Lazily load db of type DBM instance variable for DatabaseCache so the corresponding database file isn't created in the .use block for a DatabaseCache.

Library/Homebrew/cache_store.rb

@@ -14,21 +14,51 @@ class DatabaseCache
 
   def self.use(type)
     return_value = nil
-    DatabaseCache.new(type) { |db| return_value = yield(db) }
+
+    DatabaseCache.new(type) do |database_cache|
+      return_value = yield(database_cache)
+    end
+
     return_value
   end
 
+  # Lazily loaded database in read/write mode. If this method is called, a
+  # database file with be created in the `HOMEBREW_CACHE` with name
+  # corresponding to the `@type` instance variable
+  #
+  # @return [DBM] db
+  def db
+    # DBM::WRCREAT: Creates the database if it does not already exist
+    @db ||= DBM.open(cache_path, DATABASE_MODE, DBM::WRCREAT)
+  end
+
+  # Returns `true` if the cache is empty for the given `@type`
+  #
+  # @return [Boolean]
+  def empty?
+    !File.exist?(cache_path)
+  end
+
   private
 
-  # Opens and yields a database in read/write mode. Closes the database after use
+  # Opens and yields the cache. Closes the database after use if it has been
+  # loaded
   #
-  # @yield  [DBM] db
+  # @param  [Symbol] type
+  # @yield  [DatabaseCache] self
   # @return [nil]
-  def initialize(name)
-    # DBM::WRCREAT: Creates the database if it does not already exist
-    @db = DBM.open("#{HOMEBREW_CACHE}/#{name}.db", DATABASE_MODE, DBM::WRCREAT)
-    yield(@db)
-    @db.close
+  def initialize(type)
+    @type = type
+    yield(self)
+    @db&.close
+  end
+
+  # The path where the database resides in the `HOMEBREW_CACHE` for the given
+  # `@type`
+  #
+  # @return [String]
+  def cache_path
+    File.join(HOMEBREW_CACHE, "#{@type}.db")
   end
 end
 
@@ -37,10 +67,10 @@ end
 # storage mechanism
 #
 class CacheStore
-  # @param  [DBM] database_cache
+  # @param  [DBM] db
   # @return [nil]
-  def initialize(database_cache)
-    @database_cache = database_cache
+  def initialize(db)
+    @db = db
   end
 
   # Inserts new values or updates existing cached values to persistent storage
@@ -69,7 +99,7 @@ class CacheStore
   protected
 
   # @return [DBM]
-  attr_reader :database_cache
+  attr_reader :db
 
   # DBM stores ruby objects as a ruby `String`. Hence, when fetching the data,
   # to convert the ruby string back into a ruby `Hash`, the string is converted

Library/Homebrew/dev-cmd/linkage.rb

@@ -25,7 +25,7 @@ module Homebrew
         ohai "Checking #{keg.name} linkage" if ARGV.kegs.size > 1
 
         use_cache = ARGV.include?("--cached") || ENV["HOMEBREW_LINKAGE_CACHE"]
-        result = LinkageChecker.new(keg, database_cache, use_cache)
+        result = LinkageChecker.new(keg, database_cache.db, use_cache)
 
         if ARGV.include?("--test")
           result.display_test_output

Library/Homebrew/extend/os/mac/formula_cellar_checks.rb

@@ -68,7 +68,7 @@ module FormulaCellarChecks
 
     DatabaseCache.use(:linkage) do |database_cache|
       use_cache = !ENV["HOMEBREW_LINKAGE_CACHE"].nil?
-      checker = LinkageChecker.new(keg, database_cache, use_cache, formula)
+      checker = LinkageChecker.new(keg, database_cache.db, use_cache, formula)
 
       next unless checker.broken_library_linkage?
       output = <<~EOS

Library/Homebrew/formula.rb

@@ -1529,7 +1529,7 @@ class Formula
 
     undeclared_deps = DatabaseCache.use(:linkage) do |database_cache|
       use_cache = !ENV["HOMEBREW_LINKAGE_CACHE"].nil?
-      linkage_checker = LinkageChecker.new(keg, database_cache, use_cache, self)
+      linkage_checker = LinkageChecker.new(keg, database_cache.db, use_cache, self)
       linkage_checker.undeclared_deps.map { |n| Dependency.new(n) }
     end
 

Library/Homebrew/formula_installer.rb

@@ -613,7 +613,7 @@ class FormulaInstaller
     # Updates the cache for a particular formula after doing an install
     DatabaseCache.use(:linkage) do |database_cache|
       break if database_cache.empty?
-      LinkageChecker.new(keg, database_cache, false, formula)
+      LinkageChecker.new(keg, database_cache.db, false, formula)
     end
 
     # let's reset Utils.git_available? if we just installed git

Library/Homebrew/linkage_cache_store.rb

@@ -11,11 +11,11 @@ class LinkageStore < CacheStore
   HASH_LINKAGE_TYPES  = [:brewed_dylibs, :reverse_links, :broken_deps].freeze
 
   # @param  [String] keg_name
-  # @param  [DBM]    database_cache
+  # @param  [DBM]    db
   # @return [nil]
-  def initialize(keg_name, database_cache)
+  def initialize(keg_name, db)
     @keg_name = keg_name
-    super(database_cache)
+    super(db)
   end
 
   # Inserts dylib-related information into the cache if it does not exist or
@@ -40,7 +40,7 @@ class LinkageStore < CacheStore
       end
     end
 
-    database_cache[keg_name] = ruby_hash_to_json_string(
+    db[keg_name] = ruby_hash_to_json_string(
       array_values: format_array_values(array_values),
       hash_values: format_hash_values(hash_values),
     )
@@ -64,7 +64,7 @@ class LinkageStore < CacheStore
 
   # @return [nil]
   def flush_cache!
-    database_cache.delete(keg_name)
+    db.delete(keg_name)
   end
 
   private
@@ -75,15 +75,15 @@ class LinkageStore < CacheStore
   # @param  [Symbol] the type to fetch from the `LinkageStore`
   # @return [Array]
   def fetch_array_values(type)
-    return [] unless database_cache.key?(keg_name)
-    json_string_to_ruby_hash(database_cache[keg_name])["array_values"][type.to_s]
+    return [] unless db.key?(keg_name)
+    json_string_to_ruby_hash(db[keg_name])["array_values"][type.to_s]
   end
 
   # @param  [Symbol] type
   # @return [Hash]
   def fetch_hash_values(type)
-    return {} unless database_cache.key?(keg_name)
-    json_string_to_ruby_hash(database_cache[keg_name])["hash_values"][type.to_s]
+    return {} unless db.key?(keg_name)
+    json_string_to_ruby_hash(db[keg_name])["hash_values"][type.to_s]
   end
 
   # Formats the linkage data for `array_values` into a kind which can be parsed