From dfb2b32071be8003c9048128cc8cf52bf2137d30 Mon Sep 17 00:00:00 2001
From: Karen Arutyunov <karen@codesynthesis.com>
Date: Wed, 1 Dec 2021 16:16:17 +0300
Subject: Improve pkg-build diagnostics issued when unable to satisfy
 dependency constraint

---
 bpkg/pkg-build.cxx         | 81 ++++++++++++++++++++++++++++++++++++++--------
 tests/pkg-build.testscript |  9 +++---
 2 files changed, 73 insertions(+), 17 deletions(-)

diff --git a/bpkg/pkg-build.cxx b/bpkg/pkg-build.cxx
index c0374ac..d027866 100644
--- a/bpkg/pkg-build.cxx
+++ b/bpkg/pkg-build.cxx
@@ -182,9 +182,8 @@ namespace bpkg
   // Note that we return (loaded) lazy_shared_ptr in order to also convey
   // the database to which it belongs.
   //
-  static
-  vector<pair<shared_ptr<available_package>,
-              lazy_shared_ptr<repository_fragment>>>
+  static vector<pair<shared_ptr<available_package>,
+                     lazy_shared_ptr<repository_fragment>>>
   find_available (const linked_databases& dbs,
                   const package_name& name,
                   const optional<version_constraint>& c)
@@ -232,9 +231,8 @@ namespace bpkg
   // fragments, their prerequisite repositories, and their complements,
   // recursively (note: recursivity applies to complements, not prerequisites).
   //
-  static
-  vector<pair<shared_ptr<available_package>,
-              lazy_shared_ptr<repository_fragment>>>
+  static vector<pair<shared_ptr<available_package>,
+                     lazy_shared_ptr<repository_fragment>>>
   find_available (const package_name& name,
                   const optional<version_constraint>& c,
                   const config_repo_fragments& rfs,
@@ -268,6 +266,35 @@ namespace bpkg
     return r;
   }
 
+  // As above but only look for packages from a single repository fragment,
+  // its prerequisite repositories, and its complements, recursively (note:
+  // recursivity applies to complements, not prerequisites). Doesn't provide
+  // the repository fragments the packages come from.
+  //
+  // It is assumed that the repository fragment lazy pointer contains the
+  // database information.
+  //
+  static vector<shared_ptr<available_package>>
+  find_available (const package_name& name,
+                  const optional<version_constraint>& c,
+                  const lazy_shared_ptr<repository_fragment>& rf,
+                  bool prereq = true)
+  {
+    vector<shared_ptr<available_package>> r;
+
+    database& db (rf.database ());
+    for (auto& ap: filter (rf.load (), query_available (db, name, c), prereq))
+      r.emplace_back (move (ap));
+
+    if (r.empty ())
+    {
+      if (shared_ptr<available_package> ap = find_imaginary_stub (name))
+        r.emplace_back (move (ap));
+    }
+
+    return r;
+  }
+
   // As above but only look for a single package from the specified repository
   // fragment, its prerequisite repositories, and their complements,
   // recursively (note: recursivity applies to complements, not
@@ -1534,17 +1561,45 @@ namespace bpkg
             }
 
             diag_record dr (fail);
-            dr << "unknown dependency " << dn;
 
-            // We need to be careful not to print the wildcard-based
-            // constraint.
+            // Issue diagnostics differently based on the presence of
+            // available packages for the unsatisfied dependency.
             //
-            if (d.constraint && (!dep_constr || !wildcard (*dep_constr)))
-              dr << ' ' << *d.constraint;
+            // Note that there can't be any stubs, since they satisfy any
+            // constraint and we won't be here if they were.
+            //
+            vector<shared_ptr<available_package>> aps (
+              find_available (dn, nullopt /* version_constraint */, af));
+
+            if (!aps.empty ())
+            {
+              dr << "unable to satisfy dependency constraint (" << dn;
 
-            dr << " of package " << name << pdb;
+              // We need to be careful not to print the wildcard-based
+              // constraint.
+              //
+              if (d.constraint && (!dep_constr || !wildcard (*dep_constr)))
+                dr << ' ' << *d.constraint;
 
-            if (!af->location.empty () && (!dep_constr || system))
+              dr << ") of package " << name << pdb <<
+                info << "available " << dn << " versions:";
+
+              for (const shared_ptr<available_package>& ap: aps)
+                dr << ' ' << ap->version;
+            }
+            else
+            {
+              dr << "no package available for dependency " << dn
+                 << " of package " << name << pdb;
+            }
+
+            // Avoid printing this if the dependent package is external since
+            // it's more often confusing than helpful (they are normally not
+            // fetched manually).
+            //
+            if (!af->location.empty ()           &&
+                !af->location.directory_based () &&
+                (!dep_constr || system))
               dr << info << "repository " << af->location << " appears to "
                  << "be broken" <<
                 info << "or the repository state could be stale" <<
diff --git a/tests/pkg-build.testscript b/tests/pkg-build.testscript
index 48fea7b..058a08b 100644
--- a/tests/pkg-build.testscript
+++ b/tests/pkg-build.testscript
@@ -431,7 +431,7 @@ test.options += --no-progress
   :
   $clone_root_cfg;
   $* $src/libbar-1.0.0.tar.gz 2>>EOE != 0
-    error: unknown dependency libfoo of package libbar
+    error: no package available for dependency libfoo of package libbar
     info: while satisfying libbar/1.0.0
     EOE
 
@@ -443,7 +443,7 @@ test.options += --no-progress
   $cfg_create -d cfg2 &cfg2/***;
   $cfg_link -d cfg cfg2;
   $* $src/libbar-1.0.0.tar.gz +{ --config-id 1 } 2>>~%EOE% != 0
-    %error: unknown dependency libfoo of package libbar \[cfg2.\]%
+    %error: no package available for dependency libfoo of package libbar \[cfg2.\]%
     %info: while satisfying libbar/1.0.0 \[cfg2.\]%
     EOE
 
@@ -1633,7 +1633,7 @@ test.options += --no-progress
     $rep_fetch;
 
     $* libfoo 2>>~%EOE% != 0;
-      error: unknown dependency libhello >= 1.0 of package libfoo
+      error: no package available for dependency libhello of package libfoo
       %.+
       EOE
 
@@ -2052,7 +2052,8 @@ test.options += --no-progress
       $rep_fetch $rep/t0a $rep/t0c;
 
       $* libbar/1.0.0 ?libfoo/0.0.1 2>>EOE != 0
-        error: unknown dependency libfoo == 0.0.1 of package libbar
+        error: unable to satisfy dependency constraint (libfoo == 0.0.1) of package libbar
+          info: available libfoo versions: 1.0.0
         info: while satisfying libbar/1.0.0
         EOE
     }
-- 
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