Decrement SHLVL when running `exec`

`exec` removes fish from the shell "stack", so SHLVL needs to be
decremented to match. This means `exec fish` will result in the same
SHLVL in the new fish instance.

Also tweak the SHLVL logic to interpret an environment SHLVL of "3foo"
as garbage instead of as the value "3".

Fixes #1693.

1 files changed, 4 insertions, 2 deletions

diff --git a/env.cpp b/env.cpp
index 353085b8..c9f986b1 100644
--- a/env.cpp
+++ b/env.cpp
@@ -542,8 +542,10 @@ void env_init(const struct config_paths_t *paths /* or NULL */)
     wcstring nshlvl_str = L"1";
     if (! shlvl_str.missing())
     {
-        long shlvl_i = wcstol(shlvl_str.c_str(), NULL, 10);
-        if (shlvl_i >= 0)
+        wchar_t *end;
+        long shlvl_i = wcstol(shlvl_str.c_str(), &end, 10);
+        while (iswspace(*end)) ++end; /* skip trailing whitespace */
+        if (shlvl_i >= 0 && *end == '\0')
         {
             nshlvl_str = to_string<long>(shlvl_i + 1);
         }