WasmFS JS API: Implement read (#19504)

Author: jameshu15869

.circleci/config.yml

@@ -529,6 +529,7 @@ jobs:
             wasmfs.test_futimens
             wasmfs.test_unistd_links_memfs
             wasmfs.test_fcntl_open
+            wasmfs.test_fs_js_api
             wasmfs.test_freetype"
   test-wasm2js1:
     executor: bionic

emcc.py

@@ -2322,6 +2322,8 @@ def phase_linker_setup(options, state, newargs):
         '_wasmfs_unlink',
         '_wasmfs_chdir',
         '_wasmfs_rmdir',
+        '_wasmfs_read',
+        '_wasmfs_pread',
         '_wasmfs_symlink',
         '_wasmfs_chmod',
         '_wasmfs_fchmod',

src/library_wasmfs.js

@@ -143,6 +143,26 @@ FS.createPreloadedFile = FS_createPreloadedFile;
       });
     },
     // TODO: read
+    read: (fd, buffer, offset, length, position) => {
+      var seeking = typeof position != 'undefined';
+
+      var dataBuffer = _malloc(length);
+
+      var bytesRead;
+      if (seeking) {
+        bytesRead = __wasmfs_pread(fd, dataBuffer, length, position);
+      } else {
+        bytesRead = __wasmfs_read(fd, dataBuffer, length);
+      }
+      bytesRead = FS.handleError(bytesRead);
+
+      for (var i = 0; i < length; i++) {
+        buffer[offset + i] = {{{ makeGetValue('dataBuffer', 'i', 'i8')}}}
+      }
+
+      _free(dataBuffer);
+      return bytesRead;
+    },
     // TODO: write
     // TODO: allocate
     // TODO: mmap

system/lib/wasmfs/js_api.cpp

@@ -145,6 +145,32 @@ int _wasmfs_lchmod(char* path, mode_t mode) {
   return __syscall_fchmodat(AT_FDCWD, (intptr_t)path, mode, AT_SYMLINK_NOFOLLOW);
 }
 
+int _wasmfs_read(int fd, void *buf, size_t count) {
+  __wasi_iovec_t iovs[1];
+  iovs[0].buf = (uint8_t *)buf;
+  iovs[0].buf_len = count;
+
+  __wasi_size_t numBytes;
+  __wasi_errno_t err = __wasi_fd_read(fd, iovs, 1, &numBytes);
+  if (err) {
+    return -err;
+  }
+  return numBytes;
+}
+
+int _wasmfs_pread(int fd, void *buf, size_t count, off_t offset) {
+  __wasi_iovec_t iovs[1];
+  iovs[0].buf = (uint8_t *)buf;
+  iovs[0].buf_len = count;
+
+  __wasi_size_t numBytes;
+  __wasi_errno_t err = __wasi_fd_pread(fd, iovs, 1, offset, &numBytes);
+  if (err) {
+    return -err;
+  }
+  return numBytes;
+}
+
 int _wasmfs_close(int fd) {
   return __wasi_fd_close(fd);
 }

test/fs/test_fs_js_api.c

@@ -37,6 +37,51 @@ int main() {
 #endif
     );
 
+    /********** test FS.read() **********/
+    EM_ASM(
+        FS.writeFile("readtestfile", 'a=1_b=2_');
+
+        // Test read variant, checking that the file offset moves correctly.
+        var buf = new Uint8Array(8);
+        var stream = FS.open("readtestfile", "r");
+        var numRead = FS.read(stream, buf, 0, 4);
+        assert(numRead == 4);
+        assert((new TextDecoder().decode(buf.subarray(0, 4))) === "a=1_");
+        numRead = FS.read(stream, buf, 4, 4);
+        assert(numRead == 4);
+        assert((new TextDecoder().decode(buf)) == 'a=1_b=2_');
+
+        // Test pread variant.
+        stream = FS.open("readtestfile", "r");
+        var extraBuf = new Uint8Array(8);
+        numRead = FS.read(stream, extraBuf, 0, 4, 0);
+        assert(numRead == 4);
+        assert((new TextDecoder().decode(extraBuf.subarray(0, 4))) == 'a=1_');
+
+        // Check that pread did not move the file offset (Offset begins at 0).
+        var doubleBuf = new Uint8Array(8);
+        var firstNumRead = FS.read(stream, doubleBuf, 0, 4);
+        var secondNumRead = FS.read(stream, doubleBuf, 4, 4);
+        assert(firstNumRead == 4 && secondNumRead == 4);
+        assert((new TextDecoder().decode(doubleBuf)) == 'a=1_b=2_');
+
+        // Check that full read works.
+        stream = FS.open("readtestfile", "r");
+        var fullBuf = new Uint8Array(8);
+        numRead = FS.read(stream, fullBuf, 0, 8);
+        assert(numRead == 8);
+        assert((new TextDecoder().decode(fullBuf)) == 'a=1_b=2_');
+
+        FS.close(stream);
+        var ex;
+        try {
+            FS.read(stream, buf, 4, 4);
+        } catch (err) {
+            ex = err;
+        }
+        assert(ex.name === 'ErrnoError' && ex.errno == 8 /* EBADF */);
+    );
+
     /********** test FS.close() **********/
     EM_ASM(
         FS.writeFile("closetestfile", 'a=1\nb=2\n');