Merge branch 'hostname'

This addresses the discussion regarding the dependency on `hostname` and
the addition of a `$hostname` variable to replace it.

`$hostname` is a read-only, GLOBAL_ENV, non-electrified, lowercased,
non-exported variable that is read once at the start of a fish session.
The finer points of this can be debated endlessly, but this is a shared
starting point that any changes can build on (ref #4422).

Regarding performance: @krader1961 brought up some good points in #4422
regarding potential DNS timeouts (but they really don't apply except if
the host name is not hardcoded in resolv.conf, which quickly manifests
with a cascade of errors on most *nix systems in all cases), but note
that gethostname() was already being called by fish so that would be
more of a future optimization than a "must" at this point.

CHANGELOG.md

@@ -45,6 +45,7 @@ This section is for changes merged to the `major` branch that are not also merge
 - Wrapping completions (from `complete -w` or `function -w`) can now inject arguments. For example, `complete gco -w 'git checkout'` now works properly (#1976). The `alias` function has been updated to respect this behavior.
 - The `jobs` builtin now has a `-q` and `--quiet` option to silence the output.
 - fish now supports `&&`, `||`, and `!` (#4620).
+- The machine hostname, where available, is now exposed as `$hostname` which is now a reserved variable. This drops the dependency on the `hostname` executable (#4422).
 
 ## Other significant changes
 - Command substitution output is now limited to 10 MB by default (#3822).

README.md

@@ -76,7 +76,6 @@ Running fish requires:
     * `awk`
     * `find`
     * `grep`
-    * `hostname`
     * `kill`
     * `ps`
     * `sed`

share/functions/prompt_hostname.fish

@@ -1,10 +1,3 @@
-# Fetching the host name can be expensive if there is a problem with DNS or whatever subsystem the
-# hostname command uses. So cache the answer so including it in the prompt doesn't make fish seem
-# slow.
-if not set -q __fish_prompt_hostname
-    set -g __fish_prompt_hostname (hostname | string split '.')[1]
-end
-
 function prompt_hostname
-    echo $__fish_prompt_hostname
+    echo $hostname;
 end

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

@@ -44,6 +44,7 @@
 #include "utf8.h"
 #include "util.h"  // IWYU pragma: keep
 #include "wutil.h"
+#include "wcstringutil.h"
 
 #if __APPLE__
 #define FISH_NOTIFYD_AVAILABLE 1
@@ -76,7 +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"";
@@ -495,7 +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.
@@ -911,11 +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[HOSTNAME_LEN + 1] = {};
-    if (gethostname(hostname, HOSTNAME_LEN) == 0) {
-        result->assign(str2wcstring(hostname));
+    char hostname[256] = {};
+    if (gethostname(hostname, sizeof(hostname)) == 0) {
+        result.assign(str2wcstring(hostname));
+        result.assign(truncate(result, HOSTNAME_LEN));
         success = true;
     }
     return success;
@@ -931,7 +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