[WasmFS] Move O_TRUNC handling to avoid crash (#17192)

WasmFS backends like OPFS may reasonably expect that files will only be modified
while open and may be unprepared to modify unopened files. However, when
handling O_TRUNC, we truncated files before opening them in the backend. For the
OPFS backend in particular, this caused a crash because the backend did not yet
have an AccessHandle for the file. Fix the problem by deferring O_TRUNC handling
until after we have asked the backend to open the file.

Author: tlively

system/lib/wasmfs/syscalls.cpp

@@ -465,6 +465,10 @@ static __wasi_fd_t doOpen(path::ParsedParent parsed,
     return -EEXIST;
   }
 
+  // Note that we open the file before truncating it because some backends may
+  // depend on that (e.g. OPFS).
+  auto openFile = std::make_shared<OpenFileState>(0, flags, child);
+
   // If O_TRUNC, truncate the file if possible.
   if ((flags & O_TRUNC) && child->is<DataFile>()) {
     if (fileMode & WASMFS_PERM_WRITE) {
@@ -474,7 +478,6 @@ static __wasi_fd_t doOpen(path::ParsedParent parsed,
     }
   }
 
-  auto openFile = std::make_shared<OpenFileState>(0, flags, child);
   return wasmFS.getFileTable().locked().addEntry(openFile);
 }
 

tests/wasmfs/wasmfs_opfs.c

@@ -121,7 +121,17 @@ int main(int argc, char* argv[]) {
   err = stat("/opfs/working/foo.txt", &stat_buf);
   assert(err == 0);
   assert(stat_buf.st_size == 1);
-  emscripten_console_log("statted");
+  emscripten_console_log("statted while closed");
+
+  fd = open("/opfs/working/foo.txt", O_RDONLY | O_TRUNC);
+  assert(fd > 0);
+  emscripten_console_log("truncated while opening");
+  close(fd);
+
+  err = stat("/opfs/working/foo.txt", &stat_buf);
+  assert(err == 0);
+  assert(stat_buf.st_size == 0);
+  emscripten_console_log("statted after truncation");
 
   err = rename("/opfs/working/foo.txt", "/opfs/foo.txt");
   assert(err == 0);