Add IO#read_greedy

spec/std/io/io_spec.cr

@@ -486,6 +486,17 @@ describe IO do
         IO.same_content?(io1, io2).should be_false
       end
     end
+
+    it "combines multiple reads using #read_greedy" do
+      bytes = Bytes.new 7
+
+      io = SimpleIOMemory.new("Hello World", max_read: 2)
+      io.read_greedy(bytes).should eq(7)
+      bytes.should eq("Hello W".to_slice)
+      io.read_greedy(bytes).should eq(4)
+      bytes[0, 4].should eq("orld".to_slice)
+      io.read_greedy(bytes).should eq(0)
+    end
   end
 
   describe "write operations" do

src/io.cr

@@ -523,6 +523,9 @@ abstract class IO
   # slice                # => Bytes[49, 50, 51, 52, 53]
   # io.read_fully(slice) # raises IO::EOFError
   # ```
+  #
+  # `#read_greedy` also tries to fill the entire buffer if possible,
+  # but still allows the partially filled slice to be used if an early EOF was reached.
   def read_fully(slice : Bytes) : Int32
     read_fully?(slice) || raise(EOFError.new)
   end
@@ -538,11 +541,34 @@ abstract class IO
   # slice                 # => Bytes[49, 50, 51, 52, 53]
   # io.read_fully?(slice) # => nil
   # ```
+  #
+  # `#read_greedy` also tries to fill the entire buffer if possible,
+  # but still allows the partially filled slice to be used if an early EOF was reached.
   def read_fully?(slice : Bytes) : Int32?
+    count = read_greedy(slice)
+    return nil if count != slice.size
+    count
+  end
+
+  # Similar to `#read`, but with the additional guarantee that either
+  # the buffer will be entirely filled or the EOF will be reached while trying.
+  #
+  # Calling this method may result in multiple calls to `#read` if necessary.
+  #
+  # ```
+  # io = IO::Memory.new "123451234"
+  # slice = Bytes.new(5)
+  # io.read_greedy(slice) # => 5
+  # slice                 # => Bytes[49, 50, 51, 52, 53]
+  # io.read_greedy(slice) # => 4
+  # ```
+  #
+  # `#read_fully` and `#read_fully?` also try to fill the entire buffer but error on unexpected EOF.
+  def read_greedy(slice : Bytes) : Int32
     count = slice.size
     while slice.size > 0
-      read_bytes = read slice
-      return nil if read_bytes == 0
+      read_bytes = read(slice)
+      return count &- slice.size if read_bytes == 0
       slice += read_bytes
     end
     count