Fs_fake: fix offset computation

Author: vouillon

compiler/tests-io/dune

@@ -84,3 +84,17 @@
   (compilation_mode whole_program)
   (flags
    (:standard --file "accentué"))))
+
+(rule
+ (with-stdout-to
+  "file.txt"
+  (echo test)))
+
+(tests
+ (names gh1856)
+ (deps file.txt)
+ (modes js)
+ (js_of_ocaml
+  (compilation_mode whole_program)
+  (flags
+   (:standard --file file.txt))))

compiler/tests-io/gh1856.ml

@@ -0,0 +1,7 @@
+let () =
+  let ch = open_in "/static/file.txt" in
+  let b = Bytes.create 1024 in
+  while input ch b 0 1024 > 0 do
+    ()
+  done;
+  close_in ch

runtime/js/fs_fake.js

@@ -472,17 +472,19 @@ class MlFakeFd {
   write(buf, pos, len, raise_unix) {
     if (this.file && (this.flags.wronly || this.flags.rdwr)) {
       var offset = this.offset;
+      len = this.file.write(offset, buf, pos, len);
       this.offset += len;
-      return this.file.write(offset, buf, pos, len);
+      return len;
     }
     this.err_closed("write", raise_unix);
   }
 
   read(buf, pos, len, raise_unix) {
     if (this.file && !this.flags.wronly) {
       var offset = this.offset;
+      len = this.file.read(offset, buf, pos, len);
       this.offset += len;
-      return this.file.read(offset, buf, pos, len);
+      return len;
     }
     this.err_closed("read", raise_unix);
   }