Commit Graph

632 Commits

Author SHA1 Message Date
Dandelion Mané 6235febdac
Add porcelain-style classes to `RelationalView` (#424)
Based on offline design discussion with @wchargin, we've decided to
upgrade the `RelationalView` to be *the* comprehensive source for GitHub
data inside SourceCred. The `RelationalView` will contain the full
dataset, including parsed relational information (such as
cross-references between GitHub entities). Then, we will project our
GitHub graph out of the `RelationalView`.

To that end, the `RelationalView` no longer exports raw data blobs.
Instead, it exports nice classes: `Repo`, `Issue`, `Pull`, `Review`,
and `Userlike`. These classes have convenient methods for accessing both
their own data and related entities, e.g. `repo.issues()` yields all
the issues in that repo.

This is effectively a port of #170 into the v3 API. The main difference
is that in v1, the Graph contained this data store, whereas in v3, we
will use this data store to generate the graph.

This supersedes #418.

Test plan:
The snapshot tests are quite readable.
2018-06-27 15:25:20 -07:00
William Chargin e7b28b81db
Convert V3 graphs to Markov chains (#427)
Summary:
This is based on the V1 file `basicPagerank.js`. The API is necessarily
changed for the new graph format, and we export additional utilities
compared to the previous version of the module (useful for testing and
serialization). We also improve the implementation to make it simpler
and easier to understand.

Test Plan:
Unit tests included.

wchargin-branch: v3-graph-markov-chain
2018-06-27 15:24:47 -07:00
William Chargin b9c67f447f
Expose `advancedGraph` test case (#426)
Summary:
We’d like to use this test case to generate a Markov chain, which
requires that it not be local to the `graph.js` tests.

Test Plan:
Existing unit tests suffice.

wchargin-branch: expose-advanced-graph
2018-06-27 15:18:47 -07:00
William Chargin faa2f8c9d0
Copy Markov chain code from V1 to V3 (#425)
Summary:
This code is independent of the graph abstraction, and so is mostly
copied. The only change is to the structure of the test code (we now
prefer to wrap everything in a big `describe` block with an absolute
path to the module under test).

Test Plan:
Unit tests included.

wchargin-branch: v3-markov-chain
2018-06-27 15:14:42 -07:00
Dandelion Mané 659fc51d9b
Rename `findReferences` to `parseReferences` (#429)
This code is about parsing references out of text, so `parseReferences`
is a better name.

The code that consumes this logic to find all the references in the
GitHub data shall be rightly called `findReferences`

Test plan:
`yarn travis`
2018-06-27 13:21:25 -07:00
William Chargin 518d5b819c
Represent submodule commits as normal commits (#423)
Summary:
Closes #417. Submodule commits are dead; long live commits. The ontology
is now:

  - A tree includes tree entries.
  - A tree entry may have a blob as contents.
  - A tree entry may have a tree as contents.
  - A tree entry may have a commit as contents.

Test Plan:
Existing unit tests suffice, especially `#commits yields all commits`.

wchargin-branch: git-remove-submodule-commits
2018-06-27 12:01:07 -07:00
William Chargin 38c364c916
Allow Git commits to have zero or one tree (#422)
Summary:
Submodule commits need not have associated tree objects, in case the
repository to which they belong does not exist in our graph. We’d like
to represent submodule commits as actual commits, which necessitates
this change. See #417 for context.

Test Plan:
Existing unit tests suffice.

wchargin-branch: git-affine-trees
2018-06-27 11:47:39 -07:00
William Chargin dd83d7b4ab
Implement a Git graph view (#415)
Summary:
Similar in structure to the GitHub graph view.

Test Plan:
Unit tests added, with full coverage.

wchargin-branch: git-graph-view
2018-06-26 14:00:19 -07:00
William Chargin 0522894a8d
Create Git graph (#406)
Summary:
This commit adds logic to create the Git graph, modeled after the GitHub
graph creator in #405. In this commit, we do not include the
corresponding porcelain; a Git `GraphView` will be added subsequently.

Kudos to @decentralion for suggesting in #187 that I write the logic to
detect BECOMES edges against the high-level data structures. Due to that
decision, the logic and tests are copied directly from the V1 code
without change, because the high-level data structures are the same. The
new code is exactly the body of the `GraphCreator` class.

