lib/model: Handle filename conflicts on Windows.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3810
LGTM: calmh

lib/model/rwfolder.go

@@ -435,6 +435,13 @@ func (f *sendReceiveFolder) pullerIteration(ignores *ignore.Matcher) int {
 			continue
 		}
 
+		// Verify that we handle the right thing and not something whose name
+		// collides.
+		if !osutil.CheckNameConflict(f.dir, fi.Name) {
+			f.newError(fi.Name, errNameConflict)
+			continue
+		}
+
 		switch {
 		case fi.IsDeleted():
 			// A deleted file, directory or symlink
@@ -524,6 +531,13 @@ nextFile:
 			continue
 		}
 
+		// Verify that we handle the right thing and not something whose name
+		// collides.
+		if !osutil.CheckNameConflict(f.dir, fi.Name) {
+			f.newError(fi.Name, errNameConflict)
+			continue
+		}
+
 		// Check our list of files to be removed for a match, in which case
 		// we can just do a rename instead.
 		key := string(fi.Blocks[0].Hash)
@@ -1273,6 +1287,16 @@ func (f *sendReceiveFolder) copierRoutine(in <-chan copyBlocksState, pullChan ch
 					if err != nil {
 						return false
 					}
+					// The following checks are racy
+					if !osutil.IsDir(folderRoots[folder], filepath.Dir(file)) {
+						return false
+					}
+					if !osutil.CheckNameConflict(folderRoots[folder], file) {
+						return false
+					}
+					if info, err := osutil.Lstat(inFile); err != nil || !info.Mode().IsRegular() {
+						return false
+					}
 					fd, err := os.Open(inFile)
 					if err != nil {
 						return false