Introduce apt why, apt why-not

These are implemented somewhat differently from aptitudes
why and why-not commands: They produce the actual solver
trace for why a particular decision has been taken.

For the why-not case, we need to explicitly discover our
specified package, as if nothing else depends on it in
our graph, it would otherwise always be undiscovered and
conflicts not detected (see e.g. level-3 in the test).

1 files changed, 17 insertions, 0 deletions

diff --git a/cmdline/apt.cc b/cmdline/apt.cc
index af8a95fa5..a876085ce 100644
--- a/cmdline/apt.cc
+++ b/cmdline/apt.cc
@@ -16,6 +16,7 @@
 #include <apt-pkg/error.h>
 #include <apt-pkg/init.h>
 #include <apt-pkg/pkgsystem.h>
+#include <apt-pkg/solver3.h>
 #include <apt-pkg/strutl.h>
 
 #include <apt-private/private-cmndline.h>
@@ -53,6 +54,20 @@ static bool ShowHelp(CommandLine &)					/*{{{*/
    return true;
 }
 									/*}}}*/
+
+static bool DoWhy(CommandLine &CmdL) /*{{{*/
+{
+   pkgCacheFile CacheFile;
+   APT::PackageList pkgset = APT::PackageList::FromCommandLine(CacheFile, CmdL.FileList + 1);
+   bool const decision = strcmp(CmdL.FileList[0], "why") == 0;
+   if (pkgset.size() != 1)
+      return _error->PendingError() ? false : _error->Error("Only a single argument is supported at this time.");
+   if (unlikely(not CacheFile.BuildDepCache()))
+      return false;
+   for (auto pkg : pkgset)
+      std::cout << APT::Solver::InternalCliWhy(CacheFile, pkg, decision) << std::flush;
+   return not _error->PendingError();
+}
 static std::vector<aptDispatchWithHelp> GetCommands()			/*{{{*/
 {
    // advanced commands are left undocumented on purpose
@@ -81,6 +96,8 @@ static std::vector<aptDispatchWithHelp> GetCommands()			/*{{{*/
       {"modernize-sources", &ModernizeSources, _("modernize .list files to .sources files")},
       {"moo", &DoMoo, nullptr},
       {"satisfy", &DoBuildDep, _("satisfy dependency strings")},
+      {"why", &DoWhy, _("produce a reason trace for the current state of the package")},
+      {"why-not", &DoWhy, _("produce a reason trace for the current state of the package")},
 
       // for compat with muscle memory
       {"dist-upgrade", &DoDistUpgrade, nullptr},