fix(go_modules): preserve unrelated go.mod checksums in go.sum

When go get updates a dependency, it may remove /go.mod checksum lines
from go.sum for unrelated modules that are still in the dependency
graph. This causes noisy diffs that go mod tidy would revert.

Add reconcile_go_sum to restore missing /go.mod checksum lines when:
- The line belongs to a module not being updated
- The same module+version still has a zip hash entry in the updated file

The restore uses Dependabot::GoModules::Version for semver-aware sorting
to produce canonical go.sum ordering. When no lines need restoring, the
updated go.sum is returned unchanged to avoid unnecessary rewrites.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: v-HaripriyaC

go_modules/lib/dependabot/go_modules/file_updater/go_mod_updater.rb

@@ -10,6 +10,7 @@
 require "dependabot/go_modules/go_work_parser"
 require "dependabot/go_modules/replace_stubber"
 require "dependabot/go_modules/resolvability_errors"
+require "dependabot/go_modules/version"
 
 module Dependabot
   module GoModules
@@ -222,10 +223,12 @@ def update_files
               substitute_all(substitutions.invert)
             end
 
-            updated_go_sum = original_go_sum ? File.read("go.sum") : nil
+            updated_go_sum = T.let(nil, T.nilable(String))
+            updated_go_sum = reconcile_go_sum(original_go_sum, File.read("go.sum")) if original_go_sum
+
             updated_go_mod = File.read("go.mod")
 
-            { go_mod: updated_go_mod, go_sum: updated_go_sum }
+            { go_mod: updated_go_mod, go_sum: updated_go_sum }.compact
           end
         end
 
@@ -417,6 +420,115 @@ def build_module_stubs(stub_paths)
           end
         end
 
+        sig { params(original_go_sum: String, updated_go_sum: String).returns(String) }
+        def reconcile_go_sum(original_go_sum, updated_go_sum)
+          original_lines = original_go_sum.lines(chomp: true).reject(&:empty?)
+          updated_lines = updated_go_sum.lines(chomp: true).reject(&:empty?)
+          updated_set = updated_lines.to_set
+          updated_module_versions = extract_module_versions(updated_lines)
+
+          restored_lines = find_restorable_go_mod_lines(original_lines, updated_set, updated_module_versions)
+          return updated_go_sum if restored_lines.empty?
+
+          (updated_lines + restored_lines).sort! { |a, b| go_sum_line_compare(a, b) }.join("\n") + "\n"
+        end
+
+        sig do
+          params(
+            original_lines: T::Array[String],
+            updated_set: T::Set[String],
+            updated_module_versions: T::Set[String]
+          ).returns(T::Array[String])
+        end
+        def find_restorable_go_mod_lines(original_lines, updated_set, updated_module_versions)
+          original_lines.filter_map do |line|
+            next unless go_mod_checksum_line?(line)
+            next if updated_set.include?(line)
+
+            module_path = go_sum_module_path(line)
+            next unless module_path
+            next if updated_dependency_names.include?(module_path)
+
+            module_version = extract_module_version_from_go_mod_line(line)
+            next unless module_version
+            next unless updated_module_versions.include?(module_version)
+
+            line
+          end
+        end
+
+        sig { params(line: String).returns(T::Boolean) }
+        def go_mod_checksum_line?(line)
+          line.include?("/go.mod h1:")
+        end
+
+        sig { params(line: String).returns(T.nilable(String)) }
+        def go_sum_module_path(line)
+          line.split(/\s+/, 2).first
+        end
+
+        # Extracts "module/path vX.Y.Z" from a /go.mod checksum line
+        sig { params(line: String).returns(T.nilable(String)) }
+        def extract_module_version_from_go_mod_line(line)
+          match = line.match(%r{^(\S+)\s+(\S+)/go\.mod\s})
+          return nil unless match
+
+          "#{match[1]} #{match[2]}"
+        end
+
+        # Builds a set of "module/path vX.Y.Z" pairs from non-/go.mod lines in go.sum
+        sig { params(lines: T::Array[String]).returns(T::Set[String]) }
+        def extract_module_versions(lines)
+          lines.each_with_object(Set.new) do |line, set|
+            next if go_mod_checksum_line?(line)
+
+            parts = line.split(/\s+/, 3)
+            next unless parts.length >= 2
+
+            set.add("#{parts[0]} #{parts[1]}")
+          end
+        end
+
+        sig { returns(T::Set[String]) }
+        def updated_dependency_names
+          @updated_dependency_names ||= T.let(dependencies.to_set(&:name), T.nilable(T::Set[String]))
+        end
+
+        # Compares two go.sum lines using Go's module-aware sort order:
+        # sort by module path, then semver version, then /go.mod suffix last.
+        sig { params(line_a: String, line_b: String).returns(Integer) }
+        def go_sum_line_compare(line_a, line_b)
+          path_a, version_rest_a = line_a.split(/\s+/, 2)
+          path_b, version_rest_b = line_b.split(/\s+/, 2)
+
+          path_cmp = T.must((path_a || "") <=> (path_b || ""))
+          return path_cmp unless path_cmp.zero?
+
+          compare_go_versions(version_rest_a || "", version_rest_b || "")
+        end
+
+        # Compares version+suffix portions of go.sum lines using GoModules::Version.
+        sig { params(ver_a: String, ver_b: String).returns(Integer) }
+        def compare_go_versions(ver_a, ver_b)
+          a_is_gomod = ver_a.include?("/go.mod")
+          b_is_gomod = ver_b.include?("/go.mod")
+
+          # Extract raw version token (e.g., "v0.6.0" from "v0.6.0/go.mod h1:...")
+          raw_a = ver_a.split(%r{(/go\.mod)?\s}, 2).first || ""
+          raw_b = ver_b.split(%r{(/go\.mod)?\s}, 2).first || ""
+
+          ver_cmp = go_version_compare(raw_a, raw_b)
+          return ver_cmp unless ver_cmp.zero?
+
+          # Same version: zip hash line sorts before /go.mod line
+          (a_is_gomod ? 1 : 0) <=> (b_is_gomod ? 1 : 0)
+        end
+
+        sig { params(ver_a: String, ver_b: String).returns(Integer) }
+        def go_version_compare(ver_a, ver_b)
+          T.must(Dependabot::GoModules::Version.new(ver_a) <=> Dependabot::GoModules::Version.new(ver_b))
+        end
+
         # Given a go.mod file, find all `replace` directives pointing to a path
         # on the local filesystem, and return an array of pairs mapping the
         # original path to a hash of the path.

