logtail/filch: limit buffer file size to 50MB

Signed-off-by: Maisem Ali <maisem@tailscale.com>

logtail/filch/filch_test.go

@@ -57,6 +57,39 @@ func (f *filchTest) close(t *testing.T) {
 	}
 }
 
+func TestDropOldLogs(t *testing.T) {
+	const line1 = "123456789" // 10 bytes (9+newline)
+	tests := []struct {
+		write, read int
+	}{
+		{10, 10},
+		{100, 100},
+		{200, 200},
+		{250, 150},
+		{500, 200},
+	}
+	for _, tc := range tests {
+		t.Run(fmt.Sprintf("w%d-r%d", tc.write, tc.read), func(t *testing.T) {
+			filePrefix := t.TempDir()
+			f := newFilchTest(t, filePrefix, Options{ReplaceStderr: false, MaxFileSize: 1000})
+			defer f.close(t)
+			// Make filch rotate the logs 3 times
+			for i := 0; i < tc.write; i++ {
+				f.write(t, line1)
+			}
+			// We should only be able to read the last 150 lines
+			for i := 0; i < tc.read; i++ {
+				f.read(t, line1)
+				if t.Failed() {
+					t.Logf("could only read %d lines", i)
+					break
+				}
+			}
+			f.readEOF(t)
+		})
+	}
+}
+
 func TestQueue(t *testing.T) {
 	filePrefix := t.TempDir()
 	f := newFilchTest(t, filePrefix, Options{ReplaceStderr: false})