*vm.Program Not Safe

[xuanP]

Hi I'm trying to reuse compiled Program but it seems unsafe, while the document here saying it is. I reuse the program in multiple goroutines and encounter PANIC. Here's the stack, i'm using @v1.16.9:

`fatal error: concurrent map read and map write

goroutine 73084 [running]:
reflect.mapaccess(0x3f2a520?, 0x65f9ad0?, 0x4?)
/usr/local/go/src/runtime/map.go:1357 +0x19
reflect.Value.MapIndex({0x4039fc0?, 0xc005644f00?, 0xc0027bb580?}, {0x3f2a520, 0x65f9ad0, 0x82})
/usr/local/go/src/reflect/value.go:1747 +0x156
github.com/expr-lang/expr/vm/runtime.Fetch({0x4039fc0, 0xc005644f00?}, {0x3f2a520?, 0x65f9ad0?})
/opt/tiger/compile_path/pkg/mod/github.com/expr-lang/expr@v1.16.9/vm/runtime/runtime.go:55 +0x2d6
github.com/expr-lang/expr/vm.(*VM).Run(0xc0027bbad0, 0xc0026c9200, {0x43746c0?, 0xc00dec4c40?})
/opt/tiger/compile_path/pkg/mod/github.com/expr-lang/expr@v1.16.9/vm/vm.go:130 +0xc30
github.com/expr-lang/expr/vm.Run(0x7f086cd0c108?, {0x43746c0?, 0xc00dec4c40?})
/opt/tiger/compile_path/pkg/mod/github.com/expr-lang/expr@v1.16.9/vm/vm.go:25 +0x70
github.com/expr-lang/expr.Run(...)
`

[antonmedv]

Hi,

Yes, vm.Program is safe to run in multiple goroutines and is concurrent safe. Expr even has a special race test in CI to verify what it is true:

expr/.github/workflows/test.yml

Line 35 in c42572b

race:

In the provided error, I see runtime.Fetch -> reflect.Value.MapIndex calls. I suspect some sort of map is used in env and both program tries to read those values from different goroutines at the same time other goroutines writes to the map.

Make sure what your code is thread safe as well. Lock access to env while reading from it.