Extract paginatable fragments of GitHub query (#120)

Summary:
Any time that we pull fields off a connection object, we may need to
repeat the query for subsequent pages. Therefore, such fragments will be
shared across multiple queries, and also shared within a query if we
need to fetch—say—more issue comments on two or more distinct issues.
This is a perfect use case for fragments.

This commit refactors the GitHub query to be organized in terms of
fragments, without changing the format of the results.

(We also take this opportunity to factor the page limits into
constants.)

Test Plan:
After running `yarn backend`, the `fetchGithubRepoTest.sh` test passes.

wchargin-branch: extract-github-query-fragments

src/plugins/github/graphql.js

@@ -1,12 +1,27 @@
 // @flow
 
-import type {Body} from "../../graphql/queries";
+import type {Body, FragmentDefinition} from "../../graphql/queries";
 import {build} from "../../graphql/queries";
 
+/*
+ * GitHub enforces a hard limit of no more than 100 entities per page,
+ * in any single connection. GitHub also has a more global restriction
+ * on the worst-case number of nodes that could be requested by a query,
+ * which scales as the product of the page limits in any given sequence
+ * of nested connections. (For more information, see [1].) Therefore, we
+ * tune the page sizes of various entities to keep them comfortably
+ * within the global capacity.
+ *
+ * [1]: https://developer.github.com/v4/guides/resource-limitations/#node-limit
+ */
+const PAGE_SIZE_ISSUES = 100;
+const PAGE_SIZE_PRS = 100;
+const PAGE_SIZE_COMMENTS = 20;
+const PAGE_SIZE_REVIEWS = 10;
+const PAGE_SIZE_REVIEW_COMMENTS = 10;
+
 export function createQuery(): Body {
   const b = build;
-  const makePageInfo = () => b.field("pageInfo", {}, [b.field("hasNextPage")]);
-  const makeAuthor = () => b.field("author", {}, [b.fragmentSpread("whoami")]);
   const body: Body = [
     b.query(
       "FetchData",
@@ -16,66 +31,26 @@ export function createQuery(): Body {
           "repository",
           {owner: b.variable("repoOwner"), name: b.variable("repoName")},
           [
-            b.field("issues", {first: b.literal(100)}, [
+            b.field("issues", {first: b.literal(PAGE_SIZE_ISSUES)}, [
-              makePageInfo(),
+              b.fragmentSpread("issues"),
-              b.field("nodes", {}, [
-                b.field("id"),
-                b.field("title"),
-                b.field("body"),
-                b.field("number"),
-                makeAuthor(),
-                b.field("comments", {first: b.literal(20)}, [
-                  makePageInfo(),
-                  b.field("nodes", {}, [
-                    b.field("id"),
-                    makeAuthor(),
-                    b.field("body"),
-                    b.field("url"),
-                  ]),
-                ]),
-              ]),
             ]),
-            b.field("pullRequests", {first: b.literal(100)}, [
+            b.field("pullRequests", {first: b.literal(PAGE_SIZE_PRS)}, [
-              makePageInfo(),
+              b.fragmentSpread("prs"),
-              b.field("nodes", {}, [
-                b.field("id"),
-                b.field("title"),
-                b.field("body"),
-                b.field("number"),
-                makeAuthor(),
-                b.field("comments", {first: b.literal(20)}, [
-                  makePageInfo(),
-                  b.field("nodes", {}, [
-                    b.field("id"),
-                    makeAuthor(),
-                    b.field("body"),
-                    b.field("url"),
-                  ]),
-                ]),
-                b.field("reviews", {first: b.literal(10)}, [
-                  makePageInfo(),
-                  b.field("nodes", {}, [
-                    b.field("id"),
-                    b.field("body"),
-                    makeAuthor(),
-                    b.field("state"),
-                    b.field("comments", {first: b.literal(10)}, [
-                      makePageInfo(),
-                      b.field("nodes", {}, [
-                        b.field("id"),
-                        b.field("body"),
-                        b.field("url"),
-                        makeAuthor(),
-                      ]),
-                    ]),
-                  ]),
-                ]),
-              ]),
             ]),
           ]
         ),
       ]
     ),
+    ...createFragments(),
+  ];
+  return body;
+}
+
+function createFragments(): FragmentDefinition[] {
+  const b = build;
+  const makePageInfo = () => b.field("pageInfo", {}, [b.field("hasNextPage")]);
+  const makeAuthor = () => b.field("author", {}, [b.fragmentSpread("whoami")]);
+  return [
     b.fragment("whoami", "Actor", [
       b.field("__typename"),
       b.field("login"),
@@ -83,8 +58,67 @@ export function createQuery(): Body {
       b.inlineFragment("Organization", [b.field("id")]),
       b.inlineFragment("Bot", [b.field("id")]),
     ]),
+    b.fragment("issues", "IssueConnection", [
+      makePageInfo(),
+      b.field("nodes", {}, [
+        b.field("id"),
+        b.field("title"),
+        b.field("body"),
+        b.field("number"),
+        makeAuthor(),
+        b.field("comments", {first: b.literal(PAGE_SIZE_COMMENTS)}, [
+          b.fragmentSpread("comments"),
+        ]),
+      ]),
+    ]),
+    b.fragment("prs", "PullRequestConnection", [
+      makePageInfo(),
+      b.field("nodes", {}, [
+        b.field("id"),
+        b.field("title"),
+        b.field("body"),
+        b.field("number"),
+        makeAuthor(),
+        b.field("comments", {first: b.literal(PAGE_SIZE_COMMENTS)}, [
+          b.fragmentSpread("comments"),
+        ]),
+        b.field("reviews", {first: b.literal(PAGE_SIZE_REVIEWS)}, [
+          b.fragmentSpread("reviews"),
+        ]),
+      ]),
+    ]),
+    // (Note: issue comments and PR comments use the same connection type.)
+    b.fragment("comments", "IssueCommentConnection", [
+      makePageInfo(),
+      b.field("nodes", {}, [
+        b.field("id"),
+        makeAuthor(),
+        b.field("body"),
+        b.field("url"),
+      ]),
+    ]),
+    b.fragment("reviews", "PullRequestReviewConnection", [
+      makePageInfo(),
+      b.field("nodes", {}, [
+        b.field("id"),
+        b.field("body"),
+        makeAuthor(),
+        b.field("state"),
+        b.field("comments", {first: b.literal(PAGE_SIZE_REVIEW_COMMENTS)}, [
+          b.fragmentSpread("reviewComments"),
+        ]),
+      ]),
+    ]),
+    b.fragment("reviewComments", "PullRequestReviewCommentConnection", [
+      makePageInfo(),
+      b.field("nodes", {}, [
+        b.field("id"),
+        b.field("body"),
+        b.field("url"),
+        makeAuthor(),
+      ]),
+    ]),
   ];
-  return body;
 }
 
 export function createVariables(repoOwner: string, repoName: string) {