Editorial: InitializeHostDefinedRealm-related refactorings

[jmdyck]

In 9.6 InitializeHostDefinedRealm, there are two If-steps that allow the host to customize the global object and global *this* binding. If the host just wants the default (of either), that's handled by passing *undefined* to (the relevant parameter of) SetRealmGlobalObject, which takes care of setting the realm's [[GlobalObject]] and [[GlobalEnv]] appropriately.

This division of labor always struck me as a bit odd. E.g., why set _global_ to *undefined* and then have prose to explain the significance of doing so? Why not just set _global_ appropriately right there? That was the impetus for this PR. (My guess was that SetRealmGlobalObject was used by the HTML spec, but it isn't, and as far as I can tell it never was. Perhaps that was briefly someone's plan.)

This PR is split into 6 commits for ease of review. If approved, I'll do some squashing.

• I start by moving InitializeHostDefinedRealm to a better location.

  It's odd for InitializeHostDefinedRealm (clearly a realm-centric operation) to be out on its own in 9.6, when there's a "Realms" section at 9.3.

  (When Realms were introduced in ES6, the relative location of InitializeHostDefinedRealm was somewhat different and made more sense, but things have changed since then.)

• Line-break the two "If the host requires" steps for readability.

• Inline SetRealmGlobalObject into InitializeHostDefinedRealm.

  (Originally, I only hoisted out the SetRealmGlobalObject steps that assigned 'meanings' to the *undefined* parameters, but that left so little in SetRealmGlobalObject that I didn't see any reason for it to continue to exist.)

• Reorganize/simplify the result of the previous commit.

• Tweak the return of SetDefaultGlobalBindings.

  In SetDefaultGlobalBindings, _global_ is an alias for _realmRec_.[[GlobalObject]]. In InitializeHostDefinedRealm, this is _realm_.[[GlobalObject]], which is just _global_. But SetDefaultGlobalBindings returns that object, which InitializeHostDefinedRealm then assigns to _globalObj_, meaning that we now have two aliases for the same object, which is unnecessarily confusing.

  So this commit tweaks SetDefaultGlobalBindings to return ~unused~, and tweaks InitializeHostDefinedRealm to refer to _global_ rather than introducing _globalObj_.

• Inline CreateRealm into InitializeHostDefinedRealm.

  This wasn't part of my original plan, but I like it because it surfaces both the early and later settings of _realm_.[[GlobalObject]] and _realm_.[[GlobalEnv]]. This makes it really obvious that the early settings are transient. (See Editorial: More info in 'PromiseCapability Record' table #2380 (comment))

Checking the downstream specs, nobody references SetRealmGlobalObject, CreateRealm, or SetDefaultGlobalBindings.

[michaelficarra]

LGTM, though I think @bakkot may prefer to omit the second commit. In the past, he has been explicitly in support of these single-step definitions.

[bakkot]

Have I? I don't have any recollection of that.

[michaelficarra]

@bakkot Yes, in an editor call discussing #3043 you wanted to make sure we did not outright ban this form.

[bakkot]

I support being able to use the form; that doesn't mean I'm in support of every particular instance. I'm neutral in this case.

[michaelficarra]

I support being able to use the form

That's the thing I was trying to say.

[linusg]

+1 for these changes. I wonder if everything from If the host requires to Create any host-defined global object properties should be moved into a single host hook with the default implementation using these steps verbatim and the HTML spec doing its own thing - I'm happy to look into changing that in light of the discussion in #3274.

[jmdyck]

I wonder if [steps] should be moved into a single host hook ...

I have thoughts, but maybe discussion of the proposed host hook would be better in #3274?

[linusg]

Yes, I don't mean to block this PR - IMO these changes are good as is while preserving the status quo of how the global object customizations are spec'd.

[linusg]

Checking the downstream specs, nobody references SetRealmGlobalObject, CreateRealm, or SetDefaultGlobalBindings.

It's worth noting that https://tc39.es/proposal-shadowrealm/#sec-shadowrealm will need an update.