go_modules/spec/dependabot/go_modules/file_updater/go_mod_updater_spec.rb

@@ -276,6 +276,77 @@ module github.com/dependabot/vgotest
               .not_to include(%(rsc.io/quote v1.4.0/go.mod h1:))
           end
 
+          context "when go tooling removes an unrelated go.mod checksum line" do
+            let(:removed_line) do
+              "gonum.org/v1/gonum v0.16.0/go.mod h1:fef3am4MQ93R2HHpKnLk4/Tbh/s0+wqD5nfa6Pnwy4E="
+            end
+            let(:zip_line) do
+              "gonum.org/v1/gonum v0.16.0 h1:JKbmSgVMFkFMDpGCixMRJCMEMmNhrsJuJqVDPMGPnQY="
+            end
+
+            it "restores the unrelated checksum line" do
+              allow(File).to receive(:read).and_call_original
+
+              original_go_sum = fixture("projects", project_name, "go.sum") +
+                                "#{zip_line}\n#{removed_line}\n"
+              updated_go_sum = original_go_sum.lines.reject { |line| line.chomp == removed_line }.join
+
+              allow(File).to receive(:read).with("go.sum").and_return(original_go_sum, updated_go_sum)
+
+              expect(updated_go_mod_content).to include(removed_line)
+            end
+          end
+
+          context "when the removed checksum belongs to the updated dependency" do
+            let(:removed_line) { "rsc.io/quote v1.4.0/go.mod h1:omethingoldchecksum=" }
+
+            it "does not restore the removed checksum line" do
+              allow(File).to receive(:read).and_call_original
+
+              original_go_sum = <<~GOSUM
+                rsc.io/quote v1.4.0 h1:oldchecksum=
+                #{removed_line}
+                rsc.io/sampler v1.3.0/go.mod h1:anexistingchecksum=
+              GOSUM
+
+              updated_go_sum = <<~GOSUM
+                rsc.io/quote v1.5.2 h1:newchecksum=
+                rsc.io/quote v1.5.2/go.mod h1:newgomodchecksum=
+                rsc.io/sampler v1.3.0/go.mod h1:anexistingchecksum=
+              GOSUM
+
+              allow(File).to receive(:read).with("go.sum").and_return(original_go_sum, updated_go_sum)
+
+              expect(updated_go_mod_content).not_to include(removed_line)
+            end
+          end
+
+          context "when a transitive dependency version is legitimately upgraded" do
+            let(:removed_line) { "golang.org/x/sys v0.0.0-20200116001909/go.mod h1:oldtransitivechecksum=" }
+
+            it "does not restore the removed checksum line" do
+              allow(File).to receive(:read).and_call_original
+
+              original_go_sum = <<~GOSUM
+                golang.org/x/sys v0.0.0-20200116001909 h1:oldziphash=
+                #{removed_line}
+                rsc.io/quote v1.4.0 h1:oldchecksum=
+                rsc.io/quote v1.4.0/go.mod h1:oldgomod=
+              GOSUM
+
+              updated_go_sum = <<~GOSUM
+                golang.org/x/sys v0.0.0-20220731174439 h1:newziphash=
+                golang.org/x/sys v0.0.0-20220731174439/go.mod h1:newtransitivechecksum=
+                rsc.io/quote v1.5.2 h1:newchecksum=
+                rsc.io/quote v1.5.2/go.mod h1:newgomodchecksum=
+              GOSUM
+
+              allow(File).to receive(:read).with("go.sum").and_return(original_go_sum, updated_go_sum)
+
+              expect(updated_go_mod_content).not_to include(removed_line)
+            end
+          end
+
           describe "a non-existent dependency with a pseudo-version" do
             let(:project_name) { "non_existent_dependency" }