Test Plan:
Verify that the new snapshot is likely equivalent to the V1 snapshot,
using the heuristic that the two graphs have the same numbers of nodes
(59) and edges (84). (I have performed this check.)

wchargin-branch: git-v3-create-graph
2018-06-26 13:54:47 -07:00
Dandelion Mané a470f28204
Add GitHub `RelationalView` (#411)
The `RelationalView` maps the GitHub GraphQL response data into a View
class, which makes it easy to access pieces of GitHub data by their
corresponding `StructuredAddress`.

This will be a valuable companion to the graph, making it possible to
access GitHub node data like the title or body of an issue via the
issue's address. This basically is the supplement to the GitHub graph
that includes the "payloads" from our v1 Graph.

It will also make creating the GitHub graph a lot more convenient,
although I've left that for another commit.

Designed with feedback from @wchargin.

Note: The `RelationalView` objects have a `nominalAuthor` rather than
`author`, so as to distinguish between authorship in the GitHub data
model (entities have at most one author) and in the SourceCred model
(entities may have multiple authors).

Test plan:
Inspect the included snapshots for reasonability, and run unit tests.
2018-06-22 20:32:07 -07:00
Dandelion Mané 2dec8868db
Copy `github/findReferences` from v1 to v3 (#410)
The code will be refactored so that references are expressed in terms of
the GitHub node address code; the implementation is copied first so that
the review will be cleaner.

Test plan:
`yarn travis` passes.
2018-06-22 16:16:48 -07:00
William Chargin 127200f67c
Cache core graph `checkInvariants` result (#408)
Summary:
The public method `checkInvariants` on graph is now cached. The cache is
invalidated when the graph is modified via the public API. As a result
of this change, the time of `yarn ci-test --testPathPattern src/v3/`
decreases from 5.631s to 3.866s (best-of-three timing, but low variance
anyway). This effect becomes much more pronounced as higher-level APIs
check their own invariants by themselves indirectly invoking the graph’s
`checkInvariants` method many times.

Test Plan:
Existing unit tests have been adapted and extended. Tests for the
invariant checking have been updated to call the internal, uncached
method, and new tests have been added to check that the caching behavior
is correct.

wchargin-branch: cache-graph-invariants
2018-06-22 16:16:43 -07:00
Dandelion Mané a209caeec2
create the GitHub graph (#405)
This commit:
- adds `github/createGraph.js`
  - which ingests GitHub GraphQL response
  - and creates a GitHub graph
- adds `github/graphView.js`,
  - which takes a Graph
  - and validates that all GitHub specific node and edge invariants hold
    - every github node may be parsed by `github/node/fromRaw`
      - with the right node type
    - every github edge may be parsed by `github/edge/fromRaw`
      - with the right edge type
      - with the right src address prefix
      - with the right dst address prefix
    - every child node has exactly one parent
      - of the right type
  - and provides convenient porcelain methods for
    - finding repos in the graph
    - finding issues of a repo
    - finding pulls of a repo
    - finding reviews of a pull
    - finding comments of a Commentable
    - finding authors of Authorables
    - finding parent of a ChildAddress
- tests `createGraph`
  - via snapshot testing
  - by checking the GraphView invariants hold
- tests `graphView`
  - by checking individual entities in the example-git repository have
  the proper relationships
  - by checking that for every class of invariant, errors are thrown if
  the invariant is violated

Test plan:
- Extensive unit and snapshot tests added. `yarn travis` passes.
2018-06-22 13:10:19 -07:00
Dandelion Mané 24a7547e16
Use Git Commit type in GitHub mergedAs edge (#407)
Test plan:
`yarn travis` passes
2018-06-22 12:12:08 -07:00
William Chargin 448fb3e1a8
Add edge type definitions for V3 Git plugin (#404)
Summary:
This is modeled after the GitHub edge module format. In particular, the
whole length encoding garbage is directly copied. As in that module, we
decline to test the error paths.

Test Plan:
Unit tests added; run `yarn travis`. Snapshots are readable.

wchargin-branch: git-v3-edges
2018-06-20 15:49:50 -07:00
William Chargin 7c1b3ca835
Add node type definitions for V3 Git plugin (#403)
Summary:
This is modeled after the GitHub node module format, with the obvious
alterations plus a bit more type safety in the implementation of `toRaw`
(namely, we check `type` exhaustively).

