Fix log-validator checksum mismatch

Author: aarongable

blog/checksum.go

@@ -37,8 +37,16 @@ var _ io.Writer = (*checksumWriter)(nil)
 // results in a single call to the wrapped io.Writer.Write. This means that we
 // are computing writing exactly one checksum per call to slog.Logger.Handle.
 func (w *checksumWriter) Write(in []byte) (int, error) {
+	// slog handlers terminate each record with a trailing newline. rsyslog
+	// strips that newline before writing the line to disk, and the log-validator
+	// computes its checksum over the stripped line. Strip it here too so the
+	// checksum we emit covers exactly the bytes the validator will see.
+	body := in
+	if len(body) > 0 && body[len(body)-1] == '\n' {
+		body = body[:len(body)-1]
+	}
 	out := bytes.Buffer{}
-	out.WriteString(LogLineChecksum(string(in)))
+	out.WriteString(LogLineChecksum(string(body)))
 	out.WriteString(" ")
 	out.Write(in)
 	_, err := out.WriteTo(w.inner)

blog/checksum_test.go

@@ -22,9 +22,15 @@ func TestLogLineChecksum(t *testing.T) {
 		{
 			name: "simple",
 			line: "hello, world",
-			// Deterministic base64url(CRC32("hello, world"))
+			// Deterministic base64url(CRC32("hello, world")).
 			want: "OnKr_w",
 		},
+		{
+			name: "newline",
+			line: "hello, world\n",
+			// LogLineChecksum hashes every byte, so the trailing newline changes it.
+			want: "U3Qk9A",
+		},
 	}
 
 	for _, tc := range testCases {
@@ -66,11 +72,12 @@ func TestChecksumWriter(t *testing.T) {
 			want: "OnKr_w hello, world",
 		},
 		{
-			name: "complex",
-			in: `level=INFO msg="he said \"hi\"\nthen" trail=actual
-`,
-			want: `R3t8pA level=INFO msg="he said \"hi\"\nthen" trail=actual
-`,
+			name: "newline",
+			in:   "hello, world\n",
+			// The checksumWriter knows that trailing newlines are actually line
+			// terminators, not part of the line itself, so it discards them before
+			// computing the checksum. Therefore the checksum should be the same.
+			want: "OnKr_w hello, world\n",
 		},
 	}
 

blog/logger_test.go

@@ -180,7 +180,11 @@ func TestLoggerChecksum(t *testing.T) {
 	if len(parts) != 2 {
 		t.Fatalf("expected log line to have a space-separated checksum prefix, got %q", got[0])
 	}
-	if want := LogLineChecksum(parts[1]); parts[0] != want {
+	// The trailing newline is a line terminator, not part of the line, so
+	// strip it before computing the expected checksum.
+	body := strings.TrimSuffix(parts[1], "\n")
+	want := LogLineChecksum(body)
+	if parts[0] != want {
 		t.Errorf("checksum prefix %q does not match LogLineChecksum of remainder %q", parts[0], want)
 	}
 }