Merge pull request #1334 from tigerbeetle/matklad/checksum-empty

vsr: make `comptime checksum(&.{})` available everywhere

src/vsr/checksum.zig

@@ -48,11 +48,22 @@ fn seed_init() void {
 // Lazily initialize the Aegis State instead of recomputing it on each call to checksum().
 // Then, make a copy of the state and use that to hash the source input bytes.
 pub fn checksum(source: []const u8) u128 {
+    if (@inComptime()) {
+        // Aegis128 uses hardware accelerated AES via inline asm which isn't available at comptime.
+        // Use a hard-coded value instead and verify via a test.
+        if (source.len == 0) return 0x49F174618255402DE6E7E3C40D60CC83;
+    }
     var stream = ChecksumStream.init();
     stream.add(source);
     return stream.checksum();
 }
 
+test "checksum empty" {
+    var stream = ChecksumStream.init();
+    stream.add(&.{});
+    try std.testing.expectEqual(stream.checksum(), comptime checksum(&.{}));
+}
+
 pub const ChecksumStream = struct {
     state: Aegis128LMac_128,
 

src/vsr/message_header.zig

@@ -9,13 +9,7 @@ const Command = vsr.Command;
 const Operation = vsr.Operation;
 const schema = @import("../lsm/schema.zig");
 
-/// Aegis128 (used by checksum) uses hardware accelerated AES via inline asm which isn't
-/// available at comptime. Use a hard-coded value instead and verify via a test.
-const checksum_body_empty: u128 = 0x49F174618255402DE6E7E3C40D60CC83;
-
-test "empty checksum" {
-    assert(checksum_body_empty == vsr.checksum(&.{}));
-}
+const checksum_body_empty = vsr.checksum(&.{});
 
 /// Network message and journal entry header:
 /// We reuse the same header for both so that prepare messages from the primary can simply be