fix Issue 15803 - std.file should support sub-second file time precision on POSIX

std/file.d

@@ -822,6 +822,28 @@ unittest
 }
 
 
+// Reads a time field from a stat_t with full precision.
+version(Posix)
+private SysTime statTimeToStdTime(char which)(ref stat_t statbuf)
+{
+    auto unixTime = mixin(`statbuf.st_` ~ which ~ `time`);
+    long stdTime = unixTimeToStdTime(unixTime);
+
+    static if (is(typeof(mixin(`statbuf.st_` ~ which ~ `tim`))))
+        stdTime += mixin(`statbuf.st_` ~ which ~ `tim.tv_nsec`) / 100;
+    else
+    static if (is(typeof(mixin(`statbuf.st_` ~ which ~ `timensec`))))
+        stdTime += mixin(`statbuf.st_` ~ which ~ `timensec`) / 100;
+    else
+    static if (is(typeof(mixin(`statbuf.st_` ~ which ~ `time_nsec`))))
+        stdTime += mixin(`statbuf.st_` ~ which ~ `time_nsec`) / 100;
+    else
+    static if (is(typeof(mixin(`statbuf.__st_` ~ which ~ `timensec`))))
+        stdTime += mixin(`statbuf.__st_` ~ which ~ `timensec`) / 100;
+
+    return SysTime(stdTime);
+}
+
 /++
     Get the access and modified times of file or folder $(D name).
 
@@ -863,8 +885,8 @@ void getTimes(R)(R name,
             string names = null;
         cenforce(trustedStat(namez, statbuf) == 0, names, namez);
 
-        accessTime = SysTime(unixTimeToStdTime(statbuf.st_atime));
+        accessTime = statTimeToStdTime!'a'(statbuf);
-        modificationTime = SysTime(unixTimeToStdTime(statbuf.st_mtime));
+        modificationTime = statTimeToStdTime!'m'(statbuf);
     }
 }
 
@@ -1217,7 +1239,7 @@ SysTime timeLastModified(R)(R name)
             string names = null;
         cenforce(trustedStat(namez, statbuf) == 0, names, namez);
 
-        return SysTime(unixTimeToStdTime(statbuf.st_mtime));
+        return statTimeToStdTime!'m'(statbuf);
     }
 }
 
@@ -1288,7 +1310,7 @@ SysTime timeLastModified(R)(R name, SysTime returnIfMissing)
 
         return trustedStat(namez, statbuf) != 0 ?
                returnIfMissing :
-               SysTime(unixTimeToStdTime(statbuf.st_mtime));
+               statTimeToStdTime!'m'(statbuf);
     }
 }
 
@@ -1312,6 +1334,35 @@ unittest
 }
 
 
+// Tests sub-second precision of querying file times.
+// Should pass on most modern systems running on modern filesystems.
+// Exceptions:
+// - FreeBSD, where one would need to first set the
+//   vfs.timestamp_precision sysctl to a value greater than zero.
+// - OS X, where the native filesystem (HFS+) stores filesystem
+//   timestamps with 1-second precision.
+version (FreeBSD) {} else
+version (OSX) {} else
+unittest
+{
+    import core.thread;
+
+    if(exists(deleteme))
+        remove(deleteme);
+
+    SysTime lastTime;
+    foreach (n; 0..3)
+    {
+        write(deleteme, "a");
+        auto time = timeLastModified(deleteme);
+        remove(deleteme);
+        assert(time != lastTime);
+        lastTime = time;
+        Thread.sleep(10.msecs);
+    }
+}
+
+
 /**
  * Determine whether the given file (or directory) exists.
  * Params:
@@ -2844,21 +2895,21 @@ else version(Posix)
         {
             _ensureStatDone();
 
-            return SysTime(unixTimeToStdTime(_statBuf.st_ctime));
+            return statTimeToStdTime!'c'(_statBuf);
         }
 
         @property SysTime timeLastAccessed()
         {
             _ensureStatDone();
 
-            return SysTime(unixTimeToStdTime(_statBuf.st_ctime));
+            return statTimeToStdTime!'a'(_statBuf);
         }
 
         @property SysTime timeLastModified()
         {
             _ensureStatDone();
 
-            return SysTime(unixTimeToStdTime(_statBuf.st_mtime));
+            return statTimeToStdTime!'m'(_statBuf);
         }
 
         @property uint attributes()