[gparted] Increase PipeCapture maximum read size to 64K (#777973)

[Curtis Gedak <gedakc src gnome org>]

commit 6f49f3049dc8fb72ef771074db73b60a79925962
Author: Mike Fleetwood <mike fleetwood googlemail com>
Date:   Tue Mar 28 07:27:29 2017 +0100

    Increase PipeCapture maximum read size to 64K (#777973)
    
    For large output PipeCapture spends all it's time copying the capturebuf
    to callerbuf to provide a consistent view for any registered update
    callbacks.  This overhead is dependant of the size of the ever growing
    captured output and the number of times OnReadable() is called.
    Therefore increase the maximum read size to exponentially reduce this
    overhead.
    
    Time taken to read varying amounts of fsck.fat output with various
    read buffer sizes:
    
                     1 MiB   10 MiB            122 MiB
        512b  :   0.60 sec   65 sec [1:05]   17262 sec [4:47:42]
        4096b :   0.19 sec   13 sec           2157 sec [  35:57]
        64K   :   0.07 sec    1.4 sec          210 sec [   3:30]
    
    Note that this is only increasing the maximum size that can be read from
    the output of the external command.  If the command produces it's output
    slowly, such as the with progress reporting commands like mkfs.ext4,
    then only the available number of bytes is read reporting the next
    progress increment.  However if the command produces it's output
    quickly, such as when testing this bug using a modified fsck.fat
    concatenating the 122 MiB of pre-recorded output, then full buffer reads
    are performed.
    
    To ensure that a single call to OnReadable() couldn't block the UI too
    long, the time taken for OnReadable() to process a full buffer of
    various sizes was recorded as:
    
        512b  :   0.031 milliseconds
        4096b :   0.188 milliseconds
        64K   :   3.576 milliseconds
    
    Adding this amount of processing time in the UI under normal
    circumstances is not a problem.
    
    As the captured output increases, the time taken by OnReadable() becomes
    dominated by the time taken to copy the ever increasing capture buffer
    to handle it's expansion and to copy it to the caller buffer for the
    update callback.  At the end of the 122M captured fsck.fat output
    OnReadable() takes 350 milliseconds per call.  This is not a problem
    because this is an extreme case in which GParted is already hung and
    increasing the buffer size is reducing the overall hang time from over
    4 hours to a few minutes.
    
    Bug 777973 - Segmentation fault on bad disk

 src/PipeCapture.cc |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/src/PipeCapture.cc b/src/PipeCapture.cc
index 3579b2f..053c3b2 100644
--- a/src/PipeCapture.cc
+++ b/src/PipeCapture.cc
@@ -15,6 +15,7 @@
  */
 
 #include "PipeCapture.h"
+#include "Utils.h"
 
 #include <iostream>
 #include <string>
@@ -27,7 +28,7 @@
 
 namespace GParted {
 
-const size_t READBUF_SIZE = 512;
+const size_t READBUF_SIZE = 64*KIBIBYTE;
 
 PipeCapture::PipeCapture( int fd, Glib::ustring &buffer ) : fill_offset( 0 ),
                                                             cursor( 0 ),