Test Plan:
Unit tests added; run `yarn travis`.

wchargin-branch: git-v3-nodes
2018-06-20 15:40:08 -07:00
William Chargin 83151d9fac
Remove payload definitions from V3 `git/types.js` (#402)
Test Plan:
Existing Flow and unit tests suffice.

wchargin-branch: git-v3-remove-payloads
2018-06-20 15:32:12 -07:00
William Chargin 9347348dd7
Copy graph-independent V1 Git plugin code to V3 (#401)
Summary:
Many files are unchanged. Some files have had paths updated, or new
build/test targets added.

The `types.js` file includes payload type definitions. These are
technically independent of the graph abstraction (i.e., nothing from V1
is imported and the code all still works), but it of course implicitly
depends on the V1 model. For now, we include the entirety of this file,
just so that we have a clean copy operation. Subsequent commits will
strip out this extraneous code.

Suggest reviewing with the `--find-copies-harder` argument to Git’s
diffing functions.

Test Plan:
Running `yarn travis --full` passes. Running

    ./src/v3/plugins/git/demoData/synchronizeToGithub.sh --dry-run

yields “Everything up-to-date”.

wchargin-branch: git-v3-copy
2018-06-20 15:28:37 -07:00
William Chargin 281cb574d5
Greatly simplify GitHub edge tests (#400)
Summary:
We had `edgeExamples`, wherein we constructed examples of edge addresses
(not actual edges) by manual instantiation. We checked that these
matched snapshots, and then we also called `createEdge` a bunch to
create actual edges, and checked that those matched snapshots, too.
Consequently, we had twice as many snapshots as we needed, and also
defined twice as many edge addresses as we needed.

Test Plan:
Note that snapshot contents are either deleted or unchanged.

wchargin-branch: simplify-github-edge-tests
2018-06-19 16:01:26 -07:00
William Chargin b9c01d13c9
Use `edgeToParts` in the GitHub edge tests (#399)
Test Plan:
Observe that the new snapshots are easier to read. Might as well make
sure that they encode the same data as the old snapshots, too. Note that
the backslash character no longer appears in this snapshot file. :-)

wchargin-branch: use-edgeToParts
2018-06-19 15:55:10 -07:00
William Chargin aac2fc6792
Add `edgeToParts` convenience export from `Graph` (#398)
Summary:
We have `edgeToString`, which formats edges as nicely human-readable
strings. However, these strings have some quotes in them, and so when
they are themselves stringified (e.g., as part of a Jest snapshot), they
become much harder to read. We thus introduce `edgeToParts` to make our
snapshots more readable.

Test Plan:
Unit tests added; run `yarn travis`.

wchargin-branch: add-edgeToParts
2018-06-19 15:48:04 -07:00
William Chargin ea74955a66
Fix GitHub node `fromRaw` error-path test cases (#397)
Summary:
In #394, we uppercased the constants for GitHub node types. However, we
were using string literals instead of constants in the test cases. These
test cases were supposed to cover every error path, but instead ended up
just covering the “bad type” error path many times.

Any one of the following would have prevented this regression:

 1. using string constants instead of literals in the test case;
 2. throwing and checking more precise error messages; or
 3. being alerted that coverage decreased as a result of the change.

In this commit, we enact the first of these options. I’m open to adding
a coverage bot, but don’t feel strongly about it at this time.

Test Plan:
Running `yarn coverage` now shows 100% coverage for the `nodes.js`
module, whereas previously almost all `throw fail();` lines were
uncovered (and the branch coverage was just 76%).

wchargin-branch: fix-github-node-error-tests
2018-06-19 14:57:14 -07:00
Dandelion Mané ed3397f654
Add GitHub prefixes and const types (#395)
- Switch string constant node and edge types (e.g. "REPO") to exported
consts (eg `export const REPO_TYPE`).
- Add (and internally use) a `_Prefix` psuedomodule which contains
per-type address prefixes
- Test that constructing a StructuredAddress with the wrong type is an
error.

Test plan:
Unit tests pass, snapshots unchanged.

Paired with @wchargin
2018-06-14 15:01:33 -07:00
Dandelion Mané a8bf6a36bf
Add `CommentableAddress` (#396)
Test plan: Not needed.

