背景 链接到标题
公司产品最终交付形态是 ISO，在涉及一个产品的多个 OEM 场景时，会选择在标准版本的基础上，删除某些软件包，新增某些软件包的形式来减少构建时间。产品的 BaseOS 是 CentOS，包管理器是 RPM 系，也就需要使用 rpm / yum 等命令来实现。
其中新增某些软件包是使用 yumdownloader 来完成的。在 Yum Repository 中会包含同一软件包的多个版本，预期 yumdownloader 会下载 Yum Repository 中某个软件最新版本的包，比如 yumdownloader zbs-5.1.2* ，则会下载 zbs-5.1.2 大版本的最新 release 版本。
但是最近发现，从某个版本开始 yumdownloader 没有下载最新的软件包，反而停在了一个两个月之前构建的版本，于是开始调查原因。
Yumdownloader 链接到标题
yumdownloader 工具集是由 yum-utils 提供，同时还提供了 repotrack，repoquery, reposync 等有用的工具。yumdownloader 使用方式是 yumdownloader $pkg 即可。在 yum-utils 中会大量引用 yum module，因此需要同时查找两个 Git repo。
yum-utils 代码仓库地址： https://github.com/rpm-software-management/yum-utils/blob/master/yumdownloader.pyyum 代码仓库地址：https://github.com/rpm-software-management/yum
下载逻辑 链接到标题
def main(self):
# Add command line option specific to yumdownloader
self.addCmdOptions()
...
# make yumdownloader work as non root user.
if not self.setCacheDir():
self.logger.error("Error: Could not make cachedir, exiting")
sys.exit(50)
...
# Setup yum (Ts, RPM db, Repo & Sack)
self.doUtilYumSetup()
# Do the real action
self.exit_code = self.downloadPackages(opts)
- 解析命令行参数
- 检查执行用户权限
- 配置 Yum Repo 正确性
- 下载 RPM
其中前几项不是很重要，直接看 self.downloadPackages(opts) 逻辑。主要做了以下几件事情：
- 根据 PKG 列表进行 Repo 中的查询，查询出所有的软件包；
- 根据查询结果，进行解析匹配，其中精确匹配和模糊匹配会进行后续处理；
- 根据 RPM
name+arch作为 key 来将所有的 Pkg 列表转换为 Dict； - 遍历 Dict，从列表中找到 Best Package ，并将其添加到下载列表中；
- 根据下载列表信息进行下载。
Github: https://github.com/rpm-software-management/yum-utils/blob/master/yumdownloader.py#L136
def downloadPackages(self,opts):
toDownload = [] # 最终要下载的 RPM
packages = self.cmds
for pkg in packages:
toActOn = []
if not pkg or pkg[0] != '@':
pkgnames = [pkg] # 如果 PKG 名称不以 `@` 开头，那么就是单个软件包；如果以 `@` 开头，那么表示是一个 Group；
else:
# Group 处理逻辑，忽略
...
pos = self.pkgSack.returnPackages(patterns=pkgnames) # 在 Yum repo 中根据 pkgnames 来获取查询到的包列表
exactmatch, matched, unmatched = parsePackages(pos, pkgnames) # 根据 pkgnames 名称 从 pos 中解析具体的匹配结果，其中如果精确匹配则添加到 exactmatch，如果模糊匹配则添加到 matched。在本文场景下所有匹配为模糊匹配，所以在 matched 列表中
installable = (exactmatch + matched) # 最终可安装的是精确匹配和模糊匹配的集合
if not installable:
...
for newpkg in installable:
toActOn.extend(_best_convert_pkg2srcpkgs(self, opts, newpkg)) # 根据解析结果，将所有处于 installable 中的包都找到具体来源
if toActOn:
pkgGroups = self._groupPackages(toActOn) # 使用 RPM `name` 和 `arch` 针对 toActOn 列表进行初步分组，转换为 dict
for group in pkgGroups:
pkgs = pkgGroups[group]
if opts.source:
... # 根据 yumdownloader 命令参数进行额外的处理，忽略
else:
toDownload.extend(self.bestPackagesFromList(pkgs)) # 从 pkgs 列表中选择 best Package，然后添加到 toDownload 列表中
...
