1 Issues in Version Control

The issues addressed by version control are:

• History

  • How has the software changed since date-or-version-number? Who made those changes? Why were they made? Can we go back?

• Exploration

  • Can we try out a set of plausible changes without affecting the “main” software build? Even if exploration of the effects of those changes may take a long time?

• Collaboration

  • Can we have multiple developers working on the code without interfering with one another’s work?

2 Background - Changing the Code Base

ed

One of the earliest Unix text editors, ed applies a series of editing commands like ‘a’ to append to th end of a file, ‘i’ to insert a line at the current location, ‘d’ to delete the current line, etc.

• Few people use ed now
  • though its line-oriented “child”, sed is still a popular scripting tool.

diff compares two files line by line, listing the differences between them.

• Differences are listed as a series of line replacements, insertions, and/or deletions

  • Reduces each line to a hash code
  • Uses the dynamic programming algorithm for computing the Levenshtein distance between the two sequences of hash codes to obtain an approximately shortest sequence of commands

• Can emit differences as ed commands:

diff --ed file1 file2 > file12.diff
echo w file2 >> file12.diff
#
#  many days later...
#
ed file1 < file12.diff

would “rebuild” file2 from file1 and the diff.

patch takes a slightly more sophisticated approach to the idea of applying a diff output to a file

diff file1 file2 > file12.diff
  ⋮
patch file1 file12.diff

• Allows a variety of different diff variants

• Can detect if file1 has already been changed so that the line numbers and other info in the patch file file12.diff are no longer accurate.

  • Attempts to compensate

2.1 Integrating Changes

Suppose that we have two patch files created from the same base file file1

patch -o file2a file1 patchA
patch -o file2b file1 patchB

Change integration is the problem of combining both sets of changes to form a desired file file2.

patch -o file2a file1 patchA
patch -o file2b file1 patchB

How do we know which patched file is “correct”, or whether we need some combination of the changes?

Two-way procedure:

1. Compare file2a and file2b
2. Wherever the two are different, prompt the human to select the desired change.

Takes the base file into account as well as the two changed files.

patch -o file2a file1 patchA
patch -o file2b file1 patchB

Three-way procedure:

1. Compare file1 and file2a, then file1 and file2b
2. Any lines that differ from file1 in only one of the two other files can be applied automatically.
3. Wherever both file2a and file2b are different from file1, prompt the human to select the desired change.
   • This is called a conflict.

3 Approaches and Tools

Version Control Systems

If we could extend patch multiple files at once, we could, in theory, patch an entire software system to move it from version 1 to version 2, then patch it again to move to version 3, etc.

• This would be the heart of a version control system,

Version control systems distinguish between

• working directories: where programmers edit, modify and otherwise work with their source code, and
• repositories: where the history of changes to the source code is kept.

• Local version control systems manage history by setting aside directories on the same file system where the software under control is housed.

• One repository, one working directory

  • sccs, rcs

• Centralized version control systems keep the system history at a centralized location accessible via the network.

  Developers check out a copy of the current (or a desired older) version of the software onto their own machines.

• One repository, multiple working directories

  • CVS, Subversion

• Distributed version control systems allow developers to keep the full system history on their own machines.

  A central location may hold a base copy for management/distribution purposes, but this is not required.

• Many repositories, each with zero or one working directory

  • git