Improve diagnostics on empty stream in compactsize unpacking

Author: bbrtj

Changes

@@ -12,6 +12,7 @@ Revision history for Perl extension Bitcoin::Crypto.
 
 	[Fixes and improvements]
 	- fixed variable-time cryptographic operation in taproot key tweaking
+	- improved diagnostics on trying to unpack an empty stream as a compactsize
 
 4.005 Thu Apr 23, 2026
 	[Public interface changes]

lib/Bitcoin/Crypto/Util/Internal.pm

@@ -293,6 +293,10 @@ sub unpack_compactsize
 	my $partial = !!$pos_ref;
 	my $pos = $partial ? $$pos_ref : 0;
 
+	Bitcoin::Crypto::Exception->raise(
+		"cannot unpack CompactSize: no data in stream"
+	) unless $pos < length $stream;
+
 	# if the first byte is 0xfd, 0xfe or 0xff, then CompactSize contains 2, 4 or 8
 	# bytes respectively
 	my $value = ord substr $stream, $pos++, 1;

t/Util.t

@@ -260,6 +260,12 @@ subtest 'testing unpack_compactsize with leftover data' => sub {
 	}, 'Bitcoin::Crypto::Exception';
 };
 
+subtest 'testing compactsize with no data' => sub {
+	isa_ok dies {
+		unpack_compactsize('');
+	}, 'Bitcoin::Crypto::Exception';
+};
+
 subtest 'testing merkle_root' => sub {
 	my $block_100022 = [
 		[