SVN Source control issues when merging changes

Question

I have seen where changes have been made on one code file by two developers code like this:

x++

End up like this:

x++
x++

where due to carriage returns being inserted/removed (I think) one line has become silently merged as two lines of the same code (no conflicts) Everything compiles but suddenly tests are failing and weird behaviour ensues.

Should this be possible? How do I guard against it?

Answer 1

It certainly should not be possible. SVN merge usually recognizes that the same change has already been made locally. Even if line-breaks change, it should at least recognize the changed context and fail.

You can guard against this by checking the diffs before actually committing, and by automatic tests (as you already did).

Can you reproduce this behavior?

Answer 2

I've seen this where merges have gone wrong, and somebody has resolved the conflict, deleted the two merge versions and committed the main file which has still got both versions merged together (as you are supposed to tell SVN which is correct.)

Answer 3

To avoid merging problems due to line endings, just set the svn:eol-style property on those files.

Answer 4

This shouldn't be a problem, unless the developer doing the merge marks the conflict as resolved without reviewing it. SVN will always warn about a conflict.

Careful merge tracking, which is required anyway, should avoid any problems.

Also, a small test shows that SVN is smart enough to avoid conflicting if the changes being merged have been already applied already.

The following example (warning, messes up with the current directory; requires Unix-like tools) simulates the situation you just described.

# Initialize repository
svnadmin create repo
REPO_URL="file:///$PWD/repo"
svn mkdir "$REPO_URL/trunk" "$REPO_URL/branches" -m "Initialize repository structure"

# Add main program
svn co "$REPO_URL" wc1
cd wc1/trunk
cat > main.pl << "EOF"
my $x=0;
print("$x\n");
EOF
svn add main.pl
svn ci -m "Add main.pl"
cd ../..

# Create branch
svn cp "$REPO_URL/trunk" "$REPO_URL/branches/exp" -m "Create \"exp\" branch"

# Branch developer makes a change
svn co "$REPO_URL" wc2
cd wc2/branches/exp
perl -i -wpe 'print("\$x++;\n") if $. == 2' main.pl
svn ci -m "Increment x"
cd ../../..

# Trunk developer makes the same change
cd wc1/trunk
perl -i -wpe 'print("\$x++;\n") if $. == 2' main.pl
svn ci -m "Increment x"

# Merge changes from branch
svn up
svn merge --reintegrate "$REPO_URL/branches/exp" .
cat main.pl