Allow apt to run if no dpkg/status file exists

Not having a dpkg/status file used to be a hard error which from a
boostrap perspective is suspect as in the beginning, there is no
status so you would need to touch it into existence.

We make a difference between factual non-existence and inaccessibility
to catch mistakes in which the file is not readable for some reason,
the testcase test-bug-254770-segfault-if-cache-not-buildable is an
example of this.

Note that apt has already figured out at this point that this is a
Debian-like system which should have a dpkg/status file. This change
does not effect the auto-detection and is not supposed to.

2 files changed, 15 insertions, 6 deletions

diff --git a/apt-pkg/deb/debsystem.cc b/apt-pkg/deb/debsystem.cc
index 65a27f72f..9c55e0a6e 100644
--- a/apt-pkg/deb/debsystem.cc
+++ b/apt-pkg/deb/debsystem.cc
@@ -338,6 +338,13 @@ bool debSystem::AddStatusFiles(std::vector<pkgIndexFile *> &List)
    if (d->StatusFile == nullptr)
    {
       auto dpkgstatus = _config->FindFile("Dir::State::status");
+      if (dpkgstatus.empty())
+	 return true;
+      // we ignore only if the file doesn't exist, not if it is inaccessible
+      // e.g. due to permissions on parent directories as FileExists would do
+      errno = 0;
+      if (access(dpkgstatus.c_str(), R_OK) != 0 && errno == ENOENT)
+	 return true;
       _error->PushToStack();
       d->StatusFile = new debStatusIndex(std::move(dpkgstatus));
       bool const errored = _error->PendingError();
diff --git a/apt-pkg/policy.cc b/apt-pkg/policy.cc
index 5fcc11b66..68b3f5e5f 100644
--- a/apt-pkg/policy.cc
+++ b/apt-pkg/policy.cc
@@ -70,17 +70,19 @@ pkgPolicy::pkgPolicy(pkgCache *Owner) : VerPins(nullptr),
    if (DefRel.empty() == false)
    {
       bool found = false;
-      // FIXME: make ExpressionMatches static to use it here easily
-      pkgVersionMatch vm("", pkgVersionMatch::None);
       for (pkgCache::PkgFileIterator F = Cache->FileBegin(); F != Cache->FileEnd(); ++F)
       {
-	 if (vm.ExpressionMatches(DefRel, F.Archive()) ||
-	     vm.ExpressionMatches(DefRel, F.Codename()) ||
-	     vm.ExpressionMatches(DefRel, F.Version()) ||
+	 if (pkgVersionMatch::ExpressionMatches(DefRel, F.Archive()) ||
+	     pkgVersionMatch::ExpressionMatches(DefRel, F.Codename()) ||
+	     pkgVersionMatch::ExpressionMatches(DefRel, F.Version()) ||
 	     (DefRel.length() > 2 && DefRel[1] == '='))
 	    found = true;
       }
-      if (found == false)
+      // "now" is our internal archive name for the status file,
+      // which we should accept even if we have no status file at the moment
+      if (not found && pkgVersionMatch::ExpressionMatches(DefRel, "now"))
+	 found = true;
+      if (not found)
 	 _error->Error(_("The value '%s' is invalid for APT::Default-Release as such a release is not available in the sources"), DefRel.c_str());
       else
 	 CreatePin(pkgVersionMatch::Release,"",DefRel,990);