Improve dlerror message reporting

- for absolute paths, show the error message if file exists.
  this way users always get diagnostics when they try a known
  path.
- for DL_LOAD_PATH/DL_EXT permutations: if a file exists, show
  error. these paths should be complete by construction (ie,
  include extension). if dlopen fails and the file exists,
  best to let the user know.
- when throwing an error, first print the dlerror, then free the
  handle, then throw the error. this is not idea, but is the
  simplest way I can think of to actually show the error without
  leaking the errmsg.
- free the handle before throwing an error on Windows

also add some comments.

src/dlload.c

@@ -83,28 +83,45 @@ static uv_lib_t *jl_load_dynamic_library_(const char *modname, unsigned flags, i
  char *ext;
  char path[PATHBUF];
  int i;
+ uv_stat_t stbuf;
  uv_lib_t *handle = (uv_lib_t*)malloc(sizeof(uv_lib_t));
  handle->errmsg = NULL;
 
+/*
+ this branch returns handle of libjulia
+*/
  if (modname == NULL) {
 #ifdef _OS_WINDOWS_
  if (!GetModuleHandleExW(GET_MODULE_HANDLE_EX_FLAG_FROM_ADDRESS | GET_MODULE_HANDLE_EX_FLAG_UNCHANGED_REFCOUNT,
  (LPCWSTR)(&jl_load_dynamic_library),
- &handle->handle))
+ &handle->handle)) {
+ free(handle);
  jl_errorf("could not load base module", modname);
+ }
 #else
  handle->handle = dlopen(NULL,RTLD_NOW);
 #endif
  goto done;
  }
+/*
+ this branch shortcuts absolute paths
+*/
 #ifdef _OS_WINDOWS_
  else if (modname[1] == ':') {
 #else
  else if (modname[0] == '/') {
 #endif
  error = jl_uv_dlopen(modname,handle,flags);
  if (!error) goto done;
+ // bail out and show the error if file actually exists
+ if (jl_stat(modname, (char*)&stbuf) == 0) goto notfound;
+ if (handle->errmsg)
+ uv_dlclose(handle);
  }
+/*
+ this branch permutes all base paths in DL_LOAD_PATH with all extensions
+ note: skip when !jl_base_module to avoid UndefVarError(:DL_LOAD_PATH)
+*/
  else if (jl_base_module != NULL) {
  jl_array_t *DL_LOAD_PATH = (jl_array_t*)jl_get_global(jl_base_module, jl_symbol("DL_LOAD_PATH"));
  if (DL_LOAD_PATH != NULL) {
@@ -123,19 +140,26 @@ static uv_lib_t *jl_load_dynamic_library_(const char *modname, unsigned flags, i
  else
  snprintf(path, PATHBUF, "%s" PATHSEPSTRING "%s%s", dl_path, modname, ext);
  if (handle->errmsg) {
+ // free handle->errmsg, else it will leak on next uv_dlopen
  uv_dlclose(handle);
  }
+
  error = jl_uv_dlopen(path, handle, flags);
  if (!error) goto done;
+ // bail out and show the error if file actually exists
+ if (jl_stat(path, (char*)&stbuf) == 0) goto notfound;
  }
  }
  }
  }
+
+/*
+ now fall back and look in default library paths, for all extensions
+*/
  for(i=0; i < N_EXTENSIONS; i++) {
  ext = extensions[i];
  path[0] = '\0';
  handle->handle = NULL;
- /* try loading from standard library path */
  snprintf(path, PATHBUF, "%s%s", modname, ext);
  if (handle->errmsg) {
  uv_dlclose(handle);
@@ -145,23 +169,23 @@ static uv_lib_t *jl_load_dynamic_library_(const char *modname, unsigned flags, i
  }
 
 #if defined(__linux__) || defined(__FreeBSD__)
+// check map of versioned libs from "libX" to full soname "libX.so.ver"
  {
  const char *soname = jl_lookup_soname(modname, strlen(modname));
- if (handle->errmsg) {
- free(handle->errmsg);
- handle->errmsg = NULL;
- }
  error = (soname==NULL) || jl_uv_dlopen(soname, handle, flags);
  if (!error) goto done;
  }
 #endif
 
- if (throw_err) {
-  //jl_printf(JL_STDERR, "could not load module %s (%d): %s\n", modname, error, uv_dlerror(handle));
- jl_errorf("could not load module %s: %s", modname, uv_dlerror(handle));
- }
+notfound:
+ if (throw_err)
+ jl_printf(JL_STDERR, "dlerror loading '%s': %s\n", modname, uv_dlerror(handle));
+
  uv_dlclose(handle);
  free(handle);
+
+ if (throw_err)
+ jl_errorf("could not load library %s", modname);
  return NULL;
 done:
  return handle;