opa build: combine -Ox with -t wasm

[srenatus]

Optimizations, i.e. running partial eval, before compiling into Wasm is something you could want to do. However, at the moment, the only way to get that done is to do a series of calls:

$ opa build -O2 -o o2.tar.gz -e x/y x.rego
$ opa build  -t wasm -e x/y -b o2.tar.gz    
$ opa eval -fpretty -b bundle.tar.gz data.x.y
true

and the resulting bundle looks a bit funny:

$ tar tvf bundle.tar.gz                      
-rw-------  0 0      0           3 Jan  1  1970 /data.json
-rw-------  0 0      0          27 Jan  1  1970 /o2.tar.gz/optimized/x.rego
-rw-------  0 0      0      131444 Jan  1  1970 /policy.wasm
-rw-------  0 0      0          83 Jan  1  1970 /.manifest

It would be nice if you could just call opa build -O2 -t wasm -e x/y x.rego to get the steps combined.

If you run this command now, it'll panic:

$ opa build -O2 -t wasm -e x/y x.rego     
panic: runtime error: index out of range [0] with length 0

goroutine 1 [running]:
github.com/open-policy-agent/opa/internal/compiler/wasm.(*Compiler).fileAddr(...)
        /Users/stephan/Sources/open-policy-agent/opa/internal/compiler/wasm/wasm.go:1662
github.com/open-policy-agent/opa/internal/compiler/wasm.(*Compiler).runtimeErrorAbort(0x1016d0800, {0x0, 0x0, 0x0, {0x0, 0x0}, {0x0, 0x0}}, 0xc00063f028?)
        /Users/stephan/Sources/open-policy-agent/opa/internal/compiler/wasm/wasm.go:1754 +0x2b3
github.com/open-policy-agent/opa/internal/compiler/wasm.(*Compiler).compileBlock(0xc0003dd200, 0xc0002454d0?)
        /Users/stephan/Sources/open-policy-agent/opa/internal/compiler/wasm/wasm.go:1047 +0x805
github.com/open-policy-agent/opa/internal/compiler/wasm.(*Compiler).compileFunc(0xc0003dd200, 0xc0000b4cc0)
        /Users/stephan/Sources/open-policy-agent/opa/internal/compiler/wasm/wasm.go:855 +0x85e
github.com/open-policy-agent/opa/internal/compiler/wasm.(*Compiler).compileFuncs(0xc0003dd200)
        /Users/stephan/Sources/open-policy-agent/opa/internal/compiler/wasm/wasm.go:717 +0x85
github.com/open-policy-agent/opa/internal/compiler/wasm.(*Compiler).Compile(0xc0003dd200)
        /Users/stephan/Sources/open-policy-agent/opa/internal/compiler/wasm/wasm.go:311 +0x58
github.com/open-policy-agent/opa/compile.(*Compiler).compileWasm(0xc00063fb30, {0x101c2ecb0, 0xc000124008})
        /Users/stephan/Sources/open-policy-agent/opa/compile/compile.go:627 +0x2b0
github.com/open-policy-agent/opa/compile.(*Compiler).Build(0xc00063fb30, {0x101c2ecb0, 0xc000124008})
        /Users/stephan/Sources/open-policy-agent/opa/compile/compile.go:307 +0x1cb
github.com/open-policy-agent/opa/cmd.dobuild({0xc0001eb4d0, 0xc0000d6cc0, 0x0, 0x0, 0x2, {{0xc00043a100, 0x1, 0x1}, 0x1}, {0x10184bb09, ...}, ...}, ...)
        /Users/stephan/Sources/open-policy-agent/opa/cmd/build.go:313 +0x888
github.com/open-policy-agent/opa/cmd.init.1.func2(0xc0001fc900?, {0xc0000b41e0?, 0x6?, 0x6?})
        /Users/stephan/Sources/open-policy-agent/opa/cmd/build.go:220 +0x6c
github.com/spf13/cobra.(*Command).execute(0xc0001fc900, {0xc0000b4180, 0x6, 0x6})
        /Users/stephan/Sources/open-policy-agent/opa/vendor/github.com/spf13/cobra/command.go:920 +0x847
github.com/spf13/cobra.(*Command).ExecuteC(0x1025d8040)
        /Users/stephan/Sources/open-policy-agent/opa/vendor/github.com/spf13/cobra/command.go:1044 +0x3bd
github.com/spf13/cobra.(*Command).Execute(...)
        /Users/stephan/Sources/open-policy-agent/opa/vendor/github.com/spf13/cobra/command.go:968
main.main()
        /Users/stephan/Sources/open-policy-agent/opa/main.go:14 +0x25