The fetcher should not fail due to an alredy existing symlink

[ This is a follow up to a similar change that was erroneously sent to
the OE-Core list. Now there is actually an explanation below to what is happening. ]

We have occasional failures in our autobuilders where a setscene task fails, causing the original task to be run instead, but bitbake still fails with an error code in the end, causing unnecessary grief. One such case has been identified through the following error log:

The stack trace of python calls that resulted in this exception/failure was:
File: 'exec_python_func() autogenerated', lineno: 2, function: <module>

     0001:
 *** 0002:do_package_write_rpm_setscene(d)
     0003:
File: '${COREBASE}/meta/classes/package_rpm.bbclass', lineno: 757, function: do_package_write_rpm_setscene
     0753:# but we need to stop the rootfs/solver from running while we do...
     0754:do_package_write_rpm[sstate-lockfile-shared] += "${DEPLOY_DIR_RPM}/rpm.lock"
     0755:
     0756:python do_package_write_rpm_setscene () {
 *** 0757:    sstate_setscene(d)
     0758:}
     0759:addtask do_package_write_rpm_setscene
     0760:
     0761:python do_package_write_rpm () {
File: '${COREBASE}/meta/classes/sstate.bbclass', lineno: 648, function: sstate_setscene
     0644:            break
     0645:
     0646:def sstate_setscene(d):
     0647:    shared_state = sstate_state_fromvars(d)

*** 0648: accelerate = sstate_installpkg(shared_state, d)
0649: if not accelerate:
0650: raise bb.build.FuncFailed("No suitable staging package found")
0651:
0652:python sstate_task_prefunc () {
File: '${COREBASE}/meta/classes/sstate.bbclass', lineno: 297, function: sstate_installpkg

     0293:    sstatefetch = d.getVar('SSTATE_PKGNAME', True) + '_' + ss['task'] + ".tgz"
     0294:    sstatepkg = d.getVar('SSTATE_PKG', True) + '_' + ss['task'] + ".tgz"
     0295:
     0296:    if not os.path.exists(sstatepkg):

*** 0297: pstaging_fetch(sstatefetch, sstatepkg, d)
0298:
0299: if not os.path.isfile(sstatepkg):
0300: bb.note("Staging package %s does not exist" % sstatepkg)
0301: return False
File: '${COREBASE}/meta/classes/sstate.bbclass', lineno: 635, function: pstaging_fetch
0631: for srcuri in uris:
0632: localdata.setVar('SRC_URI', srcuri)
0633: try:
0634: fetcher = bb.fetch2.Fetch([srcuri], localdata, cache=False)
*** 0635: fetcher.download()
0636:
0637: # Need to optimise this, if using file:// urls, the fetcher just changes the local path
0638: # For now work around by symlinking
0639: localpath = bb.data.expand(fetcher.localpath(srcuri), localdata)
File: '${COREBASE}/poky/bitbake/lib/bb/fetch2/__init__.py', lineno: 1572, function: download
1568: localpath = ud.localpath
1569: elif m.try_premirror(ud, self.d):
1570: logger.debug(1, "Trying PREMIRRORS")
1571: mirrors = mirror_from_string(self.d.getVar('PREMIRRORS', True))
*** 1572: localpath = try_mirrors(self, self.d, ud, mirrors, False)

     1573:
     1574:                if premirroronly:
     1575:                    self.d.setVar("BB_NO_NETWORK", "1")
     1576:

File: '${COREBASE}/poky/bitbake/lib/bb/fetch2/__init__.py', lineno: 1020, function: try_mirrors
1016:
1017: uris, uds = build_mirroruris(origud, mirrors, ld)
1018:
1019: for index, uri in enumerate(uris):
*** 1020: ret = try_mirror_url(fetch, origud, uds[index], ld, check)
1021: if ret != False:
1022: return ret
1023: return None
1024:
File: '${COREBASE}/poky/bitbake/lib/bb/fetch2/__init__.py', lineno: 978, function: try_mirror_url
0974: if os.path.islink(origud.localpath):
0975: # Broken symbolic link
0976: os.unlink(origud.localpath)
0977:
*** 0978: os.symlink(ud.localpath, origud.localpath)
0979: update_stamp(origud, ld)
0980: return ud.localpath
0981:
0982: except bb.fetch2.NetworkAccess:
Exception: OSError: [Errno 17] File exists

What happens here is that two tasks simultaneously decide to download something, and both come to the conclusion that they need to create a symbolic link. And even if there is a check for whether the link already exists, there is a small window of time where both tasks see the missing link and tries to create it with the result that the second task fails as per above.

I have now managed to determine under what circumstances this occurs. This happens when two separate tasks (typically executed by two concurrent invocations of bitbake) want to retrieve the same file:// URL. This is because there is no file lock being used for file:// URLs.

In our case it was easy to recreate the failure by adding a five seconds sleep before the call to os.symlink(). Also worth noting is that we have a global sstate cache accessed via a file:// URL (it is an NFS mount) and that we use a local sstate cache shared by multiple builds. So to cause the failure, all I had to do after that was to (in two separate build directories) cleansstate a recipe which had its sstate available in the global sstate cache and then start the two builds simultaneously.

The change proposed here just causes FileExistsError exceptions from os.symlink() to be ignored. The alternative would be to actually enable file locks for file:// URLs. However, as that is more invasive and the situation happening here should be extremely rare, that does not seem motivated.

//Peter

The following changes since commit 330302711377475e2fba0c6e9871e2dff5e1ee08:

poky.ent: Removed python3-expect from the CentOS distro (2017-03-31 12:14:18 +0100)

are available in the git repository at:

git://git.yoctoproject.org/poky-contrib pkj/handle_existing_links
http://git.yoctoproject.org/cgit.cgi/poky-contrib/log/?h=pkj/handle_existing_links

Peter Kjellerstedt (1):
fetch2: Do not fail to create symbolic links if they already exist

 bitbake/lib/bb/fetch2/__init__.py | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

2.12.0