fix(pkg/stanza/input/file/reader): skip building fingerprint in case of configuration change

CHANGELOG.md

@@ -22,6 +22,7 @@
 ### 🧰 Bug fixes 🧰
 
 - `kubletetstatsreceiver`: Bring back `k8s.container.name` attribute (#10848)
+- `pkg/stanza`: Skip building fingerprint in case of configuration change (#10485)
 
 ## v0.53.0
 

pkg/stanza/operator/input/file/file_test.go

@@ -923,6 +923,92 @@ func TestFingerprintGrowsAndStops(t *testing.T) {
 	}
 }
 
+// This is same test like TestFingerprintGrowsAndStops, but with additional check for fingerprint size check
+// Test that a fingerprint:
+// - Starts empty
+// - Updates as a file is read
+// - Stops updating when the max fingerprint size is reached
+// - Stops exactly at max fingerprint size, regardless of content
+// - Do not change size after fingerprint configuration change
+func TestFingerprintChangeSize(t *testing.T) {
+	t.Parallel()
+
+	// Use a number with many factors.
+	// Sometimes fingerprint length will align with
+	// the end of a line, sometimes not. Test both.
+	maxFP := 360
+
+	// Use prime numbers to ensure variation in
+	// whether or not they are factors of maxFP
+	lineLens := []int{3, 5, 7, 11, 13, 17, 19, 23, 27}
+
+	for _, lineLen := range lineLens {
+		t.Run(fmt.Sprintf("%d", lineLen), func(t *testing.T) {
+			t.Parallel()
+			operator, _, tempDir := newTestFileOperator(t, func(cfg *Config) {
+				cfg.FingerprintSize = helper.ByteSize(maxFP)
+			}, nil)
+			defer func() {
+				require.NoError(t, operator.Stop())
+			}()
+
+			temp := openTemp(t, tempDir)
+			tempCopy := openFile(t, temp.Name())
+			fp, err := operator.NewFingerprint(temp)
+			require.NoError(t, err)
+			require.Equal(t, []byte(""), fp.FirstBytes)
+
+			splitter, err := operator.getMultiline()
+			require.NoError(t, err)
+
+			reader, err := operator.NewReader(temp.Name(), tempCopy, fp, splitter)
+			require.NoError(t, err)
+			defer reader.Close()
+
+			// keep track of what has been written to the file
+			fileContent := []byte{}
+
+			// keep track of expected fingerprint size
+			expectedFP := 0
+
+			// Write lines until file is much larger than the length of the fingerprint
+			for len(fileContent) < 2*maxFP {
+				expectedFP += lineLen
+				if expectedFP > maxFP {
+					expectedFP = maxFP
+				}
+
+				line := stringWithLength(lineLen-1) + "\n"
+				fileContent = append(fileContent, []byte(line)...)
+
+				writeString(t, temp, line)
+				reader.ReadToEnd(context.Background())
+				require.Equal(t, fileContent[:expectedFP], reader.Fingerprint.FirstBytes)
+			}
+
+			// Test fingerprint change
+			// Change fingerprint and try to read file again
+			// We do not expect fingerprint change
+			// We test both increasing and decreasing fingerprint size
+			reader.fileInput.fingerprintSize = maxFP * (lineLen / 3)
+			line := stringWithLength(lineLen-1) + "\n"
+			fileContent = append(fileContent, []byte(line)...)
+
+			writeString(t, temp, line)
+			reader.ReadToEnd(context.Background())
+			require.Equal(t, fileContent[:expectedFP], reader.Fingerprint.FirstBytes)
+
+			reader.fileInput.fingerprintSize = maxFP / 2
+			line = stringWithLength(lineLen-1) + "\n"
+			fileContent = append(fileContent, []byte(line)...)
+
+			writeString(t, temp, line)
+			reader.ReadToEnd(context.Background())
+			require.Equal(t, fileContent[:expectedFP], reader.Fingerprint.FirstBytes)
+		})
+	}
+}
+
 func TestEncodings(t *testing.T) {
 	t.Parallel()
 	cases := []struct {

pkg/stanza/operator/input/file/reader.go

@@ -226,11 +226,19 @@ func getScannerError(scanner *PositionalScanner) error {
 
 // Read from the file and update the fingerprint if necessary
 func (r *Reader) Read(dst []byte) (int, error) {
-	if len(r.Fingerprint.FirstBytes) == r.fileInput.fingerprintSize {
+	// Skip if fingerprint is already built
+	// or if fingerprint is behind Offset
+	if len(r.Fingerprint.FirstBytes) == r.fileInput.fingerprintSize || int(r.Offset) > len(r.Fingerprint.FirstBytes) {
 		return r.file.Read(dst)
 	}
 	n, err := r.file.Read(dst)
 	appendCount := min0(n, r.fileInput.fingerprintSize-int(r.Offset))
+	// return for n == 0 or r.Offset >= r.fileInput.fingerprintSize
+	if appendCount == 0 {
+		return n, err
+	}
+
+	// for appendCount==0, the following code would add `0` to fingerprint
 	r.Fingerprint.FirstBytes = append(r.Fingerprint.FirstBytes[:r.Offset], dst[:appendCount]...)
 	return n, err
 }