
Bootstrap
Overview
The Fan compiler is written in Fan itself - which presents a chicken and the egg problem. How do you compile the compiler without having a compiler? To solve this problem, the bootstrap process requires two Fan installations:
- rel: known good Fan installation (typically the last build)
- dev: development environment to build
By convention we structure our development directory tree like this:
dev/ rel/ bin/ lib/ ... fan/ bin/ lib/ src/ ...
The "rel" directory always contains the last released build. The "fan" directory contains our main development code branch. We call our top level directory "dev", but for the purposes of this discussion "fan" is the development directory.
Dev Home
By default the build assumes devHome to be the home directory of Fan installation. For example if you rebuild jfan, then the output goes into "devHome/lib/java/sys.jar". In the case of the rel installation we don't want this default because we will overwrite ourselves (which leads to some nasty problems). So you need to set the devHome property in "lib/sys.props" of your rel installation to reference the dev directory using a URI (not OS path):
fan.build.devHome=/C:/dev/fan/
If you forget to do this, then you will likely get IO exceptions - or worse it might corrupt your rel install and you will need to do a fresh installation.
Substitutes
On a clean machine with only source code, we don't have any pods compiled such as sys
, build
, or compiler
. In order to run the build scripts to compile these pods, we need to use our rel installation.
Windows Substitutes
To make this all work seamlessly on Windows, the Fan launcher will look in "sys.props" to see if a substitute runtime should be used. For example in our environment we map the following scripts to use the rel installation:
fan.runtime.substitutes= \ /C:/dev/fan/src/buildall.fan = C:\\dev\\rel \ /C:/dev/fan/src/buildboot.fan = C:\\dev\\rel \ /C:/dev/fan/src/jfan/build.fan = C:\\dev\\rel \ /C:/dev/fan/src/sys/build.fan = C:\\dev\\rel \ /C:/dev/fan/src/compiler/build.fan = C:\\dev\\rel \ /C:/dev/fan/src/build/build.fan = C:\\dev\\rel
The launcher will check if any script being run matches one of those files. If a match is made, then it will route to the alternate runtime specified. Turn on launcher debugging to see exactly what is happening under the covers.
Unix Substitutes
Unix substitution is implemented by the bootstrap build scripts using the following shebang:
#! /usr/bin/env fansubstitute
The fansubstitute script explictly sets FAN_HOME to the value of the FAN_SUBSTITUTE variable before launching. So you will need to export FAN_SUBSTITUTE to reference your "rel" installation. And of course you have to run your scripts as executables so that the shebang takes effect.
Also note that building on Unix will skip any .NET targets.
Buildall
The "buildall.fan" script is the top level build script for compiling the Fan distribution. We commonly run this command to rebuild everything and run tests on every pod:
buildall full test
The "buildall" script is executed by the rel substitute runtime and in turn launches two sub-scripts. The "buildboot.fan" script manages rebuilding the core runtime modules:
sys/build.fan jfan/build.fan nfan/build.fan compiler/build.fan build/build.fan
Once the bootstrap modules are compiled, the development environment is self hosting and can be used to compile the remainder of itself. This is done via the "buildpods.fan" script.
Dependencies
The bootstrap issue can cause some confusing dependency issues which are summarized here:
- The rel compiler will be generating the
sys
,compiler
, andbuild
pod files. This means that the rel compiler must be able to generate fcode that the dev runtime can read. It also means that the rel compiler must be able to read any new syntax used by dev versions ofsys
,compiler
, andbuild
. - The rel compiler will actually use the dev versions of the pods to resolve dependencies. For example dev compiler can reference new sys APIs defined in dev but not rel. Under the covers this works because the
compiler
andbuild
pod's build scripts specify a non-defaultdependsDir
.
Because of these restrictions, adding new language features and fcode changes require some careful planning.