[WasmFS] Fix length computation in fetch backend (#17578)

Fixes #17571

Author: kripken

src/library_wasmfs_js_file.js

@@ -37,7 +37,10 @@ mergeInto(LibraryManager.library, {
       },
       read: (file, buffer, length, offset) => {
         var fileData = wasmFS$JSMemoryFiles[file];
-        length = Math.max(0, fileData.length - offset);
+        // We can't read past the end of the file's data.
+        var dataAfterOffset = Math.max(0, fileData.length - offset);
+        // We only read as much as we were asked.
+        length = Math.min(length, dataAfterOffset);
         HEAPU8.set(fileData.subarray(offset, offset + length), buffer);
         return length;
       },

system/lib/wasmfs/syscalls.cpp

@@ -219,7 +219,12 @@ static __wasi_errno_t readAtOffset(OffsetHandling setOffset,
       }
       return -result;
     }
+
     // The read was successful.
+
+    // Backends must only return len or less.
+    assert(result <= len);
+
     bytesRead += result;
     if (result < len) {
       // The read was short, so stop here.

test/test_browser.py

@@ -5285,7 +5285,9 @@ def test(args):
   @parameterized({
     # the fetch backend works even on the main thread: we proxy to a background
     # thread and busy-wait
-    'main_thread': (['-sPTHREAD_POOL_SIZE=4'],),
+    # this test requires one thread per fetch backend, so updates to the test
+    # will require bumping this
+    'main_thread': (['-sPTHREAD_POOL_SIZE=5'],),
     # using proxy_to_pthread also works, of course
     'proxy_to_pthread': (['-sPROXY_TO_PTHREAD', '-sINITIAL_MEMORY=32MB', '-DPROXYING'],),
   })
@@ -5294,6 +5296,7 @@ def test_wasmfs_fetch_backend(self, args):
     if is_firefox() and '-sPROXY_TO_PTHREAD' not in args:
       return self.skipTest('ff hangs on the main_thread version. browser bug?')
     create_file('data.dat', 'hello, fetch')
+    create_file('small.dat', 'hello')
     create_file('test.txt', 'fetch 2')
     delete_dir('subdir')
     ensure_dir('subdir')

test/wasmfs/wasmfs_fetch.c

@@ -16,7 +16,10 @@
 #include <sys/stat.h>
 #include <unistd.h>
 
-// NOTE: Each fetch backend runs in a separate thread.
+// NOTE: Each fetch backend runs in a separate thread. When not using
+//       PROXY_TO_PTHREAD, that means we need a pool of at least one thread per
+//       backend, so updating test_browser.py may be needed when adding more
+//       here.
 
 void getUrlOrigin(char* ptr, int len);
 char url_orig[256] = {};
@@ -126,12 +129,34 @@ void test_default() {
   assert(close(fd2) == 0);
 }
 
+void test_small_reads() {
+  // Read the file in small amounts.
+  printf("Running %s...\n", __FUNCTION__);
+
+  char expected[] = "hello";
+  size_t size = 5;
+
+  backend_t backend = wasmfs_create_fetch_backend("small.dat");
+  int fd = wasmfs_create_file("/testfile3", 0777, backend);
+  char buf[size + 1];
+  for (size_t i = 0; i < size; i++) {
+    int read_now = read(fd, buf + i, 1);
+    assert(read_now == 1);
+    printf("read one byte\n");
+  }
+  buf[size] = 0;
+  assert(strcmp(buf, "hello") == 0);
+
+  assert(close(fd) == 0);
+}
+
 int main() {
   getUrlOrigin(url_orig, sizeof(url_orig));
   test_default();
   test_url_relative();
   test_url_absolute();
   test_directory_abs();
+  test_small_reads();
 
   return 0;
 }