# set localpaths
for pkg in toDownload:
pkg.repo.copy_local = True
pkg.repo.cache = 0
# use downloader from YumBase
exit_code = 0
probs = self.downloadPkgs(toDownload) # 下载具体 RPM
if probs:
exit_code = 2
for key in probs:
for error in probs[key]:
self.logger.error('%s: %s', key, error)
return exit_code
在我的场景中，问题出在查找 Best Package 步骤中，相关日志如下，可以观察到在 Yum repo 中查询到了 zbs-5.1.2 所有的 release RPM，分别为 rc1 ,rc2 一直到 rc14 ，排序方式是字母序。 parsePackages 解析出所有的 PKG 均在 matched 中，因为都是模糊匹配，最终传入 bestPackagesFromList 方法中的参数是所有到的 release RPM，预期是返回最新的 release RPM ，即 zbs-5.1.2-rc14，但是返回的是 rc7 。接下来需要调查 bestPackagesFromList 是如何判断的。
['zbs-5.1.2*']
pos: [<YumAvailablePackageSqlite : zbs-5.1.2-rc1.0.release.git.g0cb56434e.el7.SMTX.HCI.x86_64 (0x2bf6e50)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc10.0.release.git.g228f49070.el7.SMTX.HCI.x86_64 (0x2bf6dd0)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc11.0.release.git.g6d5d763ee.el7.SMTX.HCI.x86_64 (0x2bf6d90)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc12.0.release.git.g9a6400652.el7.SMTX.HCI.x86_64 (0x2bf6bd0)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc13.0.release.git.g339708733.el7.SMTX.HCI.x86_64 (0x2bf6f90)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc14.0.release.git.g42733ba17.el7.SMTX.HCI.x86_64 (0x2bf6fd0)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc2.0.release.git.gfa5212d39.el7.SMTX.HCI.x86_64 (0x2c03050)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc3.0.release.git.ge4ecabe7b.el7.SMTX.HCI.x86_64 (0x2c03090)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc4.0.release.git.g6e9ed979b.el7.SMTX.HCI.x86_64 (0x2c030d0)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc5.0.release.git.gfa8bab6ad.el7.SMTX.HCI.x86_64 (0x2c03110)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc6.0.release.git.g51f0f1277.el7.SMTX.HCI.x86_64 (0x2c03150)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc7.0.release.git.gccd6dbf2a.el7.SMTX.HCI.x86_64 (0x2c03190)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc8.0.release.git.g763bb9046.el7.SMTX.HCI.x86_64 (0x2c031d0)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc9.0.release.git.g238ab2320.el7.SMTX.HCI.x86_64 (0x2c03210)>]
exactmatch: []
matched: [<YumAvailablePackageSqlite : zbs-5.1.2-rc14.0.release.git.g42733ba17.el7.SMTX.HCI.x86_64 (0x2bf6fd0)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc13.0.release.git.g339708733.el7.SMTX.HCI.x86_64 (0x2bf6f90)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc2.0.release.git.gfa5212d39.el7.SMTX.HCI.x86_64 (0x2c03050)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc7.0.release.git.gccd6dbf2a.el7.SMTX.HCI.x86_64 (0x2c03190)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc3.0.release.git.ge4ecabe7b.el7.SMTX.HCI.x86_64 (0x2c03090)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc9.0.release.git.g238ab2320.el7.SMTX.HCI.x86_64 (0x2c03210)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc6.0.release.git.g51f0f1277.el7.SMTX.HCI.x86_64 (0x2c03150)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc4.0.release.git.g6e9ed979b.el7.SMTX.HCI.x86_64 (0x2c030d0)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc10.0.release.git.g228f49070.el7.SMTX.HCI.x86_64 (0x2bf6dd0)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc8.0.release.git.g763bb9046.el7.SMTX.HCI.x86_64 (0x2c031d0)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc12.0.release.git.g9a6400652.el7.SMTX.HCI.x86_64 (0x2bf6bd0)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc11.0.release.git.g6d5d763ee.el7.SMTX.HCI.x86_64 (0x2bf6d90)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc1.0.release.git.g0cb56434e.el7.SMTX.HCI.x86_64 (0x2bf6e50)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc5.0.release.git.gfa8bab6ad.el7.SMTX.HCI.x86_64 (0x2c03110)>]
unmatched: []
