[GR-17457] Improve performance of IO#read_nonblock when no data is available.

PullRequest: truffleruby/2635

Author: aardvark179

CHANGELOG.md

@@ -33,6 +33,7 @@ Performance:
 * Reduce the number of AST nodes created for methods and blocks (#2261).
 * Fiber-local variables are much faster now by using less synchronization.
 * Improved the performance of the exceptional case of `String#chr` (#2318, @chrisseaton).
+* Improved the performance of `IO#read_nonblock` when no data is available to be read.
 
 Changes:
 

src/main/ruby/truffleruby/core/io.rb

@@ -1654,11 +1654,14 @@ def nonblock(nonblock=true)
 
   # Normally only provided by io/nonblock
   def nonblock=(value)
-    if value
-      fcntl(F_SETFL, fcntl(F_GETFL) | NONBLOCK)
-    else
-      fcntl(F_SETFL, fcntl(F_GETFL) & ~NONBLOCK)
-    end
+    old_flags = fcntl(F_GETFL)
+    new_flags = if value
+                  old_flags | NONBLOCK
+                else
+                  old_flags & ~NONBLOCK
+                end
+    fcntl(F_SETFL, new_flags) unless old_flags == new_flags
+    self
   end
 
   ##
@@ -1831,17 +1834,12 @@ def read_nonblock(size, buffer = nil, exception: true)
       return @ibuffer.shift(size)
     end
 
-    begin
-      str = Truffle::POSIX.read_string_nonblock(self, size)
-    rescue Errno::EAGAIN
-      if exception
-        raise EAGAINWaitReadable
-      else
-        return :wait_readable
-      end
-    end
+    str = Truffle::POSIX.read_string_nonblock(self, size, exception)
 
-    if str
+    case str
+    when Symbol
+      str
+    when String
       buffer ? buffer.replace(str) : str
     else # EOF
       if exception

src/main/ruby/truffleruby/core/posix.rb

@@ -366,13 +366,14 @@ def self.read_string_at_least_one_byte(io, count)
 
   # Used in IO#read_nonblock
 
-  def self.read_string_nonblock(io, count)
+  def self.read_string_nonblock(io, count, exception)
     # must call #read_string in order to properly support polyglot STDIO.
     string, errno = read_string(io, count)
     if errno == 0
       string
     elsif TRY_AGAIN_ERRNOS.include? errno
-      raise IO::EAGAINWaitReadable
+      raise IO::EAGAINWaitReadable if exception
+      :wait_readable
     else
       Errno.handle_errno(errno)
     end