FS: lookupPath should throw an error on an empty string

It's inconsistent that lookupPath returns `node:null` when the path
is an empty string. Normally it either raises `ENOENT` or `node` is truthy.
This makes empty string into an edge case that every caller has to handle.
`stat` has special handling for the empty string case and correctly returns
ENOENT on empty string, but chmod, truncate, and mknod all crash. utime and
some other functions mishandle the empty string in an unrelated way.

Author: hoodmane

src/library_fs.js

@@ -172,7 +172,8 @@ FS.staticInit();
     // paths
     //
     lookupPath(path, opts = {}) {
-      if (!path) return { path: '', node: null };
+      if (!path)
+        throw new FS.ErrnoError({{{ cDefs.ENOENT }}});
       opts.follow_mount ??= true
 
       if (!PATH.isAbs(path)) {
@@ -948,9 +949,6 @@ FS.staticInit();
     stat(path, dontFollow) {
       var lookup = FS.lookupPath(path, { follow: !dontFollow });
       var node = lookup.node;
-      if (!node) {
-        throw new FS.ErrnoError({{{ cDefs.ENOENT }}});
-      }
       if (!node.node_ops.getattr) {
         throw new FS.ErrnoError({{{ cDefs.EPERM }}});
       }

test/stat/test_chmod.c

@@ -159,6 +159,13 @@ void test() {
   assert(s.st_mode == (S_IRUSR | S_IFREG));
 #endif // WASMFS
 
+  // assert(stat("", &s) == -1);
+  // assert(errno == ENOENT);
+  // assert(chmod("", 0777) == -1);
+  // assert(errno == ENOENT);
+  assert(chown("", 1000, 1000) == -1);
+  assert(errno == ENOENT);
+
   puts("success");
 }
 

test/stat/test_mknod.c

@@ -59,6 +59,8 @@ void test() {
   assert(S_ISCHR(s.st_mode));
 #endif // WASMFS
 
+  assert(mknod("", 0777, 0) == -1);
+  assert(errno == ENOENT);
 #endif
 
   //

test/unistd/truncate.c

@@ -86,6 +86,10 @@ int main() {
   errno = 0;
   printf("\n");
 
+  printf("truncate(empty_path, 2): %d\n", truncate("", 2));
+  printf("errno: %s\n", strerror(errno));
+  printf("\n");
+
   printf("ftruncate(readonly, 4): %d\n", ftruncate(f2, 4));
   printf("errno: %s\n", strerror(errno));
   fstat(f2, &s);

test/unistd/truncate.out

@@ -24,6 +24,9 @@ truncate(readonly, 2): -1
 errno: Permission denied
 st_size: 6
 
+truncate(empty_path, 2): -1
+errno: No such file or directory
+
 ftruncate(readonly, 4): -1
 errno: Invalid argument
 st_size: 6

test/utime/test_futimens.c

@@ -140,6 +140,11 @@ void test() {
 
   close(fd);
 
+  // TODO:
+  // printf("check utime on empty path...\n");
+  // assert(utime("", &tb) == -1);
+  // assert(errno == ENOENT); 
+
   puts("success");
 }