Limit the size of GitHub review body comment

GitHub limits the review body (main comment) to be 65,536 characters or
less. Otherwise, an error gets thrown.
This change will truncate our errors to fit within that limit, while
still trying to show the error.

app/models/pull_request.rb

@@ -19,7 +19,7 @@ def make_comments(violations, errors)
       repo_name,
       number,
       comments,
-      format_errors(errors),
+      ReviewBody.new(errors).to_s,
     )
   end
 
@@ -67,22 +67,6 @@ def build_comment(violation)
     }
   end
 
-  def format_errors(errors)
-    if errors.any?
-      error_details = errors.map { |error| build_error_details(error) }
-      ["Some files could not be reviewed due to errors:"].
-        concat(error_details).join
-    else
-      ""
-    end
-  end
-
-  def build_error_details(error)
-    summary = error.lines.first.strip.truncate(80)
-    details = error.lines.map(&:rstrip).join(COMMENT_LINE_DELIMITER)
-    "<details><summary>#{summary}</summary><pre>#{details}</pre></details>"
-  end
-
   def user_github
     @user_github ||= GithubApi.new(token)
   end

app/models/review_body.rb

@@ -0,0 +1,55 @@
+# frozen_string_literal: true
+class ReviewBody
+  MAX_BODY_LENGTH = 65536
+  SUMMARY_LENGTH = 80
+  LINE_DELIMITER = "<br>"
+  HEADER = "Some files could not be reviewed due to errors:"
+  DETAILS_FORMAT = "<details><summary>%s</summary><pre>%s</pre></details>"
+
+  def initialize(errors)
+    @errors = errors
+  end
+
+  def to_s
+    if errors.any?
+      error_details = errors.map { |error| build_error_details(error) }
+      [HEADER].concat(error_details).join
+    else
+      ""
+    end
+  end
+
+  private
+
+  attr_reader :errors
+
+  def build_error_details(error)
+    summary = error_summary(error)
+    details = error[0...(allowed_error_length - summary.length)].
+      lines.
+      map(&:rstrip).
+      join(LINE_DELIMITER)
+
+    sprintf(DETAILS_FORMAT, summary, details)
+  end
+
+  def error_summary(error)
+    error.lines.first.strip.truncate(SUMMARY_LENGTH)
+  end
+
+  def allowed_error_length
+    (MAX_BODY_LENGTH - formatting_characters_size) / errors.size
+  end
+
+  def formatting_characters_size
+    HEADER.length + all_detail_formats_length + all_lines_delimiters_length
+  end
+
+  def all_detail_formats_length
+    DETAILS_FORMAT.size * errors.size
+  end
+
+  def all_lines_delimiters_length
+    (errors.flat_map(&:lines).size - errors.size) * LINE_DELIMITER.length
+  end
+end

spec/models/pull_request_spec.rb

@@ -60,57 +60,31 @@
   end
 
   describe "#make_comments" do
-    context "when no errors in file reviews exist" do
-      it "posts a review with comments to GitHub as the Hound user" do
-        payload = payload_stub
-        github = instance_double("GithubApi", create_pull_request_review: nil)
-        pull_request = pull_request_stub(github, payload)
-        violation = violation_stub
-
-        pull_request.make_comments([violation], [])
-
-        expect(github).to have_received(:create_pull_request_review).with(
-          "org/repo",
-          payload.pull_request_number,
-          [
-            {
-              path: violation.filename,
-              position: violation.patch_position,
-              body: violation.messages.join,
-            },
-          ],
-          ""
-        )
-      end
-    end
-
-    context "when file reviews contain errors" do
-      it "posts a review with comments to GitHub as the Hound user" do
-        payload = payload_stub
-        github = instance_double("GithubApi", create_pull_request_review: nil)
-        pull_request = pull_request_stub(github, payload)
-        violation = violation_stub
-        review_errors = ["invalid config", "foo\n  bar"]
-
-        pull_request.make_comments([violation], review_errors)
-
-        expect(github).to have_received(:create_pull_request_review).with(
-          "org/repo",
-          payload.pull_request_number,
-          [
-            {
-              path: violation.filename,
-              position: violation.patch_position,
-              body: violation.messages.join,
-            },
-          ],
-          "Some files could not be reviewed due to errors:" \
+    it "posts a review with comments to GitHub as the Hound user" do
+      payload = payload_stub
+      github = instance_double("GithubApi", create_pull_request_review: nil)
+      pull_request = pull_request_stub(github, payload)
+      violation = violation_stub
+      review_errors = ["invalid config", "foo\n  bar"]
+
+      pull_request.make_comments([violation], review_errors)
+
+      expect(github).to have_received(:create_pull_request_review).with(
+        "org/repo",
+        payload.pull_request_number,
+        [
+          {
+            path: violation.filename,
+            position: violation.patch_position,
+            body: violation.messages.join,
+          },
+        ],
+        "Some files could not be reviewed due to errors:" \
           "<details><summary>invalid config</summary>" \
           "<pre>invalid config</pre></details>" \
           "<details><summary>foo</summary>" \
           "<pre>foo<br>  bar</pre></details>",
-        )
-      end
+      )
     end
   end
 

spec/models/review_body_spec.rb

@@ -0,0 +1,53 @@
+require "app/models/review_body"
+
+RSpec.describe ReviewBody do
+  describe "#to_s" do
+    context "when no errors are passed" do
+      it "returns empty string" do
+        review_body = ReviewBody.new([])
+
+        result = review_body.to_s
+
+        expect(result).to eq ""
+      end
+    end
+
+    context "with errors" do
+      it "returns formatted errors" do
+        review_body = ReviewBody.new(["invalid config", "foo\n  bar"])
+
+        result = review_body.to_s
+
+        expect(result).to eq(
+          "Some files could not be reviewed due to errors:" \
+            "<details><summary>invalid config</summary>" \
+            "<pre>invalid config</pre></details>" \
+            "<details><summary>foo</summary>" \
+            "<pre>foo<br>  bar</pre></details>",
+        )
+      end
+
+      context "when errors are too long" do
+        it "truncates the errors" do
+          stub_const("ReviewBody::MAX_BODY_LENGTH", 200)
+          review_body = ReviewBody.new(
+            [
+              "invalid config",
+              "rule is unknown\n  MyRule",
+            ],
+          )
+
+          result = review_body.to_s
+
+          expect(result).to eq(
+            "Some files could not be reviewed due to errors:" \
+              "<details><summary>invalid config</summary>" \
+              "<pre>invalid</pre></details>" \
+              "<details><summary>rule is unknown</summary>" \
+              "<pre>rule i</pre></details>",
+          )
+        end
+      end
+    end
+  end
+end