api: uv_link_errno

docs/api.md

@@ -118,6 +118,15 @@ Invoke `shutdown` from the link's [`uv_link_methods_t`][]. Acts similarly to
 `uv_shutdown()`. `cb(uv_link_t* link, int status, void* arg)` is invoked on
 completion.
 
+### int uv_link_errno(...)
+
+* `uv_link_t** link`
+* `int err` - error code, previously either returned the one of the
+  `uv_link...` methods or passed as a negative `nread` to `link->read_cb`
+
+Unprefix internal error code and set the `link` pointer to the link that
+have emitted that error.
+
 ### const char* uv_link_strerror(...)
 
 * `uv_link_t* link`

include/uv_link_t.h

@@ -125,6 +125,7 @@ static int uv_link_shutdown(uv_link_t* link, uv_link_shutdown_cb cb,
   return uv_link_propagate_shutdown(link, link, cb, arg);
 }
 
+int uv_link_errno(uv_link_t** link, int err);
 const char* uv_link_strerror(uv_link_t* link, int err);
 
 /* Link Source */

src/uv_link_t.c

@@ -221,21 +221,42 @@ int uv_link_unchain(uv_link_t* from, uv_link_t* to) {
 }
 
 
-const char* uv_link_strerror(uv_link_t* link, int err) {
+int uv_link_errno(uv_link_t** link, int err) {
   unsigned int prefix;
   int local_err;
+  uv_link_t* p;
+
+  if (err >= UV_ERRNO_MAX) {
+    *link = NULL;
+    return err;
+  }
+
+  prefix = (-err) & kErrorPrefixMask;
+  local_err = -((-err) & kErrorValueMask);
+
+  for (p = *link; p != NULL; p = p->parent) {
+    if (prefix == p->err_prefix) {
+      *link = p;
+      return local_err;
+    }
+  }
+
+  *link = NULL;
+  return err;
+}
+
+
+const char* uv_link_strerror(uv_link_t* link, int err) {
+  int local_err;
 
   if (err >= UV_ERRNO_MAX)
     return uv_strerror(err);
 
-  prefix = (-err) & kErrorPrefixMask;
-  local_err = -((-err) & kErrorValueMask);
+  local_err = uv_link_errno(&link, err);
+  if (link == NULL)
+    return NULL;
 
-  for (; link != NULL; link = link->parent)
-    if (prefix == link->err_prefix)
-      return link->methods->strerror(link, local_err);
-
-  return NULL;
+  return link->methods->strerror(link, local_err);
 }