
# Build offline
SOURCE_MIRROR_URL ?= "file:///path/to/oe-downloads"
INHERIT += " own-mirrors"
BB_GENERATE_MIRROR_TARBALLS = "1"
BB_NO_NETWORK = "1"
After that, it might be necessary to build project once when online.
After every re-configuration (different build options) the site.conf is overwritten, so I created a script to add these lines after re-configuration.

Answer 2

I believe I found the issue.
If I replace ${PV} (which was equal to v2.3.0 here) by the hash associated to that tag, then the issue stops happening.
If I interpret this correctly, it means that bitbake is able to tell if SRCREV is a hash or a tag, and that if it is a tag then do_fetch will always run git ls-remote to make sure that the tag has not been moved.