Implement os_unsetenv()

- In UNIX systems where unsetenv() is available, it is used. Otherwise
  the variables are set with the empty string.
- New check HAVE_UNSETENV for unsetenv()
- Added unit test to env_spec.lua

config/CMakeLists.txt

@@ -58,6 +58,7 @@ check_function_exists(setenv HAVE_SETENV)
 if(NOT HAVE_SETENV)
   message(SEND_ERROR "setenv() function not found on your system.")
 endif()
+check_function_exists(unsetenv HAVE_UNSETENV)
 check_function_exists(setpgid HAVE_SETPGID)
 check_function_exists(setsid HAVE_SETSID)
 check_function_exists(sigaction HAVE_SIGACTION)

config/config.h.in

@@ -37,6 +37,7 @@
 // TODO: add proper cmake check
 // #define HAVE_SELINUX 1
 #cmakedefine HAVE_SETENV
+#cmakedefine HAVE_UNSETENV
 #cmakedefine HAVE_SETPGID
 #cmakedefine HAVE_SETSID
 #cmakedefine HAVE_SIGACTION

src/nvim/os/env.c

@@ -37,6 +37,19 @@ int os_setenv(const char *name, const char *value, int overwrite)
   return setenv(name, value, overwrite);
 }
 
+/// Unset environment variable
+///
+/// For systems where unsetenv() is not available the value will be set as an
+/// empty string
+int os_unsetenv(const char *name)
+{
+#ifdef HAVE_UNSETENV
+  return unsetenv(name);
+#else
+  return os_setenv(name, "", 1);
+#endif
+}
+
 char *os_getenvname_at_index(size_t index)
 {
 # if defined(HAVE__NSGETENVIRON)

test/unit/os/env_spec.lua

@@ -3,6 +3,7 @@ local helpers = require('test.unit.helpers')
 local cimport = helpers.cimport
 local internalize = helpers.internalize
 local eq = helpers.eq
+local neq = helpers.neq
 local ffi = helpers.ffi
 local lib = helpers.lib
 local cstr = helpers.cstr
@@ -21,6 +22,10 @@ describe('env function', function()
     return env.os_setenv((to_cstr(name)), (to_cstr(value)), override)
   end
 
+  function os_unsetenv(name, value, override)
+    return env.os_unsetenv((to_cstr(name)))
+  end
+
   function os_getenv(name)
     local rval = env.os_getenv((to_cstr(name)))
     if rval ~= NULL then
@@ -68,6 +73,18 @@ describe('env function', function()
     end)
   end)
 
+  describe('os_unsetenv', function()
+    it('unsets environment variable', function()
+      local name = 'TEST_UNSETENV'
+      local value = 'TESTVALUE'
+      os_setenv(name, value, 1)
+      os_unsetenv(name)
+      neq(os_getenv(name), value)
+      -- Depending on the platform the var might be unset or set as ''
+      assert.True(os_getenv(name) == nil or os_getenv(name) == '')
+    end)
+  end)
+
   describe('os_getenvname_at_index', function()
     it('returns names of environment variables', function()
       local test_name = 'NEOVIM_UNIT_TEST_GETENVNAME_AT_INDEX_1N'