[WasmFS] Implement O_TRUNC support in open (#17176)

Author: tlively

system/lib/wasmfs/syscalls.cpp

@@ -465,6 +465,15 @@ static __wasi_fd_t doOpen(path::ParsedParent parsed,
     return -EEXIST;
   }
 
+  // If O_TRUNC, truncate the file if possible.
+  if ((flags & O_TRUNC) && child->is<DataFile>()) {
+    if (fileMode & WASMFS_PERM_WRITE) {
+      child->cast<DataFile>()->locked().setSize(0);
+    } else {
+      return -EACCES;
+    }
+  }
+
   auto openFile = std::make_shared<OpenFileState>(0, flags, child);
   return wasmFS.getFileTable().locked().addEntry(openFile);
 }

tests/unistd/truncate.c

@@ -25,7 +25,7 @@ void setup() {
     FS.chdir('working');
     FS.writeFile('towrite', 'abcdef');
     FS.writeFile('toread', 'abcdef');
-    FS.chmod('toread', 0o444);
+    FS.chmod('toread', 0444);
   );
 #else
   FILE* f = fopen("towrite", "w");
@@ -79,15 +79,25 @@ int main() {
 
   printf("truncate(2): %d\n", truncate("towrite", 2));
   printf("errno: %s\n", strerror(errno));
-  stat("towrite", &s);
+  fstat(f, &s);
+  printf("st_size: %lld\n", s.st_size);
+  memset(&s, 0, sizeof s);
+  errno = 0;
+  printf("\n");
+
+  // Only permissions of the file, not opening mode, matter for truncation.
+  printf("open(O_TRUNC)\n");
+  open("towrite", O_RDONLY | O_TRUNC);
+  printf("errno: %s\n", strerror(errno));
+  fstat(f, &s);
   printf("st_size: %lld\n", s.st_size);
   memset(&s, 0, sizeof s);
   errno = 0;
   printf("\n");
 
   printf("truncate(readonly, 2): %d\n", truncate("toread", 2));
   printf("errno: %s\n", strerror(errno));
-  stat("toread", &s);
+  fstat(f2, &s);
   printf("st_size: %lld\n", s.st_size);
   memset(&s, 0, sizeof s);
   errno = 0;
@@ -99,6 +109,15 @@ int main() {
   printf("st_size: %lld\n", s.st_size);
   memset(&s, 0, sizeof s);
   errno = 0;
+  printf("\n");
+
+  printf("open(readonly, O_TRUNC)\n");
+  open("toread", O_RDONLY | O_TRUNC);
+  printf("errno: %s\n", strerror(errno));
+  fstat(f2, &s);
+  printf("st_size: %lld\n", s.st_size);
+  memset(&s, 0, sizeof s);
+  errno = 0;
 
 #ifdef __EMSCRIPTEN__
   // Restore full permissions on all created files so that python test runner rmtree

tests/unistd/truncate.out

@@ -16,10 +16,18 @@ truncate(2): 0
 errno: No error information
 st_size: 2
 
+open(O_TRUNC)
+errno: No error information
+st_size: 0
+
 truncate(readonly, 2): -1
 errno: Permission denied
 st_size: 6
 
 ftruncate(readonly, 4): -1
 errno: Invalid argument
 st_size: 6
+
+open(readonly, O_TRUNC)
+errno: Permission denied
+st_size: 6