Fewer allocations in gem installation

For now, on a small rails app I have hanging around:

```
==> memprof.after.txt <==
Total allocated: 872.51 MB (465330 objects)
Total retained:  40.48 kB (326 objects)

==> memprof.before.txt <==
Total allocated: 890.79 MB (1494026 objects)
Total retained:  40.40 kB (328 objects)
```

Not a huge difference in memory usage, but it's a drastic improvement
in total number of allocations.

Additionally, this will pay huge dividends once
ruby/zlib#61 is merged, as it will allow us to
completely avoid allocations in the repeated calls to readpartial,
which currently accounts for most of the memory usage shown above.

lib/rubygems/installer.rb

@@ -224,11 +224,11 @@ def check_executable_overwrite(filename) # :nodoc:
 
     File.open generated_bin, "rb" do |io|
       line = io.gets
-      shebang = /^#!.*ruby/
+      shebang = /^#!.*ruby/o
 
       # TruffleRuby uses a bash prelude in default launchers
       if load_relative_enabled? || RUBY_ENGINE == "truffleruby"
-        until line.nil? || line =~ shebang do
+        until line.nil? || shebang.match?(line) do
           line = io.gets
         end
       end
@@ -239,7 +239,7 @@ def check_executable_overwrite(filename) # :nodoc:
 
       # TODO: detect a specially formatted comment instead of trying
       # to find a string inside Ruby code.
-      next unless io.gets.to_s.include?("This file was generated by RubyGems")
+      next unless io.gets&.include?("This file was generated by RubyGems")
 
       ruby_executable = true
       existing = io.read.slice(/

lib/rubygems/package.rb

@@ -357,18 +357,21 @@ def contents
 
   def digest(entry) # :nodoc:
     algorithms = if @checksums
-      @checksums.keys
-    else
-      [Gem::Security::DIGEST_NAME].compact
+      @checksums.to_h {|algorithm, _| [algorithm, Gem::Security.create_digest(algorithm)] }
+    elsif Gem::Security::DIGEST_NAME
+      { Gem::Security::DIGEST_NAME => Gem::Security.create_digest(Gem::Security::DIGEST_NAME) }
     end
 
-    algorithms.each do |algorithm|
-      digester = Gem::Security.create_digest(algorithm)
-
-      digester << entry.readpartial(16_384) until entry.eof?
+    return @digests if algorithms.nil? || algorithms.empty?
 
-      entry.rewind
+    buf = String.new(capacity: 16_384, encoding: Encoding::BINARY)
+    until entry.eof?
+      entry.readpartial(16_384, buf)
+      algorithms.each_value {|digester| digester << buf }
+    end
+    entry.rewind
 
+    algorithms.each do |algorithm, digester|
       @digests[algorithm][entry.full_name] = digester
     end
 
@@ -437,8 +440,6 @@ def extract_tar_gz(io, destination_dir, pattern = "*") # :nodoc:
 
         FileUtils.rm_rf destination
 
-        mkdir_options = {}
-        mkdir_options[:mode] = dir_mode ? 0o755 : (entry.header.mode if entry.directory?)
         mkdir =
           if entry.directory?
             destination
@@ -447,7 +448,7 @@ def extract_tar_gz(io, destination_dir, pattern = "*") # :nodoc:
           end
 
         unless directories.include?(mkdir)
-          FileUtils.mkdir_p mkdir, **mkdir_options
+          FileUtils.mkdir_p mkdir, mode: dir_mode ? 0o755 : (entry.header.mode if entry.directory?)
           directories << mkdir
         end
 
@@ -707,6 +708,7 @@ def verify_files(gem)
 
   def verify_gz(entry) # :nodoc:
     Zlib::GzipReader.wrap entry do |gzio|
+      # TODO: read into a buffer once zlib supports it
       gzio.read 16_384 until gzio.eof? # gzip checksum verification
     end
   rescue Zlib::GzipFile::Error => e

lib/rubygems/package/tar_header.rb

@@ -127,7 +127,8 @@ def self.from(stream)
   end
 
   def self.strict_oct(str)
-    return str.strip.oct if /\A[0-7]*\z/.match?(str.strip)
+    str.strip!
+    return str.oct if /\A[0-7]*\z/.match?(str)
 
     raise ArgumentError, "#{str.inspect} is not an octal string"
   end
@@ -137,7 +138,8 @@ def self.oct_or_256based(str)
     # \ff flags a negative 256-based number
     # In case we have a match, parse it as a signed binary value
     # in big-endian order, except that the high-order bit is ignored.
-    return str.unpack("N2").last if /\A[\x80\xff]/n.match?(str)
+
+    return str.unpack1("@4N") if /\A[\x80\xff]/n.match?(str)
     strict_oct(str)
   end
 
@@ -149,21 +151,23 @@ def initialize(vals)
       raise ArgumentError, ":name, :size, :prefix and :mode required"
     end
 
-    vals[:uid] ||= 0
-    vals[:gid] ||= 0
-    vals[:mtime] ||= 0
-    vals[:checksum] ||= ""
-    vals[:typeflag] = "0" if vals[:typeflag].nil? || vals[:typeflag].empty?
-    vals[:magic] ||= "ustar"
-    vals[:version] ||= "00"
-    vals[:uname] ||= "wheel"
-    vals[:gname] ||= "wheel"
-    vals[:devmajor] ||= 0
-    vals[:devminor] ||= 0
-
-    FIELDS.each do |name|
-      instance_variable_set "@#{name}", vals[name]
-    end
+    @checksum = vals[:checksum] || ""
+    @devmajor = vals[:devmajor] || 0
+    @devminor = vals[:devminor] || 0
+    @gid = vals[:gid] || 0
+    @gname = vals[:gname] || "wheel"
+    @linkname = vals[:linkname]
+    @magic = vals[:magic] || "ustar"
+    @mode = vals[:mode]
+    @mtime = vals[:mtime] || 0
+    @name = vals[:name]
+    @prefix = vals[:prefix]
+    @size = vals[:size]
+    @typeflag = vals[:typeflag]
+    @typeflag = "0" if @typeflag.nil? || @typeflag.empty?
+    @uid = vals[:uid] || 0
+    @uname = vals[:uname] || "wheel"
+    @version = vals[:version] || "00"
 
     @empty = vals[:empty]
   end