pkgGroups: {'zbs.x86_64': [<YumAvailablePackageSqlite : zbs-5.1.2-rc14.0.release.git.g42733ba17.el7.SMTX.HCI.x86_64 (0x2bf6fd0)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc13.0.release.git.g339708733.el7.SMTX.HCI.x86_64 (0x2bf6f90)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc2.0.release.git.gfa5212d39.el7.SMTX.HCI.x86_64 (0x2c03050)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc7.0.release.git.gccd6dbf2a.el7.SMTX.HCI.x86_64 (0x2c03190)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc3.0.release.git.ge4ecabe7b.el7.SMTX.HCI.x86_64 (0x2c03090)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc9.0.release.git.g238ab2320.el7.SMTX.HCI.x86_64 (0x2c03210)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc6.0.release.git.g51f0f1277.el7.SMTX.HCI.x86_64 (0x2c03150)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc4.0.release.git.g6e9ed979b.el7.SMTX.HCI.x86_64 (0x2c030d0)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc10.0.release.git.g228f49070.el7.SMTX.HCI.x86_64 (0x2bf6dd0)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc8.0.release.git.g763bb9046.el7.SMTX.HCI.x86_64 (0x2c031d0)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc12.0.release.git.g9a6400652.el7.SMTX.HCI.x86_64 (0x2bf6bd0)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc11.0.release.git.g6d5d763ee.el7.SMTX.HCI.x86_64 (0x2bf6d90)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc1.0.release.git.g0cb56434e.el7.SMTX.HCI.x86_64 (0x2bf6e50)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc5.0.release.git.gfa8bab6ad.el7.SMTX.HCI.x86_64 (0x2c03110)>]}
pkgs: [<YumAvailablePackageSqlite : zbs-5.1.2-rc14.0.release.git.g42733ba17.el7.SMTX.HCI.x86_64 (0x2bf6fd0)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc13.0.release.git.g339708733.el7.SMTX.HCI.x86_64 (0x2bf6f90)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc2.0.release.git.gfa5212d39.el7.SMTX.HCI.x86_64 (0x2c03050)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc7.0.release.git.gccd6dbf2a.el7.SMTX.HCI.x86_64 (0x2c03190)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc3.0.release.git.ge4ecabe7b.el7.SMTX.HCI.x86_64 (0x2c03090)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc9.0.release.git.g238ab2320.el7.SMTX.HCI.x86_64 (0x2c03210)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc6.0.release.git.g51f0f1277.el7.SMTX.HCI.x86_64 (0x2c03150)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc4.0.release.git.g6e9ed979b.el7.SMTX.HCI.x86_64 (0x2c030d0)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc10.0.release.git.g228f49070.el7.SMTX.HCI.x86_64 (0x2bf6dd0)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc8.0.release.git.g763bb9046.el7.SMTX.HCI.x86_64 (0x2c031d0)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc12.0.release.git.g9a6400652.el7.SMTX.HCI.x86_64 (0x2bf6bd0)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc11.0.release.git.g6d5d763ee.el7.SMTX.HCI.x86_64 (0x2bf6d90)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc1.0.release.git.g0cb56434e.el7.SMTX.HCI.x86_64 (0x2bf6e50)>, <YumAvailablePackageSqlite : zbs-5.1.2-rc5.0.release.git.gfa8bab6ad.el7.SMTX.HCI.x86_64 (0x2c03110)>]
toDownload: [<YumAvailablePackageSqlite : zbs-5.1.2-rc7.0.release.git.gccd6dbf2a.el7.SMTX.HCI.x86_64 (0x2c03190)>]
Yum 链接到标题
yumdownloader 是 yum module 的功能分装，具体的 bestPackagesFromList 是在 yum module 中实现的。bestPackagesFromList 自身先根据 pkg arch 来进行分类，其中判断依据为：
Github： https://github.com/rpm-software-management/yum/blob/master/rpmUtils/arch.py#L153:5
# dict mapping arch -> ( multicompat, best personality, biarch personality )
multilibArches = { "x86_64": ( "athlon", "x86_64", "athlon" ),
"sparc64v": ( "sparcv9v", "sparcv9v", "sparc64v" ),
"sparc64": ( "sparcv9", "sparcv9", "sparc64" ),
"ppc64": ( "ppc", "ppc", "ppc64" ),
"s390x": ( "s390", "s390x", "s390" ),
}
Github: https://github.com/rpm-software-management/yum/blob/master/yum/__init__.py#L4432
def bestPackagesFromList(self, pkglist, arch=None, single_name=False,
req=None):
"""Return the best packages from a list of packages. This
function is multilib aware, so that it will not compare
multilib to singlelib packages.
