fail instead of segfault on unreadable config files

The report mentions "apt list --upgradable", but there are others which
have inconsistent behavior ranging from segfaulting to doing something
with the partial (and hence incomplete) data. We had a recent report
about sources.list (#818628), this one mentions prefences, the obvious
next step is conf files… so the testcase is adapted to check for all
three in file and directory versions and run a bunch of commands each
time which should all have more or less the same behavior in such a case
(aka error out).

Closes: 824503
(cherry picked from commit fdf9eef4d96a18d0167708499c993e1174251e88)

1 files changed, 10 insertions, 4 deletions

diff --git a/apt-private/private-show.cc b/apt-private/private-show.cc
index 792c455a8..cd7a2a064 100644
--- a/apt-private/private-show.cc
+++ b/apt-private/private-show.cc
@@ -258,6 +258,8 @@ bool ShowPackage(CommandLine &CmdL)					/*{{{*/
    CacheSetHelperVirtuals helper(true, GlobalError::NOTICE);
    APT::CacheSetHelper::VerSelector const select = _config->FindB("APT::Cache::AllVersions", true) ?
 			APT::CacheSetHelper::ALL : APT::CacheSetHelper::CANDIDATE;
+   if (select == APT::CacheSetHelper::CANDIDATE && CacheFile.GetDepCache() == nullptr)
+      return false;
    APT::VersionList const verset = APT::VersionList::FromCommandLine(CacheFile, CmdL.FileList + 1, select, helper);
    int const ShowVersion = _config->FindI("APT::Cache::Show::Version", 1);
    for (APT::VersionList::const_iterator Ver = verset.begin(); Ver != verset.end(); ++Ver)
@@ -356,10 +358,14 @@ bool ShowSrcPackage(CommandLine &CmdL)					/*{{{*/
 bool Policy(CommandLine &CmdL)
 {
    pkgCacheFile CacheFile;
-   pkgCache *Cache = CacheFile.GetPkgCache();
-   pkgPolicy *Plcy = CacheFile.GetPolicy();
-   pkgSourceList *SrcList = CacheFile.GetSourceList();
-   if (unlikely(Cache == NULL || Plcy == NULL || SrcList == NULL))
+   pkgSourceList const * const SrcList = CacheFile.GetSourceList();
+   if (unlikely(SrcList == nullptr))
+      return false;
+   pkgCache * const Cache = CacheFile.GetPkgCache();
+   if (unlikely(Cache == nullptr))
+      return false;
+   pkgPolicy * const Plcy = CacheFile.GetPolicy();
+   if (unlikely(Plcy == nullptr))
       return false;
 
    bool OldPolicy = _config->FindI("APT::Policy", 1) < 1;