Paired with @wchargin
2018-06-14 14:52:29 -07:00
William Chargin b6eebddeb0
Use uppercase enum constants in GitHub addresses (#394)
Summary:
@decentralion wants this! :-)

Test Plan:
Verify that the case-insensitive diff is empty:
```
$ git config --global difftool.idiff.cmd 'diff -ui "$LOCAL" "$REMOTE"'
$ git difftool -y --tool idiff HEAD~1..HEAD
```

wchargin-branch: uppercase-enum
2018-06-14 13:45:55 -07:00
William Chargin 7ce4a0c32d
Use `NodeAddress.empty` and `EdgeAddress.empty` (#393)
Summary:
This fixes up all instances of `fromParts([])` that are not in
`address.js` or `address.test.js`.

Paired with @decentralion.

Test Plan:
Running `git grep --name-only -F 'fromParts([])'` yields only the two
modules listed above. Existing unit tests suffice for correctness.

wchargin-branch: use-address-empty
2018-06-14 13:11:08 -07:00
William Chargin 6ba6d885ad
Add `empty` (monoid identity) to address modules (#392)
Summary:
This can make invocations of `FooAddress.fromParts([])` a bit more
succinct.

Paired with @decentralion.

Test Plan:
Unit tests added. Run `yarn travis`.

wchargin-branch: address-empty
2018-06-14 13:06:26 -07:00
Dandelion Mané 7199586262
Add `Graph.edges` filtering by prefixing (#391)
Similar to #390, we now allow filtering the results from `Graph.edges`
by address prefixes. It's a little more complicated than #390, as we
allow filtering by src, dst, or address.

Test plan:
Unit tests added. `yarn travis` passes.

Paired with @wchargin
2018-06-14 11:59:58 -07:00
Dandelion Mané 1a08a48c03
Implement prefix filtering for `Graph.nodes` (#390)
Simple API addition to match v1/v2 semantics.
In the future, we can perf optimize this if we switch graph to
store nodes organized by shared prefixes.

Test plan:
Unit tests were added. `yarn travis` passes.

Paired with @wchargin
2018-06-14 11:18:47 -07:00
Dandelion Mané 95c5af36d9
Add methods for parsing Ids from GitHub urls (#388)
Test plan:
Unit tests added. Run `yarn travis`.

The GitHub regex code is inspired by work in #98
2018-06-13 16:25:15 -07:00
William Chargin 2491fcd3cb
Add GitHub `edges` module (#385)
Summary:
This module includes a raw edge type, a structured edge type, and edge
creation functions that take source and destination and create an edge.

Test Plan:
Unit tests added. These cover all of the successful cases, and none of
the unsuccessful cases. We plan to refactor this code Soon™, and it is
hard to see how to nicely factor the tests without just testing the same
code paths over and over.

wchargin-branch: github-edges
2018-06-13 16:19:50 -07:00
Dandelion Mané 17b390afe9
Add trait-specific GitHub address types (#387)
Will be useful in graph creation logic, and in #385

Test plan: Only change is to add types. No testing needed.

See design discussion [on discord].

Paired with @wchargin

[on discord]: https://discordapp.com/channels/453243919774253079/454007907663740939?jump=456564101183832064
2018-06-13 14:42:37 -07:00
William Chargin 25f74b89e9
Export `_githubAddress` from GitHub `nodes` (#384)
Summary:
We’ll want to use this in the upcoming `edges` module.

Test Plan:
Existing unit tests suffice.

wchargin-branch: expose-githubaddress
2018-06-13 13:53:09 -07:00
Dandelion Mané ad9ac55bef
Github Addresses: Rename `fragment` to `id` (#386)
Test plan:
`yarn travis`
2018-06-13 13:41:52 -07:00
William Chargin 748f9210a6
Rename various aspects of GitHub `nodes` module (#383)
Summary:
First, we rename the module itself from `address` to `nodes`: we’d like
to put the edge functions in a parallel `edges` module instead of
cramping it into this one, so it stands to reason that this one should
be called `nodes`.

We also rename the `GithubAddressT` type to `RawAddress`, so that the
module exports `RawAddress` and `StructuredAddress`. The functions then
have much better natural names of `toRaw` and `fromRaw`.

