update-report: allow additions/deletions from the API.

If we're installing from the API: we cannot use Git to check for differences in
packages so instead use `{formula,cask}_names.txt` to do so.

The first time this runs: we won't yet have a base state
(`{formula,cask}_names.before.txt`) to compare against so we don't output
anything and just copy the files for next time.

Library/Homebrew/cmd/update-report.rb

@@ -197,6 +197,40 @@ def output_update_report
       end
     end
 
+    # If we're installing from the API: we cannot use Git to check for #
+    # differences in packages so instead use {formula,cask}_names.txt to do so.
+    # The first time this runs: we won't yet have a base state
+    # ({formula,cask}_names.before.txt) to compare against so we don't output a
+    # anything and just copy the files for next time.
+    unless Homebrew::EnvConfig.no_install_from_api?
+      api_cache = Homebrew::API::HOMEBREW_CACHE_API
+      core_tap = CoreTap.instance
+      cask_tap = Tap.fetch("homebrew/cask")
+      [
+        [:formula, core_tap, core_tap.formula_dir],
+        [:cask,    cask_tap, cask_tap.cask_dir],
+      ].each do |type, tap, dir|
+        names_txt = api_cache/"#{type}_names.txt"
+        next unless names_txt.exist?
+
+        names_before_txt = api_cache/"#{type}_names.before.txt"
+        if names_before_txt.exist?
+          reporter = Reporter.new(
+            tap,
+            api_names_txt:        names_txt,
+            api_names_before_txt: names_before_txt,
+            api_dir_prefix:       dir,
+          )
+          if reporter.updated?
+            updated_taps << tap.name
+            hub.add(reporter, auto_update: args.auto_update?)
+          end
+        end
+
+        FileUtils.cp names_txt, names_before_txt
+      end
+    end
+
     unless updated_taps.empty?
       auto_update_header args: args
       noun = Utils.pluralize("tap", updated_taps.count)
@@ -278,9 +312,7 @@ def install_core_tap_if_necessary
     return if ENV["HOMEBREW_UPDATE_TEST"]
     return unless Homebrew::EnvConfig.no_install_from_api?
     return if Homebrew::EnvConfig.automatically_set_no_install_from_api?
-
-    core_tap = CoreTap.instance
-    return if core_tap.installed?
+    return if CoreTap.instance.installed?
 
     CoreTap.ensure_installed!
     revision = core_tap.git_head
@@ -314,18 +346,23 @@ def initialize(var_name)
     end
   end
 
-  attr_reader :tap, :initial_revision, :current_revision
-
-  def initialize(tap)
+  def initialize(tap, api_names_txt: nil, api_names_before_txt: nil, api_dir_prefix: nil)
     @tap = tap
 
-    initial_revision_var = "HOMEBREW_UPDATE_BEFORE#{tap.repo_var}"
-    @initial_revision = ENV[initial_revision_var].to_s
-    raise ReporterRevisionUnsetError, initial_revision_var if @initial_revision.empty?
+    # This is slightly involved/weird but all the #report logic is shared so it's worth it.
+    if installed_from_api?(api_names_txt, api_names_before_txt, api_dir_prefix)
+      @api_names_txt = api_names_txt
+      @api_names_before_txt = api_names_before_txt
+      @api_dir_prefix = api_dir_prefix
+    else
+      initial_revision_var = "HOMEBREW_UPDATE_BEFORE#{tap.repo_var}"
+      @initial_revision = ENV[initial_revision_var].to_s
+      raise ReporterRevisionUnsetError, initial_revision_var if @initial_revision.empty?
 
-    current_revision_var = "HOMEBREW_UPDATE_AFTER#{tap.repo_var}"
-    @current_revision = ENV[current_revision_var].to_s
-    raise ReporterRevisionUnsetError, current_revision_var if @current_revision.empty?
+      current_revision_var = "HOMEBREW_UPDATE_AFTER#{tap.repo_var}"
+      @current_revision = ENV[current_revision_var].to_s
+      raise ReporterRevisionUnsetError, current_revision_var if @current_revision.empty?
+    end
   end
 
   def report(auto_update: false)
@@ -417,7 +454,11 @@ def report(auto_update: false)
   end
 
   def updated?
-    initial_revision != current_revision
+    if installed_from_api?
+      diff.present?
+    else
+      initial_revision != current_revision
+    end
   end
 
   def migrate_tap_migration
@@ -529,11 +570,36 @@ def migrate_formula_rename(force:, verbose:)
 
   private
 
+  attr_reader :tap, :initial_revision, :current_revision, :api_names_txt, :api_names_before_txt, :api_dir_prefix
+
+  def installed_from_api?(api_names_txt = @api_names_txt, api_names_before_txt = @api_names_before_txt,
+                          api_dir_prefix = @api_dir_prefix)
+    !api_names_txt.nil? && !api_names_before_txt.nil? && !api_dir_prefix.nil?
+  end
+
   def diff
-    Utils.popen_read(
-      "git", "-C", tap.path, "diff-tree", "-r", "--name-status", "--diff-filter=AMDR",
-      "-M85%", initial_revision, current_revision
-    )
+    @diff ||= if installed_from_api?
+      # Hack `git diff` output with regexes to look like `git diff-tree` output.
+      # Yes, I know this is a bit filthy but it saves duplicating the #report logic.
+      diff_output = Utils.popen_read("git", "diff", api_names_txt, api_names_before_txt)
+      header_regex = /^(---|\+\+\+) /.freeze
+      add_delete_characters = ["+", "-"].freeze
+
+      diff_output.lines.map do |line|
+        next if line.match?(header_regex)
+        next unless add_delete_characters.include?(line[0])
+
+        line.sub(/^\+/, "A #{api_dir_prefix.basename}/")
+            .sub(/^-/,  "D #{api_dir_prefix.basename}/")
+            .sub(/$/,   ".rb")
+            .chomp
+      end.compact.join("\n")
+    else
+      Utils.popen_read(
+        "git", "-C", tap.path, "diff-tree", "-r", "--name-status", "--diff-filter=AMDR",
+        "-M85%", initial_revision, current_revision
+      )
+    end
   end
 end