lib/crypto/sha256: restore faithful re-entrant sum() (#265 unblocked)

The port shipped sum() single-shot — mutating the live hash state —
because Hare's state snapshot `let copy = *h; let h = ©` (a
deref-rhs aggregate let-init of an array-containing struct) miscompiled
in cgen. #265 fold-1 (master 4d3f846) landed the full-size aggregate
copy for that axis, so restore the faithful form: pad+finalize the
snapshot, leave the live state untouched, close() the copy.

sum() is now non-destructive — summing twice yields the same digest and
writing after a sum() continues the stream. Pinned by a new reentrant()
@test (sum-twice identical + write-after-sum continuity). NIST vectors
unchanged.

lib/crypto/sha256/sha256.ww

@@ -11,11 +11,7 @@
 // mismatch would mean cgen promoted a u32 op to 64-bit without wrapping
 // (a cgen bug to STOP+report, NOT to mask here).
 //
-// DIVERGENCES from sha256.ha:
-//   - sum() is SINGLE-SHOT, not re-entrant: Hare's state-snapshot copy
-//     hits an array-field deref-copy cgen bug (see [[sumfn]]). This is
-//     the one BEHAVIORAL divergence (blocked on a filed cgen fix); the
-//     rest below are semantics-preserving spellings.
+// DIVERGENCES from sha256.ha (all semantics-preserving spellings):
 //   - `state` embeds [[hash.hash]], whose first field is an inline
 //     io.vtable (not Hare's `stream: io::stream` pointer); see
 //     lib/hash/hash.ww. The vtable slots are wired post-construction in
@@ -168,21 +164,17 @@ fn writefn(s: io.stream, buf: []u8) (size | io.error) = {
 
 // sumfn — finalize and emit the digest. ref/hare/crypto/sha256/sha256.ha:115-143.
 //
-// DIVERGENCE (re-entrancy / blocked on a cgen bug). Hare snapshots the
-// state — `let copy = *h; let h = ©` — so sum() is non-destructive
-// and the caller can keep writing afterwards. That deref-copy of an
-// array-containing struct miscompiles in ww cgen (the copied `h`/`x`
-// arrays come back zeroed; minimal repro: `type t = struct { h: [4]u32 };
-// let c: t = *(&s);` reads c.h back wrong — array-field + pointer-deref
-// copy, a sibling of the #135/#252 array-field cgen family). Filed for
-// ken; flagged for a drew fidelity ruling. Until it lands, sum() runs on
-// the live state and is therefore SINGLE-SHOT: writing or summing again
-// after a sum() yields wrong results. Every current caller does exactly
-// one terminal sum(), so the NIST digests are unaffected (all vectors,
-// incl. the multi-block million-'a' stream, verify byte-identical). The
-// final close() still wipes the live state per Hare.
+// Re-entrant per Hare: snapshot the state (`let copy = *h; let h = ©`)
+// and pad+finalize the COPY, so the live hash is untouched and the caller
+// can keep writing or sum() again. The deref-copy of this array-containing
+// struct was blocked by a cgen bug (#265) — fold-1 (master 4d3f846) landed
+// the full-size aggregate copy for deref-rhs let-init, unblocking it. The
+// close() at the end wipes the copy (Hare's `defer hash::close(h)`), not
+// the live state.
 fn sumfn(h: *hash.hash, buf: []u8) void = {
-	let st: *state = h: *state;
+	let live: *state = h: *state;
+	let copy: state = *live;
+	let st: *state = (&copy);
 
 	let ln: size = st.ln;
 	let tmp: [64]u8 = [0...];
@@ -219,7 +211,7 @@ fn sumfn(h: *hash.hash, buf: []u8) void = {
 		i += 1;
 	};
 
-	hash.close(h);
+	hash.close((&copy): *hash.hash);
 };
 
 // block — process every whole 64-byte block in `buf`.

lib/crypto/sha256/sha256_test.ww

@@ -91,6 +91,34 @@ fn check(msg: []u8, want: str) void = {
 		"cdc76e5c9914fb9281a1c7e284d73e67f1809a48a497200e046d39ccc7112cd0");
 };
 
+// reentrant — sum() is non-destructive: it pads+finalizes a snapshot, so
+// the live hash survives. Summing twice yields the same digest, and a
+// write after a sum() continues the same stream. Pins the faithful
+// state-snapshot restored once #265 fold-1 unblocked the deref-rhs
+// aggregate copy `let copy = *h` (see sha256.ww [[sumfn]]).
+@test fn reentrant() void = {
+	let st: sha256.state = sha256.sha256();
+	let h: *hash.hash = (&st): *hash.hash;
+	hash.write(h, strings.toutf8("abc"));
+
+	let out1: [32]u8;
+	let out2: [32]u8;
+	hash.sum(h, out1[0:32]);
+	hash.sum(h, out2[0:32]);
+	if (!bytes.equal(out1[0:32], out2[0:32])) { fail(); };
+	checkbytes(out1[0:32],
+		"ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad");
+
+	// Write more after the sum(): the post-sum stream is uncorrupted, so
+	// the running digest of "abc"+"def" matches a one-shot hash of "abcdef".
+	hash.write(h, strings.toutf8("def"));
+	let out3: [32]u8;
+	hash.sum(h, out3[0:32]);
+	let want: [32]u8;
+	dohash(strings.toutf8("abcdef"), want[0:32]);
+	if (!bytes.equal(out3[0:32], want[0:32])) { fail(); };
+};
+
 // sz()/bsz() report the SHA-256 constants regardless of state.
 @test fn sizes() void = {
 	let st: sha256.state = sha256.sha256();
@@ -104,6 +132,7 @@ export fn main() i32 = {
 	signalled = 2; abc();
 	signalled = 3; twoblockpad();
 	signalled = 4; millionas();
-	signalled = 5; sizes();
+	signalled = 5; reentrant();
+	signalled = 6; sizes();
 	return 0;
 };