Add commit parent edges in the Git graph (#178)

Test Plan:
To verify the snapshot change, either believe the programmatic tests, or
use the following script to verify that the right edges were added:
```bash
#!/bin/bash
set -eu
example_repo="$(mktemp -d)"
yarn backend >/dev/null 2>/dev/null
node bin/createExampleRepo.js "${example_repo}"
expected() {
    git -C "${example_repo}" log --format='%H %P' \
        | awk '{ if (NF > 1) { print $2 " " $1 } }' \
        | sort
}
actual() {
    git diff HEAD^..HEAD | grep -A 1 -F -e src -e dst \
        | sed -n 's/^+.*"id": "\(.\+\)".*/\1 /p' \
        | tr -d $'\n' | cat - <(echo) \
        | fold -s -w 82 | sed 's/ *$//' \
        | sort
}
diff -u <(expected) <(actual)
```

wchargin-branch: graph-commit-parents

src/plugins/git/__snapshots__/createGraph.test.js.snap

@@ -33,6 +33,23 @@ Object {
         "type": "TREE",
       },
     },
+    "{\\"id\\":\\"3715ddfb8d4c4fd2a6f6af75488c82f84c92ec2f^1\\",\\"pluginName\\":\\"sourcecred/git-beta\\",\\"repositoryName\\":\\"sourcecred/example-git\\",\\"type\\":\\"HAS_PARENT\\"}": Object {
+      "dst": Object {
+        "id": "69c5aad50eec8f2a0a07c988c3b283a6490eb45b",
+        "pluginName": "sourcecred/git-beta",
+        "repositoryName": "sourcecred/example-git",
+        "type": "COMMIT",
+      },
+      "payload": Object {
+        "parentIndex": 1,
+      },
+      "src": Object {
+        "id": "3715ddfb8d4c4fd2a6f6af75488c82f84c92ec2f",
+        "pluginName": "sourcecred/git-beta",
+        "repositoryName": "sourcecred/example-git",
+        "type": "COMMIT",
+      },
+    },
     "{\\"id\\":\\"3dfb84795e07341b05fad3a0d5a55f8304b2d7d8:.gitmodules\\",\\"pluginName\\":\\"sourcecred/git-beta\\",\\"repositoryName\\":\\"sourcecred/example-git\\",\\"type\\":\\"INCLUDES\\"}": Object {
       "dst": Object {
         "id": "3dfb84795e07341b05fad3a0d5a55f8304b2d7d8:.gitmodules",
@@ -183,6 +200,23 @@ Object {
         "type": "TREE",
       },
     },
+    "{\\"id\\":\\"69c5aad50eec8f2a0a07c988c3b283a6490eb45b^1\\",\\"pluginName\\":\\"sourcecred/git-beta\\",\\"repositoryName\\":\\"sourcecred/example-git\\",\\"type\\":\\"HAS_PARENT\\"}": Object {
+      "dst": Object {
+        "id": "e8b7a8f19701cd5a25e4a097d513ead60e5f8bcc",
+        "pluginName": "sourcecred/git-beta",
+        "repositoryName": "sourcecred/example-git",
+        "type": "COMMIT",
+      },
+      "payload": Object {
+        "parentIndex": 1,
+      },
+      "src": Object {
+        "id": "69c5aad50eec8f2a0a07c988c3b283a6490eb45b",
+        "pluginName": "sourcecred/git-beta",
+        "repositoryName": "sourcecred/example-git",
+        "type": "COMMIT",
+      },
+    },
     "{\\"id\\":\\"78fc9c83023386854c6bfdc5761c0e58f68e226f:index.py\\",\\"pluginName\\":\\"sourcecred/git-beta\\",\\"repositoryName\\":\\"sourcecred/example-git\\",\\"type\\":\\"INCLUDES\\"}": Object {
       "dst": Object {
         "id": "78fc9c83023386854c6bfdc5761c0e58f68e226f:index.py",
@@ -408,6 +442,23 @@ Object {
         "type": "TREE",
       },
     },
+    "{\\"id\\":\\"8d287c3bfbf8455ef30187bf5153ffc1b6eef268^1\\",\\"pluginName\\":\\"sourcecred/git-beta\\",\\"repositoryName\\":\\"sourcecred/example-git\\",\\"type\\":\\"HAS_PARENT\\"}": Object {
+      "dst": Object {
+        "id": "c08ee3a4edea384d5291ffcbf06724a13ed72325",
+        "pluginName": "sourcecred/git-beta",
+        "repositoryName": "sourcecred/example-git",
+        "type": "COMMIT",