:param pkglist: the list of packages to return the best
packages from
:param arch: packages will be selected that are compatible
with the architecture specified by *arch*
:param single_name: whether to return a single package name
:param req: the requirement from the user
:return: a list of the best packages from *pkglist*
"""
returnlist = []
compatArchList = self.arch.get_arch_list(arch)
multiLib = []
singleLib = []
noarch = []
for po in pkglist: # 根据架构来进行筛选，x86_64 是 multiLibArch
if po.arch not in compatArchList:
continue
elif po.arch in ("noarch"):
noarch.append(po)
elif isMultiLibArch(arch=po.arch):
multiLib.append(po) # 最终所有 pkg 添加到 multiLib 中
else:
singleLib.append(po)
# we now have three lists. find the best package(s) of each
multi = self._bestPackageFromList(multiLib, req=req) # 根据不同架构找到 best package
single = self._bestPackageFromList(singleLib, req=req)
no = self._bestPackageFromList(noarch, req=req)
...
return returnlist
继续追踪 _bestPackageFromList 的实现，可以看到
Github: https://github.com/rpm-software-management/yum/blob/master/yum/__init__.py#L4409
def _bestPackageFromList(self, pkglist, req=None):
"""take list of package objects and return the best package object.
If the list is empty, return None.
Note: this is not aware of multilib so make sure you're only
passing it packages of a single arch group.
:param pkglist: the list of packages to return the best
packages from
:param req: the requirement from the user
:return: a list of the best packages from *pkglist*
"""
...
bestlist = self._compare_providers(pkglist, reqpo=None, req=req)
return bestlist[0][0]
终于找到最关键的部分： _compare_providers ，这是一个巨大的函数，300行，根据注释可以看到主要用户给 pkg 打分：
Github: https://github.com/rpm-software-management/yum/blob/master/yum/depsolve.py#L1465
def _compare_providers(self, pkgs, reqpo, req=None):
"""take the list of pkgs and score them based on the requesting package
return a dictionary of po=score"""
self.verbose_logger.log(logginglevels.DEBUG_4,
_("Running compare_providers() for %s") %(str(pkgs)))
接下来是具体的打分流程，先根据 repo id 的字母序进行过滤，如果多个 repo 均提供了同一版本的 PKG，那么会根据 repo 字母序进行选取：
# Do a NameArch filtering, based on repo. __cmp__
unique_nevra_pkgs = {}
for pkg in pkgs:
if (pkg.pkgtup in unique_nevra_pkgs and
unique_nevra_pkgs[pkg.pkgtup].repo <= pkg.repo):
continue
unique_nevra_pkgs[pkg.pkgtup] = pkg
pkgs = unique_nevra_pkgs.values()
其中 pkg.repo 的实现如下：
class Repository:
"""this is an actual repository object"""
def __init__(self, repoid):
self.id = repoid
self.quick_enable_disable = {}
self.disable()
self._xml2sqlite_local = False
def __cmp__(self, other):
""" Sort base class repos. by alphanumeric on their id, also
see __cmp__ in YumRepository(). """
if self.id > other.id:
return 1
elif self.id < other.id:
return -1
else:
return 0
初始化 pkgresults ，其中 value 是对应的分数，第一步打分是检查目标 pkg 是否已经存在于当前主机上，会通过查询 rpmdb 来获取信息，如果已经存在了，那么这种情况是升级情况，需要与当前主机上 newest 的 pkg 进行比较，如果当前主机上最新的包版本小于 pkg，则 +5 分，如果等于则 +1000 分，如果小于则 -1024 分。如果当前主机上没有，则跳过。
pkgresults = {}
penalize = set()
for pkg in pkgs:
pkgresults[pkg] = 0 # 初始化各个 pkg 的分数为 0
# hand this off to our plugins
self.plugins.run("compare_providers", providers_dict=pkgresults,
reqpo=reqpo)
for pkg in pkgresults.keys():
rpmdbpkgs = self.rpmdb.searchNevra(name=pkg.name)
if rpmdbpkgs:
