Use the fs accessor for readInvalidDerivation

[thufschmitt]

Extend FSAccessor::readFile to allow not checking that the path is a
valid one, and rewrite readInvalidDerivation using this extended
readFile.

Several places in the code use readInvalidDerivation, either because
they need to read a derivation that has been written in the store but
not registered yet, or more generally to prevent a deadlock because
readDerivation tries to lock the state, so can't be called from a
place where the lock is already held.
However, readInvalidDerivation implicitely assumes that the store is a
LocalFSStore, which isn't always the case.

The concrete motivation for this is that it's required for nix copy --from someBinaryCache to work, which is tremendously useful for the
tests.

[Ericson2314]

I think the overrides shouldn't have the default args? Elsewhere in the codebase they at least are only part of the declarations, IIRC.

[thufschmitt]

That's a good question. Triggered me to read https://stackoverflow.com/questions/6464404/virtual-function-default-arguments-behaviour which is… interesting.

The gist of it seems to that if we only call accessor.readFile(path) where accessor is declared of type FSAccessor (which is probably the case) then we don't need the default arguments for the overrides. But that looks quite fragile. Maybe I could just split the interface as:

virtual bool isValidPath(const Path& path);
virtual std::string readFileUnchecked(const Path& path);
std::string readFile(const Path& path) {
  if (!isValidPath(path)) throw InvalidPath(...);
  return readFileUnchecked(path);
}

[Ericson2314]

Hmm I read it the other way around, where up casting certainly gives you the outer default arg, but no upcasting would also give you the outer default arg if there is no inner default arg.

[thufschmitt]

Nah, just checked:

struct Foo {
  virtual void foo(int n = 1) { }
};

struct Bar : Foo {
  void foo(int n) override { }
};

int main() {
  Foo x;
  x.foo();
  Bar y;
  y.foo();
}

x.foo() typechecks fine, but y.foo() doesn't

[Ericson2314]

Sorry to have misled you with that wrong interpretation then.

[Ericson2314]

Great idea!