Fallback to getpwuid in the unlikely case $HOME is underfined

Add a homedir() helper function, and document the $kak_config
env var.

doc/pages/expansions.asciidoc

@@ -80,7 +80,10 @@ informations about Kakoune's state:
     history id of the current buffer, the history id is an integer value
     which is used to reference a specific buffer version in the undo tree
 *kak_runtime*::
-    directory containing the kak binary
+    directory containing the kak support files, determined from kakoune's
+    binary location.
+*kak_config*::
+    directory containing the user configuration
 *kak_count*::
     count parameter passed to the command
 *kak_opt_<name>*::

src/file.cc

@@ -10,13 +10,14 @@
 #include "unicode.hh"
 
 #include <cerrno>
-#include <cstring>
-#include <sys/mman.h>
-#include <fcntl.h>
-#include <unistd.h>
-#include <dirent.h>
 #include <cstdlib>
+#include <cstring>
+#include <dirent.h>
+#include <fcntl.h>
+#include <pwd.h>
+#include <sys/mman.h>
 #include <sys/select.h>
+#include <unistd.h>
 
 #if defined(__FreeBSD__)
 #include <sys/sysctl.h>
@@ -51,7 +52,7 @@ String parse_filename(StringView filename)
 {
     auto prefix = filename.substr(0_byte, 2_byte);
     if (prefix == "~" or prefix == "~/")
-        return getenv("HOME") + filename.substr(1_byte);
+        return homedir() + filename.substr(1_byte);
     return filename.str();
 }
 
@@ -105,10 +106,10 @@ String compact_path(StringView filename)
     if (prefix_match(real_filename, real_cwd))
         return real_filename.substr(real_cwd.length()).str();
 
-    const char* home = getenv("HOME");
+    const StringView home = homedir();
-    if (home)
+    if (not home.empty())
     {
-        ByteCount home_len = (int)strlen(home);
+        ByteCount home_len = home.length();
         if (real_filename.substr(0, home_len) == home)
             return "~" + real_filename.substr(home_len);
     }
@@ -125,6 +126,14 @@ StringView tmpdir()
     return "/tmp";
 }
 
+StringView homedir()
+{
+    StringView home = getenv("HOME");
+    if (home.empty())
+        return getpwuid(geteuid())->pw_dir;
+    return home;
+}
+
 bool fd_readable(int fd)
 {
     fd_set  rfds;
@@ -331,7 +340,7 @@ String find_file(StringView filename, ConstArrayView<String> paths)
     }
     if (filename.substr(0_byte, 2_byte) == "~/")
     {
-        String candidate = getenv("HOME") + filename.substr(1_byte);
+        String candidate = homedir() + filename.substr(1_byte);
         if (stat(candidate.c_str(), &buf) == 0 and S_ISREG(buf.st_mode))
             return candidate;
         return "";

src/file.hh

@@ -25,6 +25,7 @@ String real_path(StringView filename);
 String compact_path(StringView filename);
 
 StringView tmpdir();
+StringView homedir();
 
 // returns pair { directory, filename }
 std::pair<StringView, StringView> split_path(StringView path);

src/main.cc

@@ -72,7 +72,7 @@ String config_directory()
 {
     StringView config_home = getenv("XDG_CONFIG_HOME");
     if (config_home.empty())
-        return format("{}/.config/kak", getenv("HOME"));
+        return format("{}/.config/kak", homedir());
     return format("{}/kak", config_home);
 }