# We only want to count things as "installed" if they are
# older than what we are comparing, because this then an update
# so we give preference. If they are newer then obsoletes/etc.
# could play a part ... this probably needs a better fix.
newest = sorted(rpmdbpkgs)[-1]
if newest.verLT(pkg):
# give pkgs which are updates just a SLIGHT edge
# we should also make sure that any pkg
# we are giving an edge to is not obsoleted by
# something else in the transaction. :(
# there are many ways I hate this - this is but one
pkgresults[pkg] += 5
elif newest.verEQ(pkg):
# We get here from bestPackagesFromList(), give a giant
# bump to stuff that is already installed.
pkgresults[pkg] += 1000
elif newest.verGT(pkg):
# if the version we're looking at is older than what we have installed
# score it down like we would an obsoleted pkg
pkgresults[pkg] -= 1024
else:
# just b/c they're not installed pkgs doesn't mean they should
# be ignored entirely. Just not preferred
pass
O(n^2)遍历 pkgs，先获取 Yum repo 中 newest version，然后保存下来，进行比对，如果当前 pkg 不等于 newest version，则 -1024 分。当前 repo 中匹配的 pkgs 列表一共 14 个， 遍历结束后每个 pkg 的分数应该是 13 * (-1024) = -13312。如果 pkg 被 nextpkg 所废除，那么 pkg 分数继续 -1024。如果传递了 arch 相关参数，那么会根据 arch 进行比较，如果哪个 pkg 提供了当前 arch 的包，那么会 +5 分。
lpos = {}
for po in pkgs:
for nextpo in pkgs:
if po == nextpo:
continue
# If this package isn't the latest version of said package,
# treat it like it's obsoleted. The problem here is X-1
# accidentally provides FOO, so you release X-2 without the
# provide, but X-1 is still picked over a real provider.
if po.name not in lpos:
lpos[po.name] = self.pkgSack.returnNewestByName(po.name)[:1]
if not lpos[po.name] or not po.verEQ(lpos[po.name][0]):
pkgresults[po] -= 1024
obsoleted = False
if po.obsoletedBy([nextpo]):
obsoleted = True
pkgresults[po] -= 1024
self.verbose_logger.log(logginglevels.DEBUG_4,
_("%s obsoletes %s") % (nextpo, po))
if reqpo:
arches = (reqpo.arch, self.arch.bestarch)
else:
arches = (self.arch.bestarch,)
for thisarch in arches:
res = _compare_arch_distance(po, nextpo, thisarch)
if not res:
continue
self.verbose_logger.log(logginglevels.DEBUG_4,
_('archdist compared %s to %s on %s\n Winner: %s' % (po, nextpo, thisarch, res)))
if res == po:
pkgresults[po] += 5
# End of O(N*N): for nextpo in pkgs:
接下来会根据 pkg 是否存在 source rpm，是否是弱引用，是否是直接引用，是否存在冲突等进行分数的增减。
# End of O(N*N): for nextpo in pkgs:
# Respect the repository priority for each provider, the default is 80
pkgresults[po] += (100 - po.repo.compare_providers_priority) * 10
self.verbose_logger.log(logginglevels.DEBUG_4,
_('compare_providers_priority for %s is %s' % (po, po.repo.compare_providers_priority)))
if _common_sourcerpm(po, reqpo):
self.verbose_logger.log(logginglevels.DEBUG_4,
_('common sourcerpm %s and %s' % (po, reqpo)))
pkgresults[po] += 20
if _weak_req(po, reqpo):
self.verbose_logger.log(logginglevels.DEBUG_4,
_('weak req %s and %s' % (po, reqpo)))
pkgresults[po] += 666
if _info_req(po, reqpo):
self.verbose_logger.log(logginglevels.DEBUG_4,
_('informational req %s and %s' % (po, reqpo)))
pkgresults[po] += 333
if _conflict_req(po, reqpo):
self.verbose_logger.log(logginglevels.DEBUG_4,
_('conflict req %s and %s' % (po, reqpo)))
penalize.add(po)
if self.isPackageInstalled(po.base_package_name):
self.verbose_logger.log(logginglevels.DEBUG_4,
_('base package %s is installed for %s' % (po.base_package_name, po)))
pkgresults[po] += 5 # Same as before - - but off of base package name
if reqpo:
cpl = _common_prefix_len(po.name, reqpo.name)
if cpl > 2:
self.verbose_logger.log(logginglevels.DEBUG_4,
_('common prefix of %s between %s and %s' % (cpl, po, reqpo)))
pkgresults[po] += cpl*2
当基本分数进行打分完成后， 还存在多个 best pkg，那么会根据当前 OS 安装 pkg 所需依赖数量进行判定，依赖数量越少，则分数越高，最终依赖数量少的 pkg 分数 +1。此时部分 pkg 分数从 -13112 变为 -13111。