Test Plan:
Existing unit tests suffice.

wchargin-branch: rename-nodes
2018-06-12 18:09:41 -07:00
Dandelion Mané e4d9ce1565
gh plugin: use consistent concise naming for pulls (#381)
One of the slight modifications we've made in v3 is to effect the
following renames (as implemented in #380):

PullRequest -> Pull
PullRequestReview -> Review
PullRequestReviewComment -> ReviewComment

This commit just changes the rest of the github code in v3 to follow the
new convention.

Test plan:
`yarn travis --full` passes.
2018-06-12 14:15:55 -07:00
William Chargin a3f2b82073
Add snapshot test for GitHub GraphQL query (#382)
Summary:
This has two primary benefits:
  - Humans can look at this snapshot file to see what’s being queried,
    or to manually issue a query.
  - When we change the programmatically generated query, we can easily
    see what the results are in the GraphQL output. This makes it easy
    to verify that a change is correct.

Test Plan:
None.

wchargin-branch: snapshot-query
2018-06-12 14:09:58 -07:00
Dandelion Mané 773596755a
Add an address module for the GitHub plugin (#380)
Summary:
This module exposes a structured type `StructuredAddress`, an embedding
`GithubAddressT` of this type into the `NodeAddress` layer, and
functions to convert between the two.

Paired with @wchargin.

Test Plan:
Unit tests added, with full coverage. Snapshots are easily readable.
2018-06-12 11:05:09 -07:00
Dandelion Mané 0339d9f41b
Port GitHub data ingestion into v3 (#378)
This commit copies the following logic necessary for downloading GitHub
data into v3. Minimal changes have been made to accomodate the new path
structure.

Test plan:
- Manually ran plugins/github/fetchGithubRepoTest.sh and verified that
it can correctly pass and fail
- Added the v3 github repo test to `yarn travis --full`
- Ran `yarn travis --full` and it passed

Paired with @wchargin
2018-06-11 18:57:37 -07:00
William Chargin ed70947c63
Update GitHub example repository data (#379)
Summary:
We’ve added a comment directly on a pull request.

Paired with @decentralion.

Test Plan:
`yarn travis --full` passes.

wchargin-branch: update-example-github
2018-06-11 18:53:57 -07:00
William Chargin 5d3cfd82e4
Check graph invariants during tests (#372)
Summary:
Each of the invariants listed at the top of the `Graph` class is now
explicitly checked by `checkInvariants`, which is called at the end of
each `Graph` method during tests only. This is powerful: it means that
not only do our tests for `Graph` test the graph, but also any tests
that depend on `Graph`—e.g., plugin code—will give us extra invariant
testing on `Graph`. As noted in a comment, if this becomes bad for
performance, we can blacklist expensive tests or whitelist tests that we
care about.

A graph method may assume that the graph invariants hold before the
method is invoked. Within the body of a graph method, invariants may be
violated, but the method must ensure that the invariants hold
immediately before it returns or yields. A consequence of this is that
if a graph function internally calls a public function (e.g., `addEdge`
might call `hasNode` to check that the source and destination exist),
then it must ensure that the invariants hold before the internal call.
This is not an “implementation detail” or “caveat”; it is simply part of
the interface of public functions. It is legal and reasonable for
private helper functions to explicitly not expect or not guarantee that
particular invariants hold, and in this case the exception should be
documented. (This is not yet the case in any of our code.)

Finally, note that the `checkInvariants` method should not call any
public methods, because those methods in turn call `checkInvariants`. If
this becomes a huge pain, we can look into implementing some kind of
“only check invariants if the invariants are not actively being
checked”, but I’d much rather not do so if we don’t have to.

Test Plan:
Running `yarn coverage` indicates that each of the failure cases is
verified. In principle, I’d be willing to add a test that parses the
source code for `graph.js` and verifies that each `return`, `yield`, or
implicit return is preceded by an invariant check. But I don’t really
want to implement that right now.

wchargin-branch: automatic-invariants
2018-06-11 12:28:25 -07:00
Dandelion Mané c352b5b8d6
Add an `advancedGraph` test case (#377)
The `advancedGraph` is an example graph defined in `graph.test.js`.
It shows off many tricksy features, like having loop edges, multiple
edges from the same src to same dst, etc. We also provide two ways of
constructing it: `graph1` is straightforward, `graph2` adds tons of
spurious adds, removes, and odd ordering. This way we can ensure that
our functions treat `graph1` and `graph2` equivalently.

