Synchronous mode implies fsync(), which makes updates *much*
slower on some machines, such as the DrDr machine. The doc
database doesn't need to survive a catastrophic failue
(such as a power failure or OS crash), so turn synchronous
mode off.
Change `raco setup' to guard database writes in different places
by an explicit lock that is implemented by place channels. Change
corss-reference reading to fall back to just loading ".sxref"
files if the database seems to be too contended at that point.
These changes are aimed at avoiding distaerous performance when
SQLite seems not to behave well.
Also, fix break and other exception handling near the "fast abort"
path for both reads and writes.
This change makes document building --- and specially incremental
document building --- more scalable. The global duplicate-definition
check is handled by a database query, for example.
For example, the cross-reference information for the
Reference is now broken into about 16 pieces, so that
resolving a cross-reference into the Reference doesn't
require loading all cross-reference information for
the Reference.
Every document is split into two pieces, so that the title
of a document is roughly in its own piece. That way,
re-building the page of all installed documentation can be more
scalable (after some further changes).
The `xref' produced by `setup/xref' uses the database to delay
loading "out.sxref"s, which cuts 64-bit DrRacket's initial
footprint by around 50MB (i.e., about 20%).
