fix(c-api) Don't drain the entire captured stream at first read

CHANGELOG.md

@@ -19,6 +19,7 @@
 - [#2056](https://github.com/wasmerio/wasmer/pull/2056) Change back to depend on the `enumset` crate instead of `wasmer_enumset`
 
 ### Fixed
+- [#2070](https://github.com/wasmerio/wasmer/pull/2070) Do not drain the entire captured stream at first read with `wasi_env_read_stdout` or `_stderr` in the C API.
 - [#2058](https://github.com/wasmerio/wasmer/pull/2058) Expose WASI versions to C correctly.
 - [#2044](https://github.com/wasmerio/wasmer/pull/2044) Do not build C headers on docs.rs.
 

lib/c-api/src/wasm_c_api/wasi/mod.rs

@@ -13,6 +13,7 @@ use super::{
 // required due to really weird Rust resolution rules for macros
 // https://github.com/rust-lang/rust/issues/57966
 use crate::error::{update_last_error, CApiError};
+use std::cmp::min;
 use std::convert::TryFrom;
 use std::ffi::CStr;
 use std::os::raw::c_char;
@@ -275,12 +276,16 @@ pub unsafe extern "C" fn wasi_env_read_stderr(
 
 fn read_inner(wasi_file: &mut Box<dyn WasiFile>, inner_buffer: &mut [u8]) -> isize {
     if let Some(oc) = wasi_file.downcast_mut::<capture_files::OutputCapturer>() {
-        let mut num_bytes_written = 0;
-        for (address, value) in inner_buffer.iter_mut().zip(oc.buffer.drain(..)) {
+        let total_to_read = min(inner_buffer.len(), oc.buffer.len());
+
+        for (address, value) in inner_buffer
+            .iter_mut()
+            .zip(oc.buffer.drain(..total_to_read))
+        {
             *address = value;
-            num_bytes_written += 1;
         }
-        num_bytes_written
+
+        total_to_read as isize
     } else {
         -1
     }

lib/c-api/tests/test-wasi.c

@@ -58,6 +58,7 @@ int main(int argc, const char* argv[]) {
   const char* js_string = "function greet(name) { return JSON.stringify('Hello, ' + name); }; print(greet('World'));";
   wasi_config_arg(config, "--eval");
   wasi_config_arg(config, js_string);
+  wasi_config_capture_stdout(config);
 
   wasi_env_t* wasi_env = wasi_env_new(config);
   if (!wasi_env) {
@@ -125,16 +126,35 @@ int main(int argc, const char* argv[]) {
     return 1;
   }
 
-  char buffer[BUF_SIZE] = { 0 };
-  size_t result = BUF_SIZE;
-  for (size_t i = 0;
-       // TODO: this code is too clever, make the control flow more obvious here
-       result == BUF_SIZE &&
-               (result = wasi_env_read_stdout(wasi_env, buffer, BUF_SIZE));
-       ++i) {
-     printf("%.*s", BUF_SIZE, buffer);
+  {
+    FILE *memory_stream;
+    char* stdout;
+    size_t stdout_size = 0;
+
+    memory_stream = open_memstream(&stdout, &stdout_size);
+
+    if (NULL == memory_stream) {
+      printf("> Error creating a memory stream.\n");
+      return 1;
+    }
+
+    char buffer[BUF_SIZE] = { 0 };
+    size_t data_read_size = BUF_SIZE;
+
+    do {
+      data_read_size = wasi_env_read_stdout(wasi_env, buffer, BUF_SIZE);
+
+      if (data_read_size > 0) {
+        stdout_size += data_read_size;
+        fwrite(buffer, sizeof(char), data_read_size, memory_stream);
+      }
+    } while (BUF_SIZE == data_read_size);
+
+    fclose(memory_stream);
+
+    printf("WASI Stdout: `%.*s`\n", (int) stdout_size, stdout);
   }
-  printf("\n");
+
 
   wasm_extern_vec_delete(&exports);
   wasm_extern_vec_delete(&imports);