Always use cached mtime if available

Besides other things, this is required for "logical clean" in the try-run
mode to work properly:

$ b -vn clean update

3 files changed, 26 insertions, 6 deletions

diff --git a/build2/cc/compile-rule.cxx b/build2/cc/compile-rule.cxx
index 3050a37..92ba32d 100644
--- a/build2/cc/compile-rule.cxx
+++ b/build2/cc/compile-rule.cxx
@@ -823,8 +823,22 @@ namespace build2
         // compiler, options, or source file) or if the depdb is newer than
         // the target (interrupted update), then do unconditional update.
         //
+        // Note that load_mtime() can only be used in the execute phase so we
+        // have to check for a cached value manually.
+        //
+        bool u;
         timestamp mt;
-        bool u (dd.writing () || dd.mtime > (mt = mtime (tp)));
+
+        if (dd.writing ())
+          u = true;
+        else
+        {
+          if ((mt = t.mtime ()) == timestamp_unknown)
+            t.mtime (mt = mtime (tp)); // Cache.
+
+          u = dd.mtime > mt;
+        }
+
         if (u)
           mt = timestamp_nonexistent; // Treat as if it doesn't exist.
 
@@ -1045,6 +1059,9 @@ namespace build2
         // store the first in the target file (so we do touch it) and the
         // second in depdb (which is never newer that the target).
         //
+        // Perhaps when we start keeping the partially preprocessed this will
+        // fall away? Yes, please.
+        //
         md.mt = u ? timestamp_nonexistent : dd.mtime;
       }
 
@@ -4319,10 +4336,11 @@ namespace build2
         if (md.touch)
         {
           touch (tp, false, 2);
+          t.mtime (system_clock::now ());
           skip_count.fetch_add (1, memory_order_relaxed);
         }
+        // Note: else mtime should be cached.
 
-        t.mtime (md.mt);
         return *pr.first;
       }
 
diff --git a/build2/context.hxx b/build2/context.hxx
index a8d6833..5799eef 100644
--- a/build2/context.hxx
+++ b/build2/context.hxx
@@ -420,7 +420,9 @@ namespace build2
   // Note that for this mode to function properly we have to use fake mtimes.
   // Specifically, a rule that pretends to update a target must set its mtime
   // to system_clock::now() and everyone else must use this cached value. In
-  // other words, there should be no mtime re-query from the filesystem.
+  // other words, there should be no mtime re-query from the filesystem. The
+  // same is required for "logical clean" (i.e., dry-run 'clean update' in
+  // order to see all the command lines).
   //
   // At first, it may seem like we should also "dry-run" changes to depdb. But
   // that would be both problematic (some rules update it in apply() during
diff --git a/build2/rule.cxx b/build2/rule.cxx
index 79b91b3..a09e28b 100644
--- a/build2/rule.cxx
+++ b/build2/rule.cxx
@@ -55,8 +55,8 @@ namespace build2
 
         timestamp ts (mt.mtime ());
 
-        if (ts != timestamp_unknown && ts != timestamp_nonexistent)
-          return true;
+        if (ts != timestamp_unknown)
+          return ts != timestamp_nonexistent;
 
         // Otherwise, if this is not a path_target, then we don't match.
         //
@@ -86,7 +86,7 @@ namespace build2
         ts = mtime (*p);
         pt->mtime (ts);
 
-        if (ts != timestamp_unknown && ts != timestamp_nonexistent)
+        if (ts != timestamp_nonexistent)
           return true;
 
         l4 ([&]{trace << "no existing file for target " << *pt;});