Bug Fix: Handle multi-part lists properly in ReadPostingList

[martinmr]

This PR changes ReadPostingList to skip over the parts of a multi-part list so that they are not accessed the next time ReadPostingList is called. A multi-part posting list should only be accessed via the main key. Accessing the posting list via one of the other keys was causing issues during rollup and adding spurious keys to the database, which eventually caused reads to return nil in the Jepsen uid-set test.

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This change is 

[manishrjain]

Reviewable status: 0 of 2 files reviewed, 2 unresolved discussions (waiting on @manishrjain and @martinmr)

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posting/list.go, line 491 at r1 (raw file):

func (l *List) Iterate(readTs uint64, afterUid uint64, f func(obj *pb.Posting) error) error {
	if l == nil {
		return nil

I'd create an error called errorNilList, and return it for all the funcs.

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posting/mvcc.go, line 151 at r2 (raw file):

		// Trying to read a single part of a multi part list. This type of list
		// should be read once using the canonical list (with startUid equal to zero).
		return nil, nil

No nil error here. Should have a pre-defined error.

[martinmr]

Reviewable status: 0 of 2 files reviewed, 2 unresolved discussions (waiting on @manishrjain)

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posting/list.go, line 491 at r1 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

I'd create an error called errorNilList, and return it for all the funcs.

Removed the special nil handling in favor of not returning nil lists in ReadPostingList

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posting/mvcc.go, line 151 at r2 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

No nil error here. Should have a pre-defined error.

Done.

[parasssh]

Reviewable status: 0 of 2 files reviewed, 3 unresolved discussions (waiting on @manishrjain and @martinmr)

a discussion (no related file):

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[manishrjain]

Hope you ran Jepsen!

Reviewed 2 of 2 files at r3.
Reviewable status: all files reviewed, 4 unresolved discussions (waiting on @manishrjain and @martinmr)

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posting/list.go, line 1136 at r3 (raw file):

func (l *List) Facets(readTs uint64, param *pb.FacetParams, langs []string,
	listType bool) ([]*pb.Facets, error) {
	l.RLock()

revert all changes here.

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posting/mvcc.go, line 150 at r3 (raw file):

	if err != nil {
		return nil, errors.Wrapf(err, "while reading posting list with key [%v]", key)
	}

Comment here that we used to have a bug here, as caught by . This PR fixes that.

[martinmr]

Reviewable status: 0 of 2 files reviewed, 4 unresolved discussions (waiting on @manishrjain and @martinmr)

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posting/list.go, line 1136 at r3 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

revert all changes here.

Done.

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posting/mvcc.go, line 150 at r3 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

Comment here that we used to have a bug here, as caught by . This PR fixes that.

Done.