When opening broken symlink with O_CREAT, create file at target (emscripten-core#23002)

If we open a broken symlink with `O_CREAT` we should create a new file
as the target of the symlink.

Fixes: emscripten-core#23001.

Author: hoodmane

src/library_fs.js

@@ -193,8 +193,18 @@ FS.staticInit();
             break;
           }
 
-          current = FS.lookupNode(current, parts[i]);
           current_path = PATH.join2(current_path, parts[i]);
+          try {
+            current = FS.lookupNode(current, parts[i]);
+          } catch (e) {
+            // if noent_okay is true, suppress a ENOENT in the last component
+            // and return an object with an undefined node. This is needed for
+            // resolving symlinks in the path when creating a file.
+            if ((e?.errno === {{{ cDefs.ENOENT }}}) && islast && opts.noent_okay) {
+              return { path: current_path };
+            }
+            throw e;
+          }
 
           // jump to the mount's root node if this is a mountpoint
           if (FS.isMountpoint(current) && (!islast || opts.follow_mount)) {
@@ -1036,14 +1046,15 @@ FS.staticInit();
         node = path;
       } else {
         path = PATH.normalize(path);
-        try {
-          var lookup = FS.lookupPath(path, {
-            follow: !(flags & {{{ cDefs.O_NOFOLLOW }}})
-          });
-          node = lookup.node;
-        } catch (e) {
-          // ignore
-        }
+        // noent_okay makes it so that if the final component of the path
+        // doesn't exist, lookupPath returns `node: undefined`. `path` will be
+        // updated to point to the target of all symlinks.
+        var lookup = FS.lookupPath(path, {
+          follow: !(flags & {{{ cDefs.O_NOFOLLOW }}}),
+          noent_okay: true
+        });
+        node = lookup.node;
+        path = lookup.path;
       }
       // perhaps we need to create the node
       var created = false;

test/test_core.py

@@ -6125,6 +6125,10 @@ def test_unistd_links(self, args, nodefs):
 
     self.do_run_in_out_file_test('unistd/links.c', emcc_args=args)
 
+  @also_with_noderawfs
+  def test_unistd_write_broken_link(self):
+    self.do_run_in_out_file_test('unistd/test_unistd_write_broken_link.c')
+
   @no_windows('Skipping NODEFS test, since it would require administrative privileges.')
   @requires_node
   def test_unistd_symlink_on_nodefs(self):

test/unistd/test_unistd_write_broken_link.c

@@ -0,0 +1,31 @@
+#include <errno.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <string.h>
+#include <unistd.h>
+
+
+int main() {
+  int res = symlink("link_target", "link_source");
+  printf("link result: %d\n", res);
+  int src_fd = open("link_source", O_CREAT | O_WRONLY, 0777);
+  printf("source_fd: %d, errno: %d %s\n", src_fd, errno, strerror(errno));
+  write(src_fd, "abc", 3);
+  close(src_fd);
+  {
+    int target_fd = open("link_target", O_RDONLY);
+    printf("target_fd: %d, errno: %d %s\n", target_fd, errno, strerror(errno));
+    char buf[10];
+    read(target_fd, buf, 10);
+    printf("buf: '%s'\n", buf);
+    close(target_fd);
+  }
+  {
+    int target_fd = open("link_source", O_RDONLY);
+    printf("target_fd: %d, errno: %d %s\n", target_fd, errno, strerror(errno));
+    char buf[10];
+    read(target_fd, buf, 10);
+    printf("buf: '%s'\n", buf);
+    close(target_fd);
+  }
+}

test/unistd/test_unistd_write_broken_link.out

@@ -0,0 +1,6 @@
+link result: 0
+source_fd: 3, errno: 0 No error information
+target_fd: 3, errno: 0 No error information
+buf: 'abc'
+target_fd: 3, errno: 0 No error information
+buf: 'abc'

test/wasmfs/wasmfs_open.c

@@ -89,11 +89,7 @@ int main() {
   int fd5 = open("/dev/stdout/foo", O_RDWR);
   printf("Errno: %s\n", strerror(errno));
   // Both errors are valid, but in WasmFS, ENOTDIR is returned first.
-#ifdef WASMFS
   assert(errno == ENOTDIR);
-#else
-  assert(errno == ENOENT);
-#endif
 
   errno = 0;
   // Attempt to open and write to the root directory.