channelz: ensure SubChannel is always registered with parent

[daniel-weisse]

channelz.SubChannel may be missing parent information if created using channelz.RegisterSubChannel.
This is because channelz.RegisterSubChannel does not take a channelz.Channel as argument, but just the ID of this channel.
The function itself then tries to load the Channel from a package scoped channelMap.
This map does not contain any entries if channelz data collection is turned off, resulting in the SubChannel being created without a parent.
Which in turn results in the SubChannel's string representation being logged as <nil> SubChannel #1234 (1234 being some ID of the SubChannel).

This PR fixes this by restoring the behavior of v1.62.2, which instead of passing just the ID of a SubChannel's parent, passed a reference to the parent Channel, returning an error if it is nil:

grpc-go/internal/channelz/funcs.go

Lines 199 to 206 in 10baa6b

	func RegisterSubChannel(c Channel, pid *Identifier, ref string) (*Identifier, error) {
	if pid == nil {
	return nil, errors.New("a SubChannel's parent id cannot be nil")
	}
	id := IDGen.genID()
	if !IsOn() {
	return newIdentifer(RefSubChannel, id, pid), nil
	}

RELEASE NOTES: none

[codecov]

Codecov Report

Merging #7094 (3694b2e) into master (6fbcd8a) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 3694b2e differs from pull request most recent head 5f17e6e. Consider uploading reports for the commit 5f17e6e to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7094   +/-   ##
=======================================
  Coverage   81.07%   81.08%           
=======================================
  Files         345      345           
  Lines       33927    33928    +1     
=======================================
+ Hits        27508    27512    +4     
+ Misses       5241     5235    -6     
- Partials     1178     1181    +3     

Files	Coverage Δ
internal/channelz/subchannel.go	94.73% <100.00%> (+0.29%)	⬆️

... and 25 files with indirect coverage changes

[dfawley]

@daniel-weisse do you know if this is a regression with the latest release? If so we may prefer to restore the previous behavior instead.

[daniel-weisse]

The Channel and SubChannel types were introduced in v1.63.
v1.62 used the Identifier struct, which served the same purpose.

From a brief look at the v1.62.2 code, it looks like parent information was always omitted from its string representation if there was no parent:

grpc-go/internal/channelz/id.go

Lines 69 to 75 in 10baa6b

	func newIdentifer(typ RefChannelType, id int64, pid *Identifier) *Identifier {
	str := fmt.Sprintf("%s #%d", typ, id)
	if pid != nil {
	str = fmt.Sprintf("%s %s", pid, str)
	}
	return &Identifier{typ: typ, id: id, str: str, pid: pid}
	}

grpc-go/internal/channelz/id.go

Lines 48 to 50 in 10baa6b

	func (id *Identifier) String() string {
	return id.str
	}

Edit: Took another look at the code.
I think the actual regression is in how the SubChannel resource is created in v1.63.
In v1.62.2, RegisterSubChannel throws an error if no parent is provided. It then creates the (SubChannel) Identifier by passing the parent ID, even if channelz is not turned on:

grpc-go/internal/channelz/funcs.go

Lines 247 to 255 in 10baa6b

	func RegisterListenSocket(s Socket, pid *Identifier, ref string) (*Identifier, error) {
	if pid == nil {
	return nil, errors.New("a ListenSocket's parent id cannot be 0")
	}
	id := IDGen.genID()
	if !IsOn() {
	return newIdentifer(RefListenSocket, id, pid), nil
	}

In v1.63 however, the RegisterSubChannel performs no checks for the channels parent since the parent is no longer passed as an argument, but held in a packaged scoped channelMap.
When the SubChannel is created with channelz being turned off, no parent information is passed to the subchannel:

grpc-go/internal/channelz/funcs.go

Lines 146 to 150 in c68f456

	func RegisterSubChannel(pid int64, ref string) *SubChannel {
	id := IDGen.genID()
	if !IsOn() {
	return &SubChannel{ID: id}
	}

I think the correct way would be passing the parent information here, like it is done later on in the function.
This doesn't work however, since the parent Channel was never added to the channelMap db in the first place, because channelz is turned off.

One fix would be adjusting the signature of the RegisterSubChannel function to take the SubChannel's parent *Channel as an argument, instead of just its pid.

So we would go from this:

func RegisterSubChannel(pid int64, ref string) *SubChannel {

to this:

func RegisterSubChannel(parent *Channel, ref string) *SubChannel {

The function then no longer needs to load the parent from the channel map

[dfawley]

One fix would be adjusting the signature of the RegisterSubChannel function to take the SubChannel's parent *Channel as an argument, instead of just its pid.

This sounds good. Subchannels should always have parents, so I was skeptical about the initial change proposal -- we should ideally be logging both the channel+subchannel IDs in all the subchannel events.

[daniel-weisse]

Updated the PR to restore the RegisterSubChannel function to the behavior of v1.62.2.
This should ensure SubChannels are always created with a valid parent, so this information is never logged as nil

[dfawley]

Let's not return an error here since it complicates the code so much, and it should just be an invariant of the code like any other dereference. Just panic and we'll make sure RegisterSubChannel is always called properly.

Actually, this change is pretty important to get done quickly, so I created #7101 and we will be doing a patch release with it ASAP. Thank you for the fix!

[dfawley]

As mentioned in the review comment, we'll be continuing this in #7101. Sorry for that, but it needs to get done very quickly. Thank you!