[JENKINS-56114] Correct behavior for Windows Server 2016 with Docker (#3914)

* [JENKINS-56114] Correct behavior for Windows Server 2016 with Docker
- also adjust the symlink escape hatch

* Correct missing space + retrigger build

* Improve performance with a check on the logger level

* Run the specific code only under Windows

* Rename the method as it's used only for Windows

(cherry picked from commit 5562b2fec6)

core/src/main/java/hudson/FilePath.java

@@ -132,6 +132,7 @@ import org.jenkinsci.remoting.RoleChecker;
 import org.jenkinsci.remoting.RoleSensitive;
 import org.kohsuke.accmod.Restricted;
 import org.kohsuke.accmod.restrictions.NoExternalUse;
+import org.kohsuke.stapler.Function;
 import org.kohsuke.stapler.Stapler;
 
 import static hudson.FilePath.TarCompression.GZIP;
@@ -3264,9 +3265,14 @@ public final class FilePath implements Serializable {
             Path parentAbsolutePath = Util.fileToPath(parentFile.getAbsoluteFile());
             Path parentRealPath;
             try {
-                parentRealPath = parentAbsolutePath.toRealPath();
+                if (Functions.isWindows()) {
+                    parentRealPath = this.windowsToRealPath(parentAbsolutePath);
+                } else {
+                    parentRealPath = parentAbsolutePath.toRealPath();
+                }
             }
-            catch(NoSuchFileException e) {
+            catch (NoSuchFileException e) {
+                LOGGER.log(Level.FINE, String.format("Cannot find the real path to the parentFile: %s", parentAbsolutePath), e);
                 return false;
             }
 
@@ -3298,10 +3304,11 @@ public final class FilePath implements Serializable {
                 try{
                     Path child = currentFileAbsolutePath.toRealPath();
                     if (!child.startsWith(parentRealPath)) {
+                        LOGGER.log(Level.FINE, "Child [{0}] does not start with parent [{1}] => not descendant", new Object[]{ child, parentRealPath });
                         return false;
                     }
                 } catch (NoSuchFileException e) {
-                    // nonexistent file
+                    // nonexistent file / Windows Server 2016 + MSFT docker
                     // in case this folder / file will be copied somewhere else, 
                     // it becomes the responsibility of that system to check the isDescendant with the existing links
                     // we are not taking the parentRealPath to avoid possible problem
@@ -3324,6 +3331,21 @@ public final class FilePath implements Serializable {
             }
             return current;
         }
+        
+        private @Nonnull Path windowsToRealPath(@Nonnull Path path) throws IOException {
+            try {
+                return path.toRealPath();
+            }
+            catch (IOException e) {
+                if (LOGGER.isLoggable(Level.FINE)) {
+                    LOGGER.log(Level.FINE, String.format("relaxedToRealPath cannot use the regular toRealPath on %s, trying with toRealPath(LinkOption.NOFOLLOW_LINKS)", path), e);
+                }
+            }
+
+            // that's required for specific environment like Windows Server 2016, running MSFT docker
+            // where the root is a <SYMLINKD>
+            return path.toRealPath(LinkOption.NOFOLLOW_LINKS);
+        }
     }
 
     private static final SoloFilePathFilter UNRESTRICTED = SoloFilePathFilter.wrap(FilePathFilter.UNRESTRICTED);

core/src/main/java/jenkins/util/VirtualFile.java

@@ -546,7 +546,11 @@ public abstract class VirtualFile implements Comparable<VirtualFile>, Serializab
 
         private boolean isIllegalSymlink() {
             try {
-                return !this.isDescendant("");
+                String myPath = f.toPath().toRealPath().toString();
+                String rootPath = root.toPath().toRealPath().toString();
+                if (!myPath.equals(rootPath) && !myPath.startsWith(rootPath + File.separatorChar)) {
+                    return true;
+                }
             } catch (IOException x) {
                 Logger.getLogger(VirtualFile.class.getName()).log(Level.FINE, "could not determine symlink status of " + f, x);
             } catch (InvalidPathException x2) {