[fileconsumer] Deduplicate fingerprints less aggressively (#24235)

When finding files, we would check for files with duplicate fingerprints
and deduplicate them if `a.StartsWith(b)` or `b.StartsWith(a)`. This
`StartsWith` logic is useful for recognizing files that have new content
since the previous poll interval. However, when deduplicating files
observed at the same moment, the only case that we need to consider is
copy/truncate rotation. In this case, a file may have been copied but
the original has not yet been truncated. At that moment we should expect
to find two files with exactly the same fingerprint. Therefore, we do
not need to check StartsWith cases.

.chloggen/strict-dedup.yaml

@@ -0,0 +1,20 @@
+# Use this changelog template to create an entry for release notes.
+# If your change doesn't affect end users, such as a test fix or a tooling change,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: filelogreceiver
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Fix issue where files were deduplicated unnecessarily
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [24235]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:

pkg/stanza/fileconsumer/file.go

@@ -232,8 +232,7 @@ func (m *Manager) makeFingerprint(path string) (*fingerprint.Fingerprint, *os.Fi
 
 func (m *Manager) checkDuplicates(fp *fingerprint.Fingerprint) bool {
 	for i := 0; i < len(m.currentFps); i++ {
-		fp2 := m.currentFps[i]
-		if fp.StartsWith(fp2) || fp2.StartsWith(fp) {
+		if fp.Equal(m.currentFps[i]) {
 			return true
 		}
 	}

pkg/stanza/fileconsumer/internal/fingerprint/fingerprint.go

@@ -46,6 +46,24 @@ func (f Fingerprint) Copy() *Fingerprint {
 	}
 }
 
+// Equal returns true if the fingerprints have the same FirstBytes,
+// false otherwise. This does not compare other aspects of the fingerprints
+// because the primary purpose of a fingerprint is to convey a unique
+// identity, and only the FirstBytes field contributes to this goal.
+func (f Fingerprint) Equal(other *Fingerprint) bool {
+	l0 := len(other.FirstBytes)
+	l1 := len(f.FirstBytes)
+	if l0 != l1 {
+		return false
+	}
+	for i := 0; i < l0; i++ {
+		if other.FirstBytes[i] != f.FirstBytes[i] {
+			return false
+		}
+	}
+	return true
+}
+
 // StartsWith returns true if the fingerprints are the same
 // or if the new fingerprint starts with the old one
 // This is important functionality for tracking new files,

pkg/stanza/fileconsumer/internal/fingerprint/fingerprint_test.go

@@ -166,8 +166,32 @@ func TestFingerprintCopy(t *testing.T) {
 	}
 }
 
-func TestFingerprintStartsWith(t *testing.T) {
+func TestEqual(t *testing.T) {
+	empty := &Fingerprint{FirstBytes: []byte("")}
+	empty2 := &Fingerprint{FirstBytes: []byte("")}
+	hello := &Fingerprint{FirstBytes: []byte("hello")}
+	hello2 := &Fingerprint{FirstBytes: []byte("hello")}
+	world := &Fingerprint{FirstBytes: []byte("world")}
+	world2 := &Fingerprint{FirstBytes: []byte("world")}
+	helloworld := &Fingerprint{FirstBytes: []byte("helloworld")}
+	helloworld2 := &Fingerprint{FirstBytes: []byte("helloworld")}
+
+	require.True(t, empty.Equal(empty2))
+	require.True(t, hello.Equal(hello2))
+	require.True(t, world.Equal(world2))
+	require.True(t, helloworld.Equal(helloworld2))
 
+	require.False(t, hello.Equal(empty))
+	require.False(t, empty.Equal(hello))
+
+	require.False(t, hello.Equal(world))
+	require.False(t, world.Equal(hello))
+
+	require.False(t, hello.Equal(helloworld))
+	require.False(t, helloworld.Equal(hello))
+}
+
+func TestStartsWith(t *testing.T) {
 	empty := &Fingerprint{FirstBytes: []byte("")}
 	hello := &Fingerprint{FirstBytes: []byte("hello")}
 	world := &Fingerprint{FirstBytes: []byte("world")}
@@ -183,7 +207,6 @@ func TestFingerprintStartsWith(t *testing.T) {
 	require.True(t, helloworld.StartsWith(hello))
 	require.True(t, helloworld.StartsWith(helloworld))
 	require.False(t, helloworld.StartsWith(world))
-
 }
 
 // Generates a file filled with many random bytes, then
@@ -193,7 +216,7 @@ func TestFingerprintStartsWith(t *testing.T) {
 // The static file can be thought of as the present state of
 // the file, while each iteration of the growing file represents
 // a possible state of the same file at a previous time.
-func TestFingerprintStartsWith_FromFile(t *testing.T) {
+func TestStartsWith_FromFile(t *testing.T) {
 	r := rand.New(rand.NewSource(112358))
 	fingerprintSize := 10
 	fileLength := 12 * fingerprintSize