a8n: Implement `previewCampaignPlan`

Created by: mrnugget

This is part of https://github.com/sourcegraph/sourcegraph/issues/6085 and implements the GraphQL previewCampaign mutation.

It validates arguments, creates a CampaignPlan and multiple CampaignJobs (one for each repo yielded by the CampaignTypes search query).

The single CampaignType that's currently implemented is "comby", which does search&replace across repositories.

What's in this PR already works: a plan is created, jobs are launched, executed and their Diff field is updated.

There are a few things missing that I want to address in follow-up PRs in order to make reviewing easier and to unblock @felixfbecker and @eseliger:

• See comment below The diff produced by reading json-lines from the replacer service is not a valid multi-file diff. While it can be parsed again by diff.ParseMultiFile the produced []*diff.FileDiff only has a single element. That's enough to use the API for now, which is why I don't want it to block the merging of this PR. I will address this in a separate PR that also adds tests for comby (which right now only does a single HTTP request which we likely need to change)
• CampaignPlans are not cleaned up yet. I already discussed this with @tsenart.
• The CampaignPlan.status is still a dummy implementation.

Created by: codecov[bot]

Codecov Report

Merging #6265 into master will increase coverage by 0.06%. The diff coverage is 51.76%.

@@            Coverage Diff             @@
##           master    #6265      +/-   ##
==========================================
+ Coverage   39.32%   39.38%   +0.06%     
==========================================
  Files        1192     1194       +2     
  Lines       61289    61419     +130     
  Branches     5838     5838              
==========================================
+ Hits        24101    24189      +88     
- Misses      34983    35008      +25     
- Partials     2205     2222      +17

Impacted Files	Coverage Δ
internal/a8n/types.go	21.12% <ø> (ø)
cmd/frontend/graphqlbackend/codemod.go	10.05% <0%> (ø)
enterprise/pkg/a8n/resolvers/resolver.go	38.81% <0%> (+4.36%)
enterprise/pkg/a8n/run/campaign_type.go	44.82% <44.82%> (ø)
enterprise/pkg/a8n/run/runner.go	60.78% <60.78%> (ø)

Created by: mrnugget

I added the last commit 41f8d0e after our discussion in Slack about skipping repositories without a default branch.

Created by: mrnugget

Just added 374fc893fcf0a46b7290e2cd97fecf504e923787 to temporarily fix the problem of multi-file diffs.

The problem is that the go-diff package doesn't parse multi-file diffs correctly if they're missing a diff ... separator line between the single file diffs. git apply and patch, though, can handle these perfectly fine. So that is a "bug" in go-diff that I want to fix.

Until that's fixed and to make working with this branch easier, this temporary fix injects a diff ... line between the file diffs.

The resulting multi-file diff can then be parsed by go-diff in the GraphQL backend and return multiple file diffs per repository. That gives us the correct response. Here's an example from a campaign that ran across multiple repositories and replaced text in zero or more files in each repository:

