Print --solver 3.0 explanation if both internal and it failed

This can provide useful additional context.

To avoid updating the whole test suite for it, introduce a new
option `quiet::NoSolver3Explanation` and set it by default. All
the other tests that assert output will already have matching
tests for --solver 3.0 with the correct messages asserted, it
makes no sense to duplicate them.

4 files changed, 26 insertions, 1 deletions

diff --git a/apt-private/private-install.cc b/apt-private/private-install.cc
index cb9f93e9e..29e23a57e 100644
--- a/apt-private/private-install.cc
+++ b/apt-private/private-install.cc
@@ -1024,6 +1024,9 @@ bool DoCacheManipulationFromCommandLine(CommandLine &CmdL, std::vector<PseudoPkg
 	   transaction.temporaryRollback(); // Rollback to before internal solver made changes so we can dump the OG request
 	   WriteApportReport(Cache, error, errors, UpgradeMode, distUpgradeMode);
 	}
+
+	if (not res && not errors.empty() && not _config->FindB("quiet::NoSolver3Explanation", false))
+	   _error->Error("The following information from --solver 3.0 may provide additional context:\n%s", APT::String::Join(errors, "\n").c_str());
      }
      // If the 3.0 solver could not recover either, abort.
      _error->MergeWithStack();
diff --git a/doc/examples/configure-index b/doc/examples/configure-index
index d6d76930c..c36f922b6 100644
--- a/doc/examples/configure-index
+++ b/doc/examples/configure-index
@@ -33,6 +33,7 @@ quiet "<INT>" {
   NoUpdate "<BOOL>"; // never update progress information - included in -q=1
   NoProgress "<BOOL>"; // disables the 0% → 100% progress on cache generation and stuff
   NoStatistic "<BOOL>"; // no "42 kB downloaded" stats in update
+  NoSolver3Explanation "<BOOL>"; // do not print solver3 explanations when used with internal solver
   ReleaseInfoChange "<BOOL>" // don't even print the notices if the info change is allowed
   {
     Origin "<BOOL>";
diff --git a/test/integration/framework b/test/integration/framework
index d2570b6b2..3f0290569 100644
--- a/test/integration/framework
+++ b/test/integration/framework
@@ -589,6 +589,8 @@ EOF
 	echo 'DPkg::Path "";' >> aptconfig.conf
 	echo 'Dir::Bin::ischroot "/bin/false";' >> aptconfig.conf
 
+	echo 'quiet::NoSolver3Explanation "true";' > rootdir/etc/apt/apt.conf.d/disable-solver3-context
+
 	# most tests just need one signed Release file, not both
 	export APT_DONT_SIGN='Release.gpg'
 
diff --git a/test/integration/test-solver3-evaluation b/test/integration/test-solver3-evaluation
index d3521e9c6..af12698b5 100755
--- a/test/integration/test-solver3-evaluation
+++ b/test/integration/test-solver3-evaluation
@@ -16,6 +16,7 @@ insertpackage 'unstable' 'c' 'all' '3'
 
 setupaptarchive
 
+rm rootdir/etc/apt/apt.conf.d/disable-solver3-context
 mkdir rootdir/var/log/apt
 
 testsuccess aptget install y x --solver internal -s -o Dir::Apport=var/crash
@@ -217,4 +218,22 @@ APT-Pin: 100
 APT-Candidate: yes
 " cat rootdir/var/log/apt/edsp.log
 
-
+testfailureequal "Reading package lists...
+Building dependency tree...
+Package 'c' is not installed, so not removed
+Solving dependencies...
+Some packages could not be installed. This may mean that you have
+requested an impossible situation or if you are using the unstable
+distribution that some required packages have not yet been created
+or been moved out of Incoming.
+The following information may help to resolve the situation:
+
+The following packages have unmet dependencies:
+ a : Depends: c but it is not going to be installed
+E: Unable to correct problems, you have held broken packages.
+E: The following information from --solver 3.0 may provide additional context:
+   Unable to satisfy dependencies. Reached two conflicting decisions:
+   1. a:amd64=3 is selected for install
+   2. a:amd64 Depends c
+      but none of the choices are installable:
+      - c:amd64 is not selected for install" apt install -s a c- --solver internal