Fix fetchfs off by one error (#23852)

In #23669 , I tried to implement length clipping to prevent reads past
the end of a file. Unfortunately I had an extra, unnecessary +1 term in
some places which led to issues including problems when the whole file
fit into a single chunk. This patch corrects the clipping.

Author: JoeOsborn

src/lib/libwasmfs_fetch.js

@@ -51,7 +51,7 @@ addToLibrary({
             chunks: [],
             chunkSize: chunkSize
           };
-          len = Math.min(len, size-offset+1) | 0;
+          len = Math.min(len, size-offset);
         } else {
           // may as well/forced to download the whole file
           var wholeFileReq = await fetch(url);
@@ -134,7 +134,7 @@ addToLibrary({
           return failedResponse.status === 404 ? -{{{ cDefs.ENOENT }}} : -{{{ cDefs.EBADF }}};
         }
         var fileInfo = wasmFS$JSMemoryRanges[file];
-        length = Math.min(length, fileInfo.size-offset+1) | 0;
+        length = Math.min(length, fileInfo.size-offset);
         // As above, we check the length just in case offset was beyond size and length is now negative.
         if (length <= 0) {
           return 0;

test/wasmfs/wasmfs_fetch.c

@@ -171,7 +171,7 @@ void test_small_chunks() {
   buf[size] = 0;
   printf("buf %s\n",buf);
   assert(strcmp(buf, "hello") == 0);
-  assert(read(fd, buf+size-1, 1024) == 1);
+  assert(read(fd, buf, 1024) == 0);
 
   assert(close(fd) == 0);
 
@@ -263,6 +263,23 @@ void test_nonexistent() {
   assert(close(fd) == 0);
 }
 
+void test_big_chunks() {
+  printf("Running %s...\n", __FUNCTION__);
+
+  char expected[] = "hello";
+  size_t size = 5;
+
+  backend_t backend = wasmfs_create_fetch_backend("small.dat", 16);
+  int fd;
+  char buf[size + 1];
+  fd = wasmfs_create_file("/testfile11", 0777, backend);
+  read_chunks_check(fd, buf, size, 2);
+  buf[size] = 0;
+  printf("buf %s\n",buf);
+  assert(strcmp(buf, "hello") == 0);
+  assert(close(fd) == 0);
+}
+
 int main() {
   getUrlOrigin(url_orig, sizeof(url_orig));
   test_default();
@@ -273,6 +290,7 @@ int main() {
   test_small_chunks();
   test_small_chunks_divisor_of_size();
   test_nonexistent();
+  test_big_chunks();
 
   return 0;
 }