t: avoid perl's pack/unpack "Q" specifier

The perl script introduced by 86b008e (t: add library for munging
chunk-format files, 2023-10-09) uses pack("Q") and unpack("Q") to read
and write 64-bit values ("quadwords" in perl parlance) from the on-disk
chunk files. However, some builds of perl may not support 64-bit
integers at all, and throw an exception here. While some 32-bit
platforms may still support 64-bit integers in perl (such as our linux32
CI environment), others reportedly don't (the NonStop 32-bit builds).

We can work around this by treating the 64-bit values as two 32-bit
values. We can't ever combine them into a single 64-bit value, but in
practice this is OK. These are representing file offsets, and our files
are much smaller than 4GB. So the upper half of the 64-bit value will
always be 0.

We can just introduce a few helper functions which perform the
translation and double-check our assumptions.

Reported-by: Randall S. Becker <[email protected]>
Signed-off-by: Jeff King <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: peff

t/lib-chunk/corrupt-chunk-file.pl

@@ -21,14 +21,38 @@ sub copy {
 	return $buf;
 }
 
+# Some platforms' perl builds don't support 64-bit integers, and hence do not
+# allow packing/unpacking quadwords with "Q". The chunk format uses 64-bit file
+# offsets to support files of any size, but in practice our test suite will
+# only use small files. So we can fake it by asking for two 32-bit values and
+# discarding the first (most significant) one, which is equivalent as long as
+# it's just zero.
+sub unpack_quad {
+	my $bytes = shift;
+	my ($n1, $n2) = unpack("NN", $bytes);
+	die "quad value exceeds 32 bits" if $n1;
+	return $n2;
+}
+sub pack_quad {
+	my $n = shift;
+	my $ret = pack("NN", 0, $n);
+	# double check that our original $n did not exceed the 32-bit limit.
+	# This is presumably impossible on a 32-bit system (which would have
+	# truncated much earlier), but would still alert us on a 64-bit build
+	# of a new test that would fail on a 32-bit build (though we'd
+	# presumably see the die() from unpack_quad() in such a case).
+	die "quad round-trip failed" if unpack_quad($ret) != $n;
+	return $ret;
+}
+
 # read until we find table-of-contents entry for chunk;
 # note that we cheat a bit by assuming 4-byte alignment and
 # that no ToC entry will accidentally look like a header.
 #
 # If we don't find the entry, copy() will hit EOF and exit
 # (which should cause the caller to fail the test).
 while (copy(4) ne $chunk) { }
-my $offset = unpack("Q>", copy(8));
+my $offset = unpack_quad(copy(8));
 
 # In clear mode, our length will change. So figure out
 # the length by comparing to the offset of the next chunk, and
@@ -38,11 +62,11 @@ sub copy {
 	my $id;
 	do {
 		$id = copy(4);
-		my $next = unpack("Q>", get(8));
+		my $next = unpack_quad(get(8));
 		if (!defined $len) {
 			$len = $next - $offset;
 		}
-		print pack("Q>", $next - $len + length($bytes));
+		print pack_quad($next - $len + length($bytes));
 	} while (unpack("N", $id));
 }