+      },
+      "payload": Object {
+        "parentIndex": 1,
+      },
+      "src": Object {
+        "id": "8d287c3bfbf8455ef30187bf5153ffc1b6eef268",
+        "pluginName": "sourcecred/git-beta",
+        "repositoryName": "sourcecred/example-git",
+        "type": "COMMIT",
+      },
+    },
     "{\\"id\\":\\"[{\\\\\\"id\\\\\\":\\\\\\"2f7155e359fd0ecb96ffdca66fa45b6ed5792809:README.txt\\\\\\",\\\\\\"pluginName\\\\\\":\\\\\\"sourcecred/git-beta\\\\\\",\\\\\\"repositoryName\\\\\\":\\\\\\"sourcecred/example-git\\\\\\",\\\\\\"type\\\\\\":\\\\\\"TREE_ENTRY\\\\\\"},{\\\\\\"id\\\\\\":\\\\\\"0fb31858c8e3710be77e1dbb8880acf8a5543d82\\\\\\",\\\\\\"pluginName\\\\\\":\\\\\\"sourcecred/git-beta\\\\\\",\\\\\\"repositoryName\\\\\\":\\\\\\"sourcecred/example-git\\\\\\",\\\\\\"type\\\\\\":\\\\\\"BLOB\\\\\\"}]\\",\\"pluginName\\":\\"sourcecred/git-beta\\",\\"repositoryName\\":\\"sourcecred/example-git\\",\\"type\\":\\"HAS_CONTENTS\\"}": Object {
       "dst": Object {
         "id": "0fb31858c8e3710be77e1dbb8880acf8a5543d82",
@@ -1053,6 +1104,57 @@ Object {
         "type": "TREE",
       },
     },
+    "{\\"id\\":\\"c08ee3a4edea384d5291ffcbf06724a13ed72325^1\\",\\"pluginName\\":\\"sourcecred/git-beta\\",\\"repositoryName\\":\\"sourcecred/example-git\\",\\"type\\":\\"HAS_PARENT\\"}": Object {
+      "dst": Object {
+        "id": "c2b51945e7457546912a8ce158ed9d294558d294",
+        "pluginName": "sourcecred/git-beta",
+        "repositoryName": "sourcecred/example-git",
+        "type": "COMMIT",
+      },
+      "payload": Object {
+        "parentIndex": 1,
+      },
+      "src": Object {
+        "id": "c08ee3a4edea384d5291ffcbf06724a13ed72325",
+        "pluginName": "sourcecred/git-beta",
+        "repositoryName": "sourcecred/example-git",
+        "type": "COMMIT",
+      },
+    },
+    "{\\"id\\":\\"d160cca97611e9dfed642522ad44408d0292e8ea^1\\",\\"pluginName\\":\\"sourcecred/git-beta\\",\\"repositoryName\\":\\"sourcecred/example-git\\",\\"type\\":\\"HAS_PARENT\\"}": Object {
+      "dst": Object {
+        "id": "8d287c3bfbf8455ef30187bf5153ffc1b6eef268",
+        "pluginName": "sourcecred/git-beta",
+        "repositoryName": "sourcecred/example-git",
+        "type": "COMMIT",
+      },
+      "payload": Object {
+        "parentIndex": 1,
+      },
+      "src": Object {
+        "id": "d160cca97611e9dfed642522ad44408d0292e8ea",
+        "pluginName": "sourcecred/git-beta",
+        "repositoryName": "sourcecred/example-git",
+        "type": "COMMIT",
+      },
+    },
+    "{\\"id\\":\\"e8b7a8f19701cd5a25e4a097d513ead60e5f8bcc^1\\",\\"pluginName\\":\\"sourcecred/git-beta\\",\\"repositoryName\\":\\"sourcecred/example-git\\",\\"type\\":\\"HAS_PARENT\\"}": Object {
+      "dst": Object {
+        "id": "d160cca97611e9dfed642522ad44408d0292e8ea",
+        "pluginName": "sourcecred/git-beta",
+        "repositoryName": "sourcecred/example-git",
+        "type": "COMMIT",
+      },
+      "payload": Object {
+        "parentIndex": 1,
+      },
+      "src": Object {
+        "id": "e8b7a8f19701cd5a25e4a097d513ead60e5f8bcc",
+        "pluginName": "sourcecred/git-beta",
+        "repositoryName": "sourcecred/example-git",
+        "type": "COMMIT",
+      },
+    },
   },
   "nodes": Object {
     "{\\"id\\":\\"0fb31858c8e3710be77e1dbb8880acf8a5543d82\\",\\"pluginName\\":\\"sourcecred/git-beta\\",\\"repositoryName\\":\\"sourcecred/example-git\\",\\"type\\":\\"BLOB\\"}": Object {

