From 4ad1a241281d3e8fd0b28388c0191c044ac3d788 Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Wed, 21 Nov 2018 12:40:09 +0200
Subject: More backwards modification time experimentation

---
 build2/cc/link-rule.cxx |  3 ++-
 build2/depdb.cxx        | 47 ++++++++++++++++++++++++++++++++++-------------
 build2/depdb.hxx        |  2 +-
 3 files changed, 37 insertions(+), 15 deletions(-)

diff --git a/build2/cc/link-rule.cxx b/build2/cc/link-rule.cxx
index 981358e..2581d6f 100644
--- a/build2/cc/link-rule.cxx
+++ b/build2/cc/link-rule.cxx
@@ -2076,7 +2076,7 @@ namespace build2
       timestamp dd_tt (system_clock::now ());
 #endif
 
-      dd.close ();
+      timestamp dd_cl (dd.close ());
 
       // If nothing changed, then we are done.
       //
@@ -2569,6 +2569,7 @@ namespace build2
         if (dd_mt > tp_mt)
           fail << "backwards modification times:\n"
                << dd_tt    << " window start\n"
+               << dd_cl    << " write mtime\n"
                << dd.mtime << " close mtime\n"
                << dd_mt    << " " << dd.path.string () << '\n'
                << tp_mt    << " " << tp.string () << '\n'
diff --git a/build2/depdb.cxx b/build2/depdb.cxx
index f0b0d21..088b355 100644
--- a/build2/depdb.cxx
+++ b/build2/depdb.cxx
@@ -6,6 +6,10 @@
 
 #include <libbutl/filesystem.mxx> // file_mtime()
 
+#ifdef _WIN32
+#  include <libbutl/win32-utility.hxx>
+#endif
+
 #include <build2/diagnostics.hxx>
 
 using namespace std;
@@ -201,7 +205,7 @@ namespace build2
       fs_.put ('\n');
   }
 
-  void depdb::
+  timestamp depdb::
   close ()
   {
     // If we are at eof, then it means all lines are good, there is the "end
@@ -259,34 +263,51 @@ namespace build2
     // getting the old mtime if we ask for it too soon. For such cases we are
     // going to just set it ourselves.
     //
-#if defined(_WIN32) || defined(__FreeBSD__)
-#  define BUILD2_NEED_MTIME_FIX
-#endif
-
-#ifdef BUILD2_NEED_MTIME_FIX
-    timestamp mt;
+#ifdef _WIN32
+    timestamp mt (timestamp_unknown);
 #endif
 
     if (state_ == state::write)
     {
-#ifdef BUILD2_NEED_MTIME_FIX
-      mt = system_clock::now ();
+#ifdef _WIN32
+      FILETIME ft;
+      GetSystemTimeAsFileTime (&ft);
+
+      // See libbutl/filesystem.cxx for details.
+      //
+      uint64_t nsec ((static_cast<uint64_t> (ft.dwHighDateTime) << 32) |
+                     ft.dwLowDateTime);
+      nsec -= 11644473600ULL * 10000000;
+      nsec *= 100;
+
+      mt = timestamp (
+        chrono::duration_cast<duration> (chrono::nanoseconds (nsec)));
 #endif
+
       fs_.put ('\0'); // The "end marker".
     }
 
     fs_.close ();
 
-#ifdef BUILD2_NEED_MTIME_FIX
+#if defined(_WIN32) || defined(__FreeBSD__)
     if (state_ == state::write)
     {
-      // Save the original returned time for debugging.
-      //
-      mtime = file_mtime (path);
+      mtime = file_mtime (path); // Save for debugging.
 
+#ifdef _WIN32
       if (mtime < mt)
+      {
         file_mtime (path, mt);
+        assert (file_mtime (path) == mt);
+      }
+#endif
     }
 #endif
+
+#ifdef _WIN32
+    return mt;
+#else
+    return timestamp_unknown;
+#endif
   }
 }
diff --git a/build2/depdb.hxx b/build2/depdb.hxx
index 7c9e464..ffb3b89 100644
--- a/build2/depdb.hxx
+++ b/build2/depdb.hxx
@@ -90,7 +90,7 @@ namespace build2
     // may be left in the old/currupt state. Note that in the read mode this
     // function will "chop off" lines that haven't been read.
     //
-    void
+    timestamp
     close ();
 
     // Read the next line. If the result is not NULL, then it is a pointer to
-- 
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