fileObserver: detect file existence when adding reactor

When startingContent for non-existent file is added, detect
the non-existence of the file to avoid restart-loops.

pkg/controller/fileobserver/observer_polling.go

@@ -57,7 +57,11 @@ func (o *pollingObserver) AddReactor(reaction ReactorFn, startingFileContent map
 			// in case the file does not exists but empty string is specified as initial content, without this
 			// the content will be hashed and reaction will trigger as if the content changed.
 			if len(startingContent) == 0 {
-				o.files[f] = fileHashAndState{exists: true}
+				var fileExists bool
+				if fileExists, err = isFile(f); err != nil {
+					panic(fmt.Sprintf("unexpected error while adding reactor for %#v: %v", files, err))
+				}
+				o.files[f] = fileHashAndState{exists: fileExists}
 				o.reactors[f] = append(o.reactors[f], reaction)
 				continue
 			}
@@ -165,22 +169,15 @@ type fileHashAndState struct {
 
 func calculateFileHash(path string) (fileHashAndState, error) {
 	result := fileHashAndState{}
-	stat, err := os.Stat(path)
-	if err != nil {
+	if exists, err := isFile(path); !exists || err != nil {
 		return result, err
 	}
 
-	// this is fatal
-	if stat.IsDir() {
-		return result, fmt.Errorf("you can watch only files, %s is a directory", path)
-	}
-
 	f, err := os.Open(path)
 	if err != nil {
 		return result, err
 	}
 	defer f.Close()
-
 	// at this point we know for sure the file exists and we can read its content even if that content is empty
 	result.exists = true
 
@@ -207,3 +204,20 @@ func calculateHash(content io.Reader) (string, bool, error) {
 	}
 	return hex.EncodeToString(hasher.Sum(nil)), false, nil
 }
+
+func isFile(path string) (bool, error) {
+	stat, err := os.Stat(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return false, nil
+		}
+		return false, err
+	}
+
+	// this is fatal
+	if stat.IsDir() {
+		return false, fmt.Errorf("%s is a directory", path)
+	}
+
+	return true, nil
+}

pkg/controller/fileobserver/observer_polling_test.go

@@ -138,6 +138,14 @@ func TestObserverPolling(t *testing.T) {
 			evaluateActions: observedNoChanges,
 			startWithNoFile: true,
 		},
+		{
+			// This is what controllercmd.NewCommandWithContext currently does to avoid races
+			name:              "start with non-existing file with no change, force no starting hashing",
+			setInitialContent: true,
+			startFileContent:  emptyContent,
+			evaluateActions:   observedNoChanges,
+			startWithNoFile:   true,
+		},
 		{
 			name:              "start with non-existing file that is created as empty file",
 			evaluateActions:   observedSingleFileCreated,