wasip3: Limit randomness instead of trapping (#12794)

This updates the behavior of the randomness-generating interfaces in
WASIp3 to account for recent spec changes, notably the ability for the
guest to receive less random bytes than requested to limit allocations
the host is forced to do.

cc WebAssembly/WASI#901

Author: alexcrichton

crates/wasi/src/p3/random/host.rs

@@ -2,16 +2,12 @@ use crate::p3::bindings::random::{insecure, insecure_seed, random};
 use crate::random::WasiRandomCtx;
 use cap_rand::Rng;
 use cap_rand::distributions::Standard;
-use wasmtime::bail;
 
 impl random::Host for WasiRandomCtx {
     fn get_random_bytes(&mut self, len: u64) -> wasmtime::Result<Vec<u8>> {
-        if len > self.max_size {
-            bail!("requested len {len:?} exceeds limit {}", self.max_size);
-        }
         Ok((&mut self.random)
             .sample_iter(Standard)
-            .take(len as usize)
+            .take(len.min(self.max_size) as usize)
             .collect())
     }
 
@@ -22,12 +18,9 @@ impl random::Host for WasiRandomCtx {
 
 impl insecure::Host for WasiRandomCtx {
     fn get_insecure_random_bytes(&mut self, len: u64) -> wasmtime::Result<Vec<u8>> {
-        if len > self.max_size {
-            bail!("requested len {len:?} exceeds limit {}", self.max_size);
-        }
         Ok((&mut self.insecure_random)
             .sample_iter(Standard)
-            .take(len as usize)
+            .take(len.min(self.max_size) as usize)
             .collect())
     }
 

tests/all/cli_tests.rs

@@ -2844,18 +2844,15 @@ start a print 1234
 
         for rand in ["random", "insecure"] {
             run_wasmtime(&["run", "-Sp3", "-Wcomponent-model-async", c, rand, "256"])?;
-            assert!(
-                run_wasmtime(&[
-                    "run",
-                    "-Sp3",
-                    "-Wcomponent-model-async",
-                    "-Smax-random-size=255",
-                    c,
-                    rand,
-                    "256"
-                ])
-                .is_err()
-            );
+            run_wasmtime(&[
+                "run",
+                "-Sp3",
+                "-Wcomponent-model-async",
+                "-Smax-random-size=255",
+                c,
+                rand,
+                "256",
+            ])?;
         }
 
         Ok(())