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Remove generic std::optional<T> suppport from worker proto

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See comment for rational; I think it's good to leave a comment lest
anyone is tempted to add such a sum-type instance again.

Fixes #4113
This commit is contained in:
John Ericson 2020-10-07 12:48:35 +00:00
parent d761485010
commit 57d960dcd1
1 changed files with 16 additions and 32 deletions
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48
src/libstore/worker-protocol.hh
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@ -83,7 +83,6 @@ MAKE_WORKER_PROTO(, StorePath);
MAKE_WORKER_PROTO(, ContentAddress);
MAKE_WORKER_PROTO(template<typename T>, std::set<T>);
MAKE_WORKER_PROTO(template<typename T>, std::optional<T>);
#define X_ template<typename K, typename V>
#define Y_ std::map<K, V>
@ -91,6 +90,22 @@ MAKE_WORKER_PROTO(X_, Y_);
#undef X_
#undef Y_
/* These use the empty string for the null case, relying on the fact
that the underlying types never serialize to the empty string.
We do this instead of a generic std::optional<T> instance because
ordinal tags (0 or 1, here) are a bit of a compatability hazard. For
the same reason, we don't have a std::variant<T..> instances (ordinal
tags 0...n).
We could the generic instances and then these as specializations for
compatability, but that's proven a bit finnicky, and also makes the
worker protocol harder to implement in other languages where such
specializations may not be allowed.
*/
MAKE_WORKER_PROTO(, std::optional<StorePath>);
MAKE_WORKER_PROTO(, std::optional<ContentAddress>);
template<typename T>
std::set<T> read(const Store & store, Source & from, Phantom<std::set<T>> _)
{
@ -134,37 +149,6 @@ void write(const Store & store, Sink & out, const std::map<K, V> & resMap)
}
}
template<typename T>
std::optional<T> read(const Store & store, Source & from, Phantom<std::optional<T>> _)
{
auto tag = readNum<uint8_t>(from);
switch (tag) {
case 0:
return std::nullopt;
case 1:
return read(store, from, Phantom<T> {});
default:
throw Error("got an invalid tag bit for std::optional: %#04x", (size_t)tag);
}
}
template<typename T>
void write(const Store & store, Sink & out, const std::optional<T> & optVal)
{
out << (uint64_t) (optVal ? 1 : 0);
if (optVal)
worker_proto::write(store, out, *optVal);
}
/* Specialization which uses and empty string for the empty case, taking
advantage of the fact these types always serialize to non-empty strings.
This is done primarily for backwards compatability, so that T <=
std::optional<T>, where <= is the compatability partial order, T is one of
the types below.
*/
MAKE_WORKER_PROTO(, std::optional<StorePath>);
MAKE_WORKER_PROTO(, std::optional<ContentAddress>);
}
}