Question

When I use Subversion in Cygwin to update some repository, some directories update with success, while some other one gets a failure with the error message:

svn: E200030: sqlite: disk I/O error

When doing svn update again for the same repository, a different directory can get the same error. Sometimes, there is a SVN instruction after the above error message.

Answer 1

This happened due to a change someone wanted in Cygwin’s SQLite package. I was the maintainer of that package when this question was asked, and I made the change that caused this symptom.

The change was released as Cygwin SQLite version 3.7.12.1-1, and it fixed that one person’s problem, but it had this bad side effect of preventing Cygwin’s Subversion package from cooperating with native Windows Subversion implementations.

What Happened?

The core issue here is that Subversion 1.7 changed the working copy on-disk format. Part of that change involves a new SQLite database file, .svn/wc.db. Now, in order to implement SQLite’s concurrency guarantees, SQLite locks the database file while it is accessing it.

That’s all fine and sensible, but you run into a problem when you try to mix Windows native and POSIX file locking semantics. On Windows, file locking almost always means mandatory locking, but on Linux systems — which Cygwin is trying to emulate — locking usually means advisory locking instead.

That helps understand where the “disk I/O error” comes from.

The Cygwin SQLite 3.7.12.1-1 change was to build the library in “Unix mode” instead of “Cygwin mode.” In Cygwin mode, the library uses Windows native file locking, which goes against the philosophy of Cygwin: where possible, Cygwin packages call POSIX functions instead of direct to the Windows API, so that cygwin1.dll can provide the proper POSIX semantics.

POSIX advisory file locking is exactly what you want with SQLite when all the programs accessing the SQLite DBs in question are built with Cygwin, which is the default assumption within Cygwin. But, when you run a Windows native Subversion program like TortoiseSVN alongside a pure POSIX Cygwin svn, you get a conflict. When the TortoiseSVN Windows Explorer shell extension has the .svn/wc.db file locked with a mandatory lock and Cygwin svn comes along and tries an advisory lock on it, it fails immediately. Cygwin svn assumes a lock attempt will either succeed immediately or block until it can succeed, so it incorrectly interprets the lock failure as a disk I/O error.

How Did We Solve This Dilemma?

Within Cygwin, we always try to play nice with Windows native programs where possible. The trick was to find a way to do that, while still playing nice with Cygwin programs, too.

Not everyone agreed that we should attempt this. “Cygwin SQLite is part of Cygwin, so it only needs to work well with other Cygwin programs,” one group would say. The counterpartisans would reply, “Cygwin runs on Windows, so it has to perform well with other Windows programs.”

Fortunately, we came up with a way to make both groups happy.

As part of the Cygwin SQLite 3.7.17-x packaging effort, I tested a new feature that Corinna Vinschen added to cygwin1.dll version 1.7.19. It allowed a program to request mandatory file locking through the BSD file locking APIs. My part of the change was to make Cygwin SQLite turn this feature on and off at the user’s direction, allowing the same package to meet the needs of both the Cygwin-centric and Windows-native camps.

This Cygwin DLL feature was further improved in 1.7.20, and I released Cygwin SQLite 3.7.13-3 using the finalized locking semantics. This version allowed a choice of three locking strategies: POSIX advisory locking, BSD advisory locking, and BSD/Cygwin mandatory locking. So far, the latter strategy has proven to be completely compatible with native Windows locking.

Later, when Jan Nijtmans took over maintenance of Cygwin SQLite, he further enhanced this mechanism by fully integrating it with the SQLite VFS layer. This allowed a fourth option: the native Windows locking that Cygwin SQLite used to use before we started on this journey. This is mostly a hedge against the possibility that the BSD/Windows locking strategy doesn’t cooperate cleanly with a native Windows SQLite program. So far as I know, no one has ever needed to use this option, but it’s nice to know it’s there.

Alternate Remedy

If the conflict you’re having is between Cygwin’s command line svn and the TortoiseSVN Windows Explorer shell extension, there’s another option to fix it. TortoiseSVN ships with native Windows Subversion command-line programs as well. If you put these in your PATH ahead of Cygwin’s bin directory, you shouldn’t run into this problem at all.