fix(3p/nix/hash): initialize HashSink.ctx

Fixup for CL 1492 (addcba11b0)

Additionally, add a test to verify functionality of HashSink.

Change-Id: I2a74b925a1b93ed4d3add29021d759c93e813424
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1507
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>

third_party/nix/src/libutil/hash.cc

@@ -361,7 +361,10 @@ Hash hashFile(HashType ht, const Path& path) {
   return hash;
 }
 
-HashSink::HashSink(HashType ht) : ht(ht), ctx(), bytes(0) { start(ht, *ctx); }
+HashSink::HashSink(HashType ht)
+    : ht(ht), ctx(std::make_unique<hash::Ctx>()), bytes(0) {
+  start(ht, *ctx);
+}
 
 HashSink::~HashSink() { bufPos = 0; }
 

third_party/nix/src/tests/hash_test.cc

@@ -80,4 +80,22 @@ TEST(HashTest, SHA256DecodeFail) {
       HasSubstr("invalid base-16"));
 }
 
+TEST(HashSink, SHA256) {
+  HashSink sink(htSHA256);
+
+  sink.write(reinterpret_cast<const unsigned char*>("fo"), 2);
+  HashResult partial = sink.currentHash();
+  EXPECT_EQ(partial.first.to_string(Base16),
+            "sha256:"
+            "9c3aee7110b787f0fb5f81633a36392bd277ea945d44c874a9a23601aefe20cf");
+  EXPECT_EQ(partial.second, 2);
+
+  sink.write(reinterpret_cast<const unsigned char*>("o"), 1);
+  HashResult end = sink.finish();
+  EXPECT_EQ(end.first.to_string(Base16),
+            "sha256:"
+            "2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae");
+  EXPECT_EQ(end.second, 3);
+}
+
 }  // namespace nix