add regression test for already fixed bug

closes #6781

test/stage1/behavior.zig

@@ -53,6 +53,7 @@ comptime {
     _ = @import("behavior/bugs/5413.zig");
     _ = @import("behavior/bugs/5474.zig");
     _ = @import("behavior/bugs/5487.zig");
+    _ = @import("behavior/bugs/6781.zig");
     _ = @import("behavior/bugs/394.zig");
     _ = @import("behavior/bugs/421.zig");
     _ = @import("behavior/bugs/529.zig");

test/stage1/behavior/bugs/6781.zig

@@ -0,0 +1,74 @@
+const std = @import("std");
+const assert = std.debug.assert;
+
+const segfault = true;
+
+pub const JournalHeader = packed struct {
+    hash_chain_root: u128 = undefined,
+    prev_hash_chain_root: u128,
+    checksum: u128 = undefined,
+    magic: u64,
+    command: u32,
+    size: u32,
+
+    pub fn calculate_checksum(self: *const JournalHeader, entry: []const u8) u128 {
+        assert(entry.len >= @sizeOf(JournalHeader));
+        assert(entry.len == self.size);
+
+        const checksum_offset = @byteOffsetOf(JournalHeader, "checksum");
+        const checksum_size = @sizeOf(@TypeOf(self.checksum));
+        assert(checksum_offset == 0 + 16 + 16);
+        assert(checksum_size == 16);
+
+        var target: [32]u8 = undefined;
+        std.crypto.hash.Blake3.hash(entry[checksum_offset + checksum_size ..], target[0..], .{});
+        return @bitCast(u128, target[0..checksum_size].*);
+    }
+
+    pub fn calculate_hash_chain_root(self: *const JournalHeader) u128 {
+        const hash_chain_root_size = @sizeOf(@TypeOf(self.hash_chain_root));
+        assert(hash_chain_root_size == 16);
+
+        const prev_hash_chain_root_offset = @byteOffsetOf(JournalHeader, "prev_hash_chain_root");
+        const prev_hash_chain_root_size = @sizeOf(@TypeOf(self.prev_hash_chain_root));
+        assert(prev_hash_chain_root_offset == 0 + 16);
+        assert(prev_hash_chain_root_size == 16);
+
+        const checksum_offset = @byteOffsetOf(JournalHeader, "checksum");
+        const checksum_size = @sizeOf(@TypeOf(self.checksum));
+        assert(checksum_offset == 0 + 16 + 16);
+        assert(checksum_size == 16);
+
+        assert(prev_hash_chain_root_offset + prev_hash_chain_root_size == checksum_offset);
+
+        const header = @bitCast([@sizeOf(JournalHeader)]u8, self.*);
+        const source = header[prev_hash_chain_root_offset .. checksum_offset + checksum_size];
+        assert(source.len == prev_hash_chain_root_size + checksum_size);
+        var target: [32]u8 = undefined;
+        std.crypto.hash.Blake3.hash(source, target[0..], .{});
+        if (segfault) {
+            return @bitCast(u128, target[0..hash_chain_root_size].*);
+        } else {
+            var array = target[0..hash_chain_root_size].*;
+            return @bitCast(u128, array);
+        }
+    }
+
+    pub fn set_checksum_and_hash_chain_root(self: *JournalHeader, entry: []const u8) void {
+        self.checksum = self.calculate_checksum(entry);
+        self.hash_chain_root = self.calculate_hash_chain_root();
+    }
+};
+
+test "fixed" {
+    var buffer = [_]u8{0} ** 65536;
+    var entry = std.mem.bytesAsValue(JournalHeader, buffer[0..@sizeOf(JournalHeader)]);
+    entry.* = .{
+        .prev_hash_chain_root = 0,
+        .magic = 0,
+        .command = 0,
+        .size = 64 + 128,
+    };
+    entry.set_checksum_and_hash_chain_root(buffer[0..entry.size]);
+    try std.io.null_writer.print("{}\n", .{entry});
+}