# If we have more than one "best", see what would happen if we picked
# each package ... ie. what things do they require that _aren't_ already
# installed/to-be-installed. In theory this can screw up due to:
# pkgA => requires pkgX
# pkgB => requires pkgY, requires pkgZ
# ...but pkgX requires 666 other things. Going recursive is
# "non-trivial" though, python != prolog. This seems to do "better"
# from simple testing though.
bestnum = max(pkgresults.values()) # 将当前 pkg 分数最大的置为 bestnum
rec_depsolve = {}
for po in pkgs:
if pkgresults[po] != bestnum: # 如果当前 pkg 分数不等于最高分，跳过
continue
rec_depsolve[po] = 0
if len(rec_depsolve) > 1: # 如果仍有多个 pkg，则进行依赖判定
for po in rec_depsolve:
fake_txmbr = TransactionMember(po)
# Note that this is just requirements, so you could also have
# 4 requires for a single package. This might be fixable, if
# needed, but given the above it's probably better to leave it
# like this.
reqs = self._checkInstall(fake_txmbr) # 检查安装 pkg 所需依赖
rec_depsolve[po] = len(reqs) # 将依赖数量置为当前 pkg 的分数
bestnum = min(rec_depsolve.values()) # 找到依赖数量最少的分数作为 bestnum
self.verbose_logger.log(logginglevels.DEBUG_4,
_('requires minimal: %d') % bestnum)
for po in rec_depsolve:
if rec_depsolve[po] == bestnum:
self.verbose_logger.log(logginglevels.DEBUG_4,
_(' Winner: %s') % po)
pkgresults[po] += 1 # 将依赖数量结果填充会 pkgresults 中，依赖数量最少的分数 +1
else:
num = rec_depsolve[po]
self.verbose_logger.log(logginglevels.DEBUG_4,
_(' Loser(with %d): %s') % (num, po))
将当前分数最高的置为 bestnum，遍历 pkgs，如果当前 pkg 分数等于 bestnum，则将其分数 +1000，并将其分数 +(pkg.name)*-1 。如果 -13111 + 1000 +(-3) = -12114。
# We don't want to decide to use a "shortest first", if something else
# has told us to pick something else. But we want to pick between
# multiple "best" packages. So we spike all the best packages (so
# only those can win) and then bump them down by package name length.
bestnum = max(pkgresults.values())
for po in pkgs:
if pkgresults[po] != bestnum:
continue
pkgresults[po] += 1000
pkgresults[po] += (len(po.name)*-1)
# Bump down any packages that we identified as "last-resort" in such a
# way that they all score below the worst overall score whilst keeping
# their relative differences.
shift = max(pkgresults.values()) - min(pkgresults.values()) + 1
for po in penalize:
pkgresults[po] -= shift
bestorder = sorted(pkgresults.items(),
key=lambda x: (x[1], x[0]), reverse=True)
self.verbose_logger.log(logginglevels.DEBUG_4,
_('Best Order: %s' % str(bestorder)))
return bestorder
最终根据 pkg 分数进行重新排序，返回第一个结果。
总结 链接到标题
发现这个问题最初是猜测是 Yum Repository 配置问题，阅读代码之后判定是 RPM 在某个版本依赖发生了改变，增加了某些依赖项，导致了 Yum 打分认为其分数较低，从而无法通过 yumdownloader 下载最新的 RPM。与相关同事确认，rc8 版本开始增加了部分依赖，调查结束。