Social Interactions around Cross-System Bug Fixings: the Case of FreeBSD and OpenBSD

1. Social Interactions around
Cross-System Bug Fixings:
The Case of
FreeBSD and OpenBSD
Gerardo Canfora, Luigi Cerulo,
Marta Cimitile, Massimiliano Di Penta
dipenta@unisannio.it

2. Context
 Source code is often reused across different systems
 Unixes (FreeBSD, OpenBSD, Linux)
 Office applications (NeoOffice, OpenOffice)
 Desktop environment apps (KDE or GNOME apps)
 Maintenance might require to propagate bug fixings
 We call this “Cross System Bug Fixing” (CSBF)
 Example:
 FreeBSD, 1996/01/19, file ip_icmp.h:
– “Added definitions for ICMP router discovery. Reviewed by:
wollman
 OpenBSD, 1996/08/02, file ip_icmp.h:
– “ICMP Router Discovery definitions; from FreeBSD”

3. What we propose
 A method to track CSBFs
 A study on the social characteristics
and development activity made by
CSBF committers
 degree, betweenness, brokerage
 commits, lines changed

4. Detecting CSBF - I
 Step 1: mining cross-referencing commits
 openbsd, atphy.c,2008/09/25 20:47:16,brad,
Add a driver for the Attansic F1 PHY. From FreeBSD via
kevlo@
 Step 2: mine commits previously performed on files
with same name in the other system
 freebsd,atphy.c,2008/05/19 01:12:10,yongari,
Add Attansic/Atheros F1 PHY driver.
 openbsd, atphy.c,2008/09/25 20:47:16,brad,
Add a driver for the Attansic F1 PHY. From FreeBSD via
kevlo@

5. Detecting CSBF - II
 Step 3: compute file similarity with clone detection
 CCFinder
 Threshold: at least 10% of cloned lines
 Step 4: take the previous change with the highest
textual similarity in the commit note
 Use of Vector Space models
 Cosine similarity; threshold (0.20) to filter out unrelated
commits
Add Attansic/Atheros F1 PHY driver.
= 0.72
Add a driver for the Attansic F1 PHY. From FreeBSD via kevlo@

6. Building Committers' Network
 We extract communication from mailing
lists
 Bug fixing mailing lists
 Heuristic similar to the one of Bird et al.
[2006] to map inconsistent namings /
emails
 Also, to map committer Ids to mailing list
names/emails
 Nodes of the network labeled as:
 Committer / other mailing list contributors
 CSBFs committer

7. Empirical Study
 Goal: analyze the phenomenon of CSBFs
 Purpose: understanding its relevance with
respect to the social characteristics of the
involved developers
 Context: CVS repositories and mailing lists
archives of FreeBSD and OpenBSD
 Period: 1993-2009 (FreeBSD), 1998-2009
(OpenBSD)
 Commits: 119,000 (FreeBSD), 70,000 (OpenBSD)

8. Research Questions
 RQ1: How do the source code committers
and contributors of the two systems
overlap?
 RQ2: How frequent is the phenomenon of
CSBFs?
 RQ3: Who are the contributors involved in
CSBFs?
 RQ4: Are mailing list contributors involved
in CSBFs more active than others?

9. RQ1 – Team overlap
FreeBSD OpenBSD Both
Committers 383 211 26
Mailing list contribs 8035 3843 359
Committers and 213 122 17
mailing list contributors
The two projects have less than 10% of
common contributors →
the development team of Free and
Open BSD is really different

10. RQ2 – Commit filtering
1000 933
900
800
700
600
500 439
400
296
300
200 133 120
100
59
0
FreeBSD OpenBSD
Referring commits Cloned files Linked commits
At the end of the filtering not that many but...

11. RQ2 – Cloned lines in CSBF files
C source files header files
 Percentage smaller for .h files
 Use of preprocessor conditional to make header files system-
dependent
 #if defined(__FreeBSD__)

12. RQ3 – CSBF Graph (excerpt)
Blue/cyan: FreeBSD
Red/orange: OpenBSD
Yellow: common

13. RQ3: social characteristics
 Importance in terms of
 (in/out) degree: number of (incoming/outcoming)
communication links
 Betweenness: number of communications for which the
node is in the short path
 Brokerage metrics: useful to analyze the
communication between two clusters
B is a coordinator
B is a gatekeeper
B is a representative

14. RQ3 – social characteristics
Representative
Gatekeeper
12
Coordinator /10
10
Betweenness / 1000
8
Out-degree
Column 1
6
In-degree Column 2
Column 3
4
Degree
2 0 5 10 15 20 25 30 35 40 45 50
0
Row 1 CSBF
Row 2 Others
Row 3 Row 4
 All differences statistically significant
 High effect size (Cohen d>1)
 Contributors involved in CSBF have a higher importance in
the communication and in the flow of communication
between systems

15. RQ3 – committers with highest
social metrics

16. RQ4 – change activity of CSBF
committers and others
LOC added/removed Commits
40000 1500
1000
20000
500
0 0
FreeBSD OpenBSD FreeBSD OpenBSD
CSBF Others CSBF Others
 All differences statistically significant
 High effect size (Cohen d∼1)
 Contributors involved in CSBF are more active
than others

17. Conclusions and Work-in-Progress
 We proposed method to mine CSBF
 We reported a study on FreeBSD and OpenBSD where:
 Development team is almost disjoint
 There is a small, though not negligible portion of CSBF
 Committers involved in CSBF have
– Higher social importance
– Higher brokerage level
– Higher activity in source code commits
 Work-in-progress:
 Better approaches to identify implicit CSBF, tracking and
linking changes occurring on both systems
 More extensive study on less obvious cases