{
  "data": {
    "previewCampaignPlan": {
      "__typename": "CampaignPlan",
      "id": "Q2FtcGFpZ25QbGFuOjEx",
      "type": "comby",
      "arguments": "{\"scopeQuery\":\"repo:gorilla\",\"matchTemplate\":\"example.com\",\"rewriteTemplate\":\"sourcegraph.com\"}",
      "status": {
        "completedCount": 0,
        "pendingCount": 99,
        "state": "ERRORED",
        "errors": [
          "this is just a skeleton api"
        ]
      },
      "changesets": {
        "nodes": [
          {
            "diff": {
              "fileDiffs": {
                "totalCount": 0,
                "nodes": []
              }
            },
            "repository": {
              "name": "ghe.sgdev.org/sourcegraph/gorilla-sessions",
              "url": "/ghe.sgdev.org/sourcegraph/gorilla-sessions"
            }
          },
          {
            "diff": {
              "fileDiffs": {
                "totalCount": 4,
                "nodes": [
                  {
                    "oldPath": "example_cors_method_middleware_test.go",
                    "newPath": "example_cors_method_middleware_test.go",
                    "hunks": [
                      {
                        "body": " \tfmt.Println(rw.Header().Get(\"Access-Control-Allow-Origin\"))\n \t// Output:\n \t// GET,PUT,PATCH,OPTIONS\n-\t// http://example.com\n+\t// http://sourcegraph.com\n }\n",
                        "oldRange": {
                          "startLine": 33
                        },
                        "newRange": {
                          "startLine": 33
                        }
                      }
                    ]
                  },
                  {
                    "oldPath": "README.md",
                    "newPath": "README.md",
                    "hunks": [
                      {
                        "body": " \n Setting the same matching conditions again and again can be boring, so we have a way to group several routes that share the same requirements. We call it \"subrouting\".\n \n-For example, let's say we have several URLs that should only match when the host is `www.example.com`. Create a route for that host and get a \"subrouter\" from it:\n+For example, let's say we have several URLs that should only match when the host is `www.sourcegraph.com`. Create a route for that host and get a \"subrouter\" from it:\n \n ```go\n r := mux.NewRouter()\n",
                        "oldRange": {
                          "startLine": 150
                        },
                        "newRange": {
                          "startLine": 150
                        }
                      },
                      {
                        "body": " s.HandleFunc(\"/articles/{category}/{id:[0-9]+}\", ArticleHandler)\n ```\n \n-The three URL paths we registered above will only be tested if the domain is `www.example.com`, because the subrouter is tested first. This is not only convenient, but also optimizes request matching. You can create subrouters combining any attribute matchers accepted by a route.\n+The three URL paths we registered above will only be tested if the domain is `www.sourcegraph.com`, because the subrouter is tested first. This is not only convenient, but also optimizes request matching. You can create subrouters combining any attribute matchers accepted by a route.\n \n Subrouters can be used to create domain or path \"namespaces\": you define subrouters in a central place and then parts of the app can register its paths relatively to a given subrouter.\n \n",
                        "oldRange": {
                          "startLine": 165
                        },
                        "newRange": {
                          "startLine": 165
                        }
                      }
                    ]
                  },
                  {
                    "oldPath": "route.go",
                    "newPath": "route.go",
                    "hunks": [
                      {
                        "body": " // For example:\n //\n //     r := mux.NewRouter()\n-//     r.Host(\"www.example.com\")\n+//     r.Host(\"www.sourcegraph.com\")\n //     r.Host(\"{subdomain}.domain.com\")\n //     r.Host(\"{subdomain:[a-z]+}.domain.com\")\n //\n",
                        "oldRange": {
                          "startLine": 284
                        },
                        "newRange": {
                          "startLine": 284
                        }
                      },
                      {
                        "body": " // It will test the inner routes only if the parent route matched. For example:\n //\n //     r := mux.NewRouter()\n-//     s := r.Host(\"www.example.com\").Subrouter()\n+//     s := r.Host(\"www.sourcegraph.com\").Subrouter()\n //     s.HandleFunc(\"/products/\", ProductsHandler)\n //     s.HandleFunc(\"/products/{key}\", ProductHandler)\n //     s.HandleFunc(\"/articles/{category}/{id:[0-9]+}\"), ArticleHandler)\n",
                        "oldRange": {
                          "startLine": 455
                        },
                        "newRange": {
                          "startLine": 455
                        }
                      }
                    ]
                  },
                  {
                    "oldPath": "mux_test.go",
                    "newPath": "mux_test.go",
                    "hunks": [
                      {
                        "body": " \t\t\tpath:            \"\",\n \t\t\tquery:           \"foo=bar&baz=ding\",\n \t\t\tpathTemplate:    `/api`,\n-\t\t\thostTemplate:    `www.example.com`,\n+\t\t\thostTemplate:    `www.sourcegraph.com`,\n \t\t\tqueriesTemplate: \"foo=bar,baz=ding\",\n \t\t\tqueriesRegexp:   \"^foo=bar$,^baz=ding$\",\n \t\t\tshouldMatch:     true,\n",
                        "oldRange": {
                          "startLine": 782
                        },
                        "newRange": {
                          "startLine": 782
                        }
                      },
                      {
                        "body": " \t\t\tpath:            \"\",\n \t\t\tquery:           \"foo=bar&baz=ding\",\n \t\t\tpathTemplate:    `/api`,\n-\t\t\thostTemplate:    `www.example.com`,\n+\t\t\thostTemplate:    `www.sourcegraph.com`,\n \t\t\tqueriesTemplate: \"foo=bar,baz=ding\",\n \t\t\tqueriesRegexp:   \"^foo=bar$,^baz=ding$\",\n \t\t\tshouldMatch:     true,\n",
                        "oldRange": {
                          "startLine": 796
                        },
                        "newRange": {
                          "startLine": 796
                        }
                      }
                    ]
                  }
                ]
              }
            },
            "repository": {
              "name": "ghe.sgdev.org/sourcegraph/gorilla-mux",
              "url": "/ghe.sgdev.org/sourcegraph/gorilla-mux"
            }
          },
          {
            "diff": {
              "fileDiffs": {
                "totalCount": 1,
                "nodes": [
                  {
                    "oldPath": "x_net_proxy.go",
                    "newPath": "x_net_proxy.go",
                    "hunks": [
                      {
                        "body": " \n // AddFromString parses a string that contains comma-separated values\n // specifying hosts that should use the bypass proxy. Each value is either an\n-// IP address, a CIDR range, a zone (*.example.com) or a host name\n+// IP address, a CIDR range, a zone (*.sourcegraph.com) or a host name\n // (localhost). A best effort is made to parse the string and errors are\n // ignored.\n func (p *proxy_PerHost) AddFromString(s string) {\n",
                        "oldRange": {
                          "startLine": 94
                        },
                        "newRange": {
                          "startLine": 94
                        }
                      }
                    ]
                  }
                ]
              }
            },
            "repository": {
              "name": "ghe.sgdev.org/sourcegraph/gorilla-websocket",
              "url": "/ghe.sgdev.org/sourcegraph/gorilla-websocket"
            }
          },
          {
            "diff": {
              "fileDiffs": {
                "totalCount": 0,
                "nodes": []
              }
            },
            "repository": {
              "name": "ghe.sgdev.org/sourcegraph/gorillalabs-sparkling",
              "url": "/ghe.sgdev.org/sourcegraph/gorillalabs-sparkling"
            }
          }
        ]
      }
    }
  }
}