OpenZFS Bug Ported to Zig

Someone on IRC shared this link with me: An (almost) catastrophic OpenZFS bug and the humans that made it (and Rust is here too) They wanted to know, would Zig catch this? I went through the trouble of porting the code snippet to Zig, so I thought I'd go the next step and turn it into a small blog post. Original snippet, in C: /* * This code converts an asize into the largest psize that can safely be written * to an allocation of that size for this vdev. * * Note that this function will not take into account the effect of gang * headers, which also modify the ASIZE of the DVAs. It is purely a reverse of * the psize_to_asize function. */ static uint64_t vdev_raidz_asize_to_psize(vdev_t *vd, uint64_t asize, uint64_t txg) { vdev_raidz_t *vdrz = vd->vdev_tsd; uint64_t psize; uint64_t ashift = vd->vdev_top->vdev_ashift; uint64_t cols = vdrz->vd_original_width; uint64_t nparity = vdrz->vd_nparity; cols = vdev_raidz_get_logical_width(vdrz, txg); ASSERT0(asize % (1 << ashift)); psize = (asize >> ashift); psize -= nparity * DIV_ROUND_UP(psize, cols); psize <<= ashift; return (asize); } The blog post author encourages us to try to spot the fail before the answer is revealed. I couldn't do it before getting bored, so I ported the code to Zig instead: const std = @import("std"); const divCeil = std.math.divCeil; const assert = std.debug.assert; const vdev_t = @import("the_rest_of_the_software.zig").vdev_t; /// This code converts an asize into the largest psize that can safely be written /// to an allocation of that size for this vdev. /// /// Note that this function will not take into account the effect of gang /// headers, which also modify the ASIZE of the DVAs. It is purely a reverse of /// the psize_to_asize function. fn vdev_raidz_asize_to_psize(vd: *vdev_t, asize: u64, txg: u64) u64 { const vdrz = vd.vdev_tsd; const ashift = vd.vdev_top.vdev_ashift; const cols = vdrz.vd_original_width; const nparity = vdrz.vd_nparity; const cols = vdrz.get_logical_width(txg); assert(asize % (1 << ashift)); const asize_shifted = (asize >> ashift); const parity_adjusted = asize_shifted - nparity * (divCeil(asize_shifted, cols) catch unreachable); const psize = parity_adjusted << ashift; return asize; } Let's start with the code formatting tool alone, zig fmt: andy@bark ~/tmp> zig fmt bug.zig --ast-check bug.zig:18:11: error: redeclaration of local constant 'cols' const cols = vdrz.get_logical_width(txg); ^~~~ bug.zig:15:11: note: previous declaration here const cols = vdrz.vd_original_width; ^~~~ Hmm, alright. That's definitely strange. The original blog post didn't notice that the initialization value of cols is dead code. That doesn't look like a logic error though. Let's fix it and try again: @@ -12,7 +12,6 @@ fn vdev_raidz_asize_to_psize(vd: *vdev_t, asize: u64, txg: u64) u64 { const vdrz = vd.vdev_tsd; const ashift = vd.vdev_top.vdev_ashift; - const cols = vdrz.vd_original_width; const nparity = vdrz.vd_nparity; const cols = vdrz.get_logical_width(txg); andy@bark ~/tmp> zig fmt bug.zig --ast-check bug.zig:23:11: error: unused local constant const psize = parity_adjusted << ashift; ^~~~~ Bingo. We didn't even have to use the type checker. Thanks for reading my blog post.