Implement $hostname variable

The value is not electrified or tied and is read-only. It isn't cached
in the get_hostname_identifier() function as the ENV_GLOBAL $hostname
will cache it for its duration.

src/env.cpp

@@ -322,7 +322,7 @@ bool string_set_contains(const T &set, const wchar_t *val) {
 
 /// Check if a variable may not be set using the set command.
 static bool is_read_only(const wchar_t *val) {
-    const string_set_t env_read_only = {L"PWD", L"SHLVL", L"_", L"history", L"status", L"version", L"pid"};
+    const string_set_t env_read_only = {L"PWD", L"SHLVL", L"_", L"history", L"status", L"version", L"pid", L"hostname"};
     return string_set_contains(env_read_only, val);
 }
 
@@ -972,6 +972,11 @@ void env_init(const struct config_paths_t *paths /* or NULL */) {
     // Set the $pid variable (%self replacement)
     env_set_one(L"pid", ENV_GLOBAL, to_string<long>(getpid()));
 
+    // Set the $hostname variable
+    wcstring hostname = L"fish";
+    get_hostname_identifier(hostname);
+    env_set_one(L"hostname", ENV_GLOBAL, hostname);
+
     // Set up SHLVL variable. Not we can't use env_get because SHLVL is read-only, and therefore was
     // not inherited from the environment.
     wcstring nshlvl_str = L"1";

src/env_universal_common.cpp

@@ -77,7 +77,6 @@
     "changes will be overwritten.\n"
 
 static wcstring get_machine_identifier();
-static bool get_hostname_identifier(wcstring *result);
 
 static wcstring vars_filename_in_directory(const wcstring &wdir) {
     if (wdir.empty()) return L"";
@@ -496,7 +495,7 @@ bool env_universal_t::load(callback_data_list_t &callbacks) {
         // Silently "upgraded."
         tried_renaming = true;
         wcstring hostname_id;
-        if (get_hostname_identifier(&hostname_id)) {
+        if (get_hostname_identifier(hostname_id)) {
             const wcstring hostname_path = wdirname(vars_path) + L'/' + hostname_id;
             if (0 == wrename(hostname_path, vars_path)) {
                 // We renamed - try again.
@@ -912,14 +911,14 @@ static bool get_mac_address(unsigned char macaddr[MAC_ADDRESS_MAX_LEN]) { return
 #endif
 
 /// Function to get an identifier based on the hostname.
-static bool get_hostname_identifier(wcstring *result) {
+bool get_hostname_identifier(wcstring &result) {
     //The behavior of gethostname if the buffer size is insufficient differs by implementation and libc version
     //Work around this by using a "guaranteed" sufficient buffer size then truncating the result.
     bool success = false;
     char hostname[256] = {};
     if (gethostname(hostname, sizeof(hostname)) == 0) {
-        result->assign(str2wcstring(hostname));
-        result->assign(truncate(*result, HOSTNAME_LEN));
+        result.assign(str2wcstring(hostname));
+        result.assign(truncate(result, HOSTNAME_LEN));
         success = true;
     }
     return success;
@@ -935,7 +934,7 @@ wcstring get_machine_identifier() {
         for (size_t i = 0; i < MAC_ADDRESS_MAX_LEN; i++) {
             append_format(result, L"%02x", mac_addr[i]);
         }
-    } else if (!get_hostname_identifier(&result)) {
+    } else if (!get_hostname_identifier(result)) {
         result.assign(L"nohost");  // fallback to a dummy value
     }
     return result;

src/env_universal_common.h

@@ -28,6 +28,7 @@ struct callback_data_t {
 
 typedef std::vector<struct callback_data_t> callback_data_list_t;
 
+bool get_hostname_identifier(wcstring &result);
 /// Class representing universal variables.
 class env_universal_t {
     var_table_t vars;  // current values