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Chicory WASM "Unreachable Instruction" Issue - RESOLVED

Problem

When evaluating flags with context in Chicory (Java WASM runtime), the WASM module was crashing with:

com.dylibso.chicory.runtime.TrapException: Trapped on unreachable instruction

This only occurred when providing evaluation context like:

{
  "sources": "flags/allFlags.json,rawflags/selector-flags.json",
  "email": "ballmer@macrosoft.com",
  "injectedmetadata": "set"
}

Root Cause

The panic was caused by std::time::SystemTime::now() in the context enrichment function (evaluation.rs:231).

In WASM without WASI support (like Chicory), SystemTime::now() panics instead of returning an error. Since this project uses panic = "abort" (Cargo.toml:55), the panic translates to the WASM unreachable instruction, causing Chicory to throw a TrapException.

The code flow was:

  1. Call evaluate() with context
  2. Enter enrich_context() to add $flagd.timestamp
  3. Call SystemTime::now() → PANIC
  4. Panic → unreachable → Chicory TrapException

Solution

Instead of relying on WASM's unavailable system time, we now use a host function that the WASM runtime must provide.

Changes Made

  1. Added host function import (lib.rs:44-58):

    #[cfg(target_family = "wasm")]
#[link(wasm_import_module = "host")]
extern "C" {
    fn host_get_current_time() -> u64;
}

  2. Created wrapper function (lib.rs:84-99):

    • In WASM: Calls host function with panic catch (defaults to 0)
    • In native code: Uses SystemTime::now() for tests/CLI

  3. Updated context enrichment (evaluation.rs:228-230):

    let timestamp = crate::get_current_time();

WASM Import Declaration

Verified with wasm-objdump:

Import[9]:
 - func[0] <host.get_current_time_unix_seconds> <- host.get_current_time_unix_seconds

How to Fix Your Java Code

You must provide the host function when loading the WASM module:

import com.dylibso.chicory.runtime.HostFunction;
import com.dylibso.chicory.wasm.types.Value;
import com.dylibso.chicory.wasm.types.ValueType;
import java.util.List;

// Define the host function
HostFunction getCurrentTime = new HostFunction(
    "host",                              // Module name (must match WASM import)
    "get_current_time_unix_seconds",    // Function name (must match WASM import)
    List.of(),                           // No parameters
    List.of(ValueType.I64),             // Returns i64
    (Instance instance, Value... args) -> {
        // Return current Unix timestamp in seconds
        long currentTimeSeconds = System.currentTimeMillis() / 1000;
        return new Value[] { Value.i64(currentTimeSeconds) };
    }
);

// Add to module when instantiating
Module module = Module.builder(wasmBytes)
    .withHostFunction(getCurrentTime)
    .build();

Instance instance = module.instantiate();

Complete Working Example

See HOST_FUNCTIONS.md for a complete Java example showing:

  • Loading the WASM module with host function
  • Updating flag state
  • Evaluating flags with context
  • Memory management (alloc/dealloc)

Testing the Fix

  1. Rebuild the WASM module:

    cargo build --target wasm32-unknown-unknown --no-default-features --release --lib

  2. Update your Java code to provide the host function (see above)

  3. The evaluation should now work with context:

    String context = "{\"email\":\"ballmer@macrosoft.com\"}";
// Should now evaluate successfully without TrapException

  4. Verify $flagd.timestamp is available in targeting rules:

    {
  "targeting": {
    "if": [
      {">": [{"var": "$flagd.timestamp"}, 1700000000]},
      "variant1",
      "variant2"
    ]
  }
}

Backward Compatibility

If the host function is not provided:

  • The WASM module catches the panic and defaults $flagd.timestamp to 0
  • Evaluation continues without errors
  • Time-based targeting rules will not work correctly

Related Files

  • HOST_FUNCTIONS.md - Complete implementation guide for all languages
  • README.md - Updated with host function requirements
  • src/lib.rs - Host function import and wrapper
  • src/evaluation.rs - Context enrichment using host function

Prevention for Future

All time-sensitive operations in WASM should:

  1. Use host functions instead of std::time::SystemTime
  2. Wrap in catch_unwind if panics are possible
  3. Provide sensible defaults when unavailable

This pattern should be used for any other system-dependent functionality in WASM.