RFC: Introduce build rules for building GWT applications.

I’m looking into adding build rules for building GWT applications. I’m curious what folks think the arguments to the rules should look like.

• Do we need a special gwt_library() rule, or can we just use java_library()? My intuition is that if you use JSNI in your Java code, then you need to classify that as a gwt_library().
• How do you deal with dependencies that have dual implementations: one for the JRE and one for JavaScript? For example, you may have a subgraph of dependencies that transitively depend on Guava. Then at the top, you have a gwt_binary() target and a java_binary() target? How do you ensure that the gwt_binary() pulls in guava-gwt-17.0.jar while the java_binary() pulls in guava-17.0.jar? One option is to have parallel versions of each subgraph, but that seems messy.
• How can we design gwt_binary() (or this whole system of rules) so that it is easy to run the GWT app in developer mode? Ideally, a developer should not have to run buck build after every code modification in order to reload the GWT app. The existing edit-refresh-test cycle needs to be preserved.

[davido]

@bolinfest Thanks for adding -Dgwt.normalizeTimestamps=true and changing the output file to .zip.

Are there any other regressions in moving to gwt_binary()?

I have an issue with recovering from compile errors in GWT code:

• conduct an error in GWT code
• compile with buck build //gerrit-gwtui:ui_safari, note that output file is not created.
• revert the error
• call the target again:

$ buck build //gerrit-gwtui:ui_safari && file `buck targets --show_output //gerrit-gwtui:ui_safari | awk '{print $2}'`
Watchman not found, please install when using buckd.
See https://github.com/facebook/watchman for details.
[-] PARSING BUILD FILES...FINISHED 0,6s
[-] BUILDING...FINISHED 0,3s
Log:
Watchman not found, please install when using buckd.
See https://github.com/facebook/watchman for details.
[-] PARSING BUILD FILES...FINISHED 0,6s
buck-out/gen/gerrit-gwtui/__gwt_binary_ui_safari__/ui_safari.zip: ERROR: cannot open `buck-out/gen/gerrit-gwtui/__gwt_binary_ui_safari__/ui_safari.zip' (No such file or directory)

To recover from this situation, i’ve found that only buck clean helps so far:

$ buck clean 
Watchman not found, please install when using buckd.
See https://github.com/facebook/watchman for details.
davido@linux-ucwl:~/projects/gerrit (upgrade-buck %)$ buck build //gerrit-gwtui:ui_safari && file `buck targets --show_output //gerrit-gwtui:ui_safari | awk '{print $2}'`
Watchman not found, please install when using buckd.
See https://github.com/facebook/watchman for details.
[-] PARSING BUILD FILES...FINISHED 0,6s
[-] BUILDING...FINISHED 1,2s
Watchman not found, please install when using buckd.
See https://github.com/facebook/watchman for details.
[-] PARSING BUILD FILES...FINISHED 0,6s
buck-out/gen/gerrit-gwtui/__gwt_binary_ui_safari__/ui_safari.zip: Zip archive data, at least v1.0 to extract Java Jar file data (zip)

The output file seems not to be created from the cache after GWT error. We are using the following caching strategy:

[cache]
  mode = dir
  dir = buck-out/cache

[sdwilsh]

We now have integration tests for this provided by @shs96c as well. If someone wanted to submit a pull requests for documentation for these rules, we’d happily take it.