prevent trying to get a commit that can't exist

common/lib/dependabot/file_fetchers/base.rb

@@ -85,7 +85,11 @@ def commit
       def clone_repo_contents
         @clone_repo_contents ||=
           _clone_repo_contents(target_directory: repo_contents_path)
-      rescue Dependabot::SharedHelpers::HelperSubprocessFailed
+      rescue Dependabot::SharedHelpers::HelperSubprocessFailed => e
+        if e.message.include?("fatal: Remote branch #{target_branch} not found in upstream origin")
+          raise Dependabot::BranchNotFound, target_branch
+        end
+
         raise Dependabot::RepoNotFound, source
       end
 

common/spec/dependabot/file_fetchers/base_spec.rb

@@ -1553,6 +1553,16 @@ def fetch_files
           expect { subject }.to raise_error(Dependabot::RepoNotFound)
         end
       end
+
+      context "when the branch can't be found" do
+        let(:branch) do
+          "notfound"
+        end
+
+        it "raises a not found error" do
+          expect { subject }.to raise_error(Dependabot::BranchNotFound)
+        end
+      end
     end
   end
 end

updater/lib/dependabot/file_fetcher_job.rb

@@ -8,12 +8,15 @@
 module Dependabot
   class FileFetcherJob < BaseJob
     def perform_job
+      @base_commit_sha = nil
+
       begin
         connectivity_check if ENV["ENABLE_CONNECTIVITY_CHECK"] == "1"
         clone_repo_contents
-        base_commit_sha
+        @base_commit_sha = file_fetcher.commit || "unknown"
         dependency_files
       rescue StandardError => e
+        @base_commit_sha ||= "unknown"
         if Octokit::RATE_LIMITED_ERRORS.include?(e.class)
           remaining = rate_limit_error_remaining(e)
           logger_error("Repository is rate limited, attempting to retry in " \
@@ -22,14 +25,14 @@ def perform_job
           logger_error("Error during file fetching; aborting")
         end
         handle_file_fetcher_error(e)
-        service.mark_job_as_processed(job_id, base_commit_sha)
+        service.mark_job_as_processed(job_id, @base_commit_sha)
         clear_repo_contents_path
         return
       end
 
       File.write(Environment.output_path, JSON.dump(
                                             base64_dependency_files: base64_dependency_files.map(&:to_h),
-                                            base_commit_sha: base_commit_sha
+                                            base_commit_sha: @base_commit_sha
                                           ))
 
       save_job_details
@@ -40,7 +43,7 @@ def save_job_details
 
       File.write(Environment.job_path, JSON.dump(
                                          base64_dependency_files: base64_dependency_files.map(&:to_h),
-                                         base_commit_sha: base_commit_sha,
+                                         base_commit_sha: @base_commit_sha,
                                          job: job_definition["job"]
                                        ))
     end
@@ -84,15 +87,6 @@ def job
       @job ||= Job.new(attrs)
     end
 
-    def base_commit_sha
-      @base_commit_sha ||= file_fetcher.commit || "unknown"
-    rescue StandardError
-      # If an error occurs, set the commit SHA instance variable (so that we
-      # don't raise when recording the error later) and re-raise
-      @base_commit_sha = "unknown"
-      raise
-    end
-
     def file_fetcher
       return @file_fetcher if defined? @file_fetcher
 

updater/spec/dependabot/file_fetcher_job_spec.rb

@@ -186,6 +186,27 @@
         expect(Dir.exist?(Dependabot::Environment.repo_contents_path)).to be_truthy
         expect(Dir.empty?(Dependabot::Environment.repo_contents_path)).to be_truthy
       end
+
+      context "when the fetcher raises a BranchNotFound error while cloning" do
+        before do
+          allow_any_instance_of(Dependabot::GoModules::FileFetcher).
+            to receive(:clone_repo_contents).
+            and_raise(Dependabot::BranchNotFound, "my_branch")
+        end
+
+        it "tells the backend about the error (and doesn't re-raise it)" do
+          expect(api_client).
+            to receive(:record_update_job_error).
+            with(
+              job_id,
+              error_details: { "branch-name": "my_branch" },
+              error_type: "branch_not_found"
+            )
+          expect(api_client).to receive(:mark_job_as_processed)
+
+          expect { perform_job }.to output(/Error during file fetching; aborting/).to_stdout_from_any_process
+        end
+      end
     end
 
     context "when the connectivity check is enabled", vcr: true do