Test plan:
New unit tests are added verifying that `equals`, `merge`, and
`to/fromJSON` handle the advanced graph appropriately.
2018-06-11 12:08:53 -07:00
Dandelion Mané 5fc0d42c1f Implement `Graph.toJSON` and `Graph.fromJSON` (#374)
The serialization scheme uses `IndexedEdge`s:

```js
type Integer = number;
type IndexedEdge = {|
  Address: EdgeAddressT,
  srcIndex: Integer,
  dstIndex: Integer,
|}
```

The nodes are first sorted. Then, we generate indexed edges from the
regular edges by replacing each node address with its index in the
sorted order. This encoding reduces the number of addresses serialized
from `n + 3e` to `n + e` (where `n` is the number of nodes and `e` is
the number of edges).

This is based on work in #295, but in contrast to that PR, we do not
index the in-memory representations of graphs. Only the JSON
representation is indexed.

Test plan:
Unit tests added. A snapshot test is also included, both to make it easy
to inspect an example of a JSON-serialized graph, and to ensure
backwards-compatibility. (The snapshot likely should not change
independent of the VERSION string.)
2018-06-11 11:57:29 -07:00
Dandelion Mané 6177f6c740
Reimplement `Graph.copy` using `Graph.merge` (#376)
* Implement `Graph.merge`

Tests are mostly copied over from the v2, as implemented in #320.
Some new tests were added, e.g. checking that Merge correctly handles
10 small graphs combined.

Test plan:
See unit tests.

* Reimplement `Graph.copy` using `Graph.merge`

Test plan:
Existing unit tests suffice

Suggested by @wchargin
2018-06-11 10:55:52 -07:00
Dandelion Mané 46751d2707
Implement `Graph.merge` (#375)
Tests are mostly copied over from the v2, as implemented in #320.
Some new tests were added, e.g. checking that Merge correctly handles
10 small graphs combined.

Test plan:
See unit tests.
2018-06-11 10:55:30 -07:00
Dandelion Mané 5fde1c10a5
Copy `v1/util` to `v3/util` (#373)
No changes made to the code - it's a straight copy.

Test plan:
Unit tests are included.
2018-06-11 10:42:25 -07:00
William Chargin 831f5f571c
Make invariants more precise (#371)
Summary:
The previously listed invariants were weak on two counts. First, it was
unstated that the keys of `_inEdges`, `_outEdges`, and `_nodes` should
coincide. Second, the “exactly once” condition on edge inclusion had the
unintentional effect that edge absent in `_edges` but present twice or
more in each of `_inEdges` and `_outEdges` would not violate the
invariant.

Test Plan:
Stay tuned.

wchargin-branch: strengthen-invariants
2018-06-08 15:19:25 -07:00
Dandelion Mané d9e2850eb3
Implement `Graph.copy` (#370)
The implementation is quite simple. The tests are somewhat more
comprehensive than in v2 or v1. We now test that copies are equal to the
original in a variety of situations.

Test plan:
Unit tests added.
2018-06-08 15:19:13 -07:00
Dandelion Mané feef119250
Add and implement `Graph.equals` (#369)
It turns out we forgot to add this to the API, so I added it. I also
implemented it. The tests are pretty thorough; as an added innovation
over our previous tests (e.g. in #312 and #61), we now consistently test
that equality is commutative.

In contrast to our previous implementations, this one is massively
simpler. That's an upside of using primitive ES6 data structures to
store all of the graph's information... which is itself an upside of not
trying to store arbitrary additional information in the graph. Now we
can just do a deep equality check on the underlying nodes set and edges
map!

We might be able to performance tune this method by taking advantage of
the structure of our nodes and edges. This should suffice for now,
though.

Paired with @wchargin

Test plan:
Unit tests were added. Run `yarn travis`
2018-06-08 14:17:57 -07:00
Dandelion Mané 4a441eb287
Implement `Graph.neighbors` (#368)
This is the change that puts the Graph into `Graph` :) We add `_inEdges`
and `_outEdges`, and use them to identify the neighbors of a given
`node`.

The API is implemented pretty uncontroversially. (We've done this a few
times before: see #319, #162). As with other iterators, we check for
comodification and error if this has occurred.

The tests cover some interesting cases like absent nodes, loops, and
multiple edges with the same src and dst.

Test plan:
Unit tests have been added. Run `yarn travis`.

Paired with: @wchargin
2018-06-08 13:34:43 -07:00