src/plugins/git/createGraph.js

@@ -18,8 +18,10 @@ import {
   TREE_ENTRY_NODE_TYPE,
   INCLUDES_EDGE_TYPE,
   HAS_CONTENTS_EDGE_TYPE,
+  HAS_PARENT_EDGE_TYPE,
   HAS_TREE_EDGE_TYPE,
   GIT_PLUGIN_NAME,
+  hasParentEdgeId,
   includesEdgeId,
   treeEntryId,
 } from "./types";
@@ -61,7 +63,7 @@ class GitGraphCreator {
       address: this.makeAddress(TREE_NODE_TYPE, commit.treeHash),
       payload: {},
     };
-    const edge = {
+    const hasTreeEdge = {
       address: this.makeAddress(
         HAS_TREE_EDGE_TYPE,
         edgeID(commitNode.address, treeNode.address)
@@ -70,10 +72,27 @@ class GitGraphCreator {
       dst: treeNode.address,
       payload: {},
     };
-    return new Graph()
+    const result = new Graph()
       .addNode(commitNode)
       .addNode(treeNode)
-      .addEdge(edge);
+      .addEdge(hasTreeEdge);
+    commit.parentHashes.forEach((parentHash, index) => {
+      const oneBasedParentIndex = index + 1;
+      const parentAddress = this.makeAddress(COMMIT_NODE_TYPE, parentHash);
+      const parentEdge = {
+        address: this.makeAddress(
+          HAS_PARENT_EDGE_TYPE,
+          hasParentEdgeId(commit.hash, oneBasedParentIndex)
+        ),
+        src: commitNode.address,
+        dst: parentAddress,
+        payload: {
+          parentIndex: oneBasedParentIndex,
+        },
+      };
+      result.addEdge(parentEdge);
+    });
+    return result;
   }
 
   treeGraph(tree: Tree) {

src/plugins/git/createGraph.test.js

@@ -8,6 +8,7 @@ import {
   COMMIT_NODE_TYPE,
   GIT_PLUGIN_NAME,
   HAS_CONTENTS_EDGE_TYPE,
+  HAS_PARENT_EDGE_TYPE,
   HAS_TREE_EDGE_TYPE,
   INCLUDES_EDGE_TYPE,
   TREE_ENTRY_NODE_TYPE,
@@ -41,13 +42,22 @@ describe("createGraph", () => {
         id: hash,
       };
       expect(graph.node(address)).toEqual({address, payload: {}});
-      expect(graph.neighborhood(address)).toHaveLength(1);
       expect(
         graph.neighborhood(address, {
           nodeType: TREE_NODE_TYPE,
           edgeType: HAS_TREE_EDGE_TYPE,
         })
       ).toHaveLength(1);
+      expect(
+        graph.neighborhood(address, {
+          nodeType: COMMIT_NODE_TYPE,
+          edgeType: HAS_PARENT_EDGE_TYPE,
+          direction: "OUT",
+        })
+      ).toHaveLength(data.commits[hash].parentHashes.length);
+      expect(graph.neighborhood(address, {direction: "OUT"})).toHaveLength(
+        1 + data.commits[hash].parentHashes.length
+      );
     });
   });
 

src/plugins/git/types.js

@@ -55,6 +55,28 @@ export type NodeType =
 
 // Edges
 
+// CommitNode -> CommitNode
+export const HAS_PARENT_EDGE_TYPE: "HAS_PARENT" = "HAS_PARENT";
+export type HasParentEdgePayload = {|
+  +parentIndex: number, // one-based
+|};
+export function hasParentEdgeId(
+  childCommitHash: Hash,
+  oneBasedParentIndex: number
+) {
+  if (
+    !isFinite(oneBasedParentIndex) ||
+    oneBasedParentIndex !== Math.floor(oneBasedParentIndex) ||
+    oneBasedParentIndex < 1
+  ) {
+    throw new Error(
+      "Expected positive integer parent index, " +
+        `but got: ${String(oneBasedParentIndex)}`
+    );
+  }
+  return `${childCommitHash}^${String(oneBasedParentIndex)}`;
+}
+
 // CommitNode -> TreeNode
 export const HAS_TREE_EDGE_TYPE: "HAS_TREE" = "HAS_TREE";
 export type HasTreeEdgePayload = {||};
@@ -76,12 +98,14 @@ export type HasContentsEdgePayload = {||};
 
 export type EdgeType =
   | typeof HAS_TREE_EDGE_TYPE
+  | typeof HAS_PARENT_EDGE_TYPE
   | typeof INCLUDES_EDGE_TYPE
   | typeof BECOMES_EDGE_TYPE
   | typeof HAS_CONTENTS_EDGE_TYPE;
 
 export type EdgePayload =
   | HasTreeEdgePayload
+  | HasParentEdgePayload
   | IncludesEdgePayload
   | BecomesEdgePayload
   | HasContentsEdgePayload;