From 2dd1f9fa5a7427096b74aa85258ec2feeb93c104 Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Thu, 1 Jun 2023 12:22:26 +0200
Subject: Diagnose null include, operation-specific variable values

While assigning null directly is unlikely, it's fairly easy via a variable
expansion. Real-world example:

./: exe{tensor}: include = $config.Eigen.unsupported
---
 libbuild2/target.cxx | 74 ++++++++++++++++++++++++++++++++--------------------
 libbuild2/target.hxx |  4 +--
 2 files changed, 47 insertions(+), 31 deletions(-)

diff --git a/libbuild2/target.cxx b/libbuild2/target.cxx
index 6b2c702..4634688 100644
--- a/libbuild2/target.cxx
+++ b/libbuild2/target.cxx
@@ -566,16 +566,25 @@ namespace build2
     context& ctx (t.ctx);
 
     include_type r (include_type::normal);
-
-    if (const string* v = cast_null<string> (p.vars[ctx.var_include]))
     {
-      if      (*v == "false")   r = include_type::excluded;
-      else if (*v == "true")    r = include_type::normal;
-      else if (*v == "adhoc")   r = include_type::adhoc;
-      else if (*v == "posthoc") r = include_type::posthoc;
-      else
-        fail << "invalid " << *ctx.var_include << " variable value "
-             << "'" << *v << "' specified for prerequisite " << p;
+      lookup l (p.vars[ctx.var_include]);
+
+      if (l.defined ())
+      {
+        if (l->null)
+          fail << "null " << *ctx.var_include << " variable value specified "
+               << "for prerequisite " << p;
+
+        const string& v (cast<string> (*l));
+
+        if      (v == "false")   r = include_type::excluded;
+        else if (v == "true")    r = include_type::normal;
+        else if (v == "adhoc")   r = include_type::adhoc;
+        else if (v == "posthoc") r = include_type::posthoc;
+        else
+          fail << "invalid " << *ctx.var_include << " variable value '"
+               << v << "' specified for prerequisite " << p;
+      }
     }
 
     // Handle operation-specific override (see var_include documentation
@@ -601,31 +610,40 @@ namespace build2
          ? ctx.current_outer_oif
          : ctx.current_inner_oif)->id].ovar;
 
-      if (ovar != nullptr && (l = p.vars[*ovar]))
+      if (ovar != nullptr)
       {
-        // Maybe we should optimize this for the common cases (bool, path,
-        // name)? But then again we don't expect many such overrides. Plus
-        // will complicate the diagnostics below.
-        //
-        ns = reverse (*l, storage, true /* reduce */);
+        l = p.vars[*ovar];
 
-        if (ns.size () == 1)
+        if (l.defined ())
         {
-          const name& n (ns[0]);
+          if (l->null)
+            fail << "null " << *ovar << " variable value specified for "
+                 << "prerequisite " << p;
+
+          // Maybe we should optimize this for the common cases (bool, path,
+          // name)? But then again we don't expect many such overrides. Plus
+          // will complicate the diagnostics below.
+          //
+          ns = reverse (*l, storage, true /* reduce */);
 
-          if (n.simple ())
+          if (ns.size () == 1)
           {
-            const string& v (n.value);
+            const name& n (ns[0]);
 
-            if (v == "false")
-              r1 = false;
-            else if (v == "true")
-              r1 = true;
+            if (n.simple ())
+            {
+              const string& v (n.value);
+
+              if (v == "false")
+                r1 = false;
+              else if (v == "true")
+                r1 = true;
+            }
           }
-        }
 
-        if (r1 && !*r1)
-          r = include_type::excluded;
+          if (r1 && !*r1)
+            r = include_type::excluded;
+        }
       }
     }
 
@@ -646,8 +664,8 @@ namespace build2
         // Note: we have to delay this until the meta-operation callback above
         // had a chance to override it.
         //
-        fail << "unrecognized " << *ovar << " variable value "
-             << "'" << ns << "' specified for prerequisite " << p;
+        fail << "unrecognized " << *ovar << " variable value '" << ns
+             << "' specified for prerequisite " << p;
       }
     }
 
diff --git a/libbuild2/target.hxx b/libbuild2/target.hxx
index d01e8b7..d4ad780 100644
--- a/libbuild2/target.hxx
+++ b/libbuild2/target.hxx
@@ -1472,9 +1472,7 @@ namespace build2
   }
 
   include_type
-  include (action, const target&,
-           const prerequisite_member&,
-           lookup* = nullptr);
+  include (action, const target&, const prerequisite_member&, lookup* = nullptr);
 
   // A "range" that presents a sequence of prerequisites (e.g., from
   // group_prerequisites()) as a sequence of prerequisite_member's. For each
-- 
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