Use uv_os_homedir and current directory as backup when $HOME is not set (#13657)

Close https://github.com/neovim/neovim/issues/8614

Use uv_os_homedir and current directory as backup if HOME env is not set.
Allocate homedir only if var is not NULL.

src/nvim/os/env.c

@@ -394,13 +394,21 @@ void os_get_hostname(char *hostname, size_t size)
 }
 
 /// To get the "real" home directory:
-///   - get value of $HOME
+///   1. get value of $HOME
+///   2. if $HOME is not set, try the following
+/// For Windows:
+///   1. assemble homedir using HOMEDRIVE and HOMEPATH
+///   2. try os_homedir()
+///   3. resolve a direct reference to another system variable
+///   4. guess C drive
 /// For Unix:
-///   - go to that directory
-///   - do os_dirname() to get the real name of that directory.
-/// This also works with mounts and links.
-/// Don't do this for Windows, it will change the "current dir" for a drive.
+///   1. try os_homedir()
+///   2. go to that directory
+///     This also works with mounts and links.
+///     Don't do this for Windows, it will change the "current dir" for a drive.
+///   3. fall back to current working directory as a last resort
 static char *homedir = NULL;
+static char *os_homedir(void);
 
 void init_homedir(void)
 {
@@ -430,7 +438,7 @@ void init_homedir(void)
     }
   }
   if (var == NULL) {
-    var = os_getenv("USERPROFILE");
+    var = os_homedir();
   }
 
   // Weird but true: $HOME may contain an indirect reference to another
@@ -440,6 +448,7 @@ void init_homedir(void)
     const char *p = strchr(var + 1, '%');
     if (p != NULL) {
       vim_snprintf(os_buf, (size_t)(p - var), "%s", var + 1);
+      var = NULL;
       const char *exp = os_getenv(os_buf);
       if (exp != NULL && *exp != NUL
           && STRLEN(exp) + STRLEN(p) < MAXPATHL) {
@@ -458,8 +467,12 @@ void init_homedir(void)
   }
 #endif
 
-  if (var != NULL) {
 #ifdef UNIX
+  if (var == NULL) {
+    var = os_homedir();
+  }
+
+  if (var != NULL) {
     // Change to the directory and get the actual path.  This resolves
     // links.  Don't do it when we can't return.
     if (os_dirname((char_u *)os_buf, MAXPATHL) == OK && os_chdir(os_buf) == 0) {
@@ -470,11 +483,37 @@ void init_homedir(void)
         EMSG(_(e_prev_dir));
       }
     }
+  }
+
+  // Fall back to current working directory if home is not found
+  if ((var == NULL || *var == NUL)
+      && os_dirname((char_u *)os_buf, sizeof(os_buf)) == OK) {
+    var = os_buf;
+  }
 #endif
+  if (var != NULL) {
     homedir = xstrdup(var);
   }
 }
 
+static char homedir_buf[MAXPATHL];
+
+static char *os_homedir(void)
+{
+  homedir_buf[0] = NUL;
+  size_t homedir_size = MAXPATHL;
+  uv_mutex_lock(&mutex);
+  // http://docs.libuv.org/en/v1.x/misc.html#c.uv_os_homedir
+  int ret_value = uv_os_homedir(homedir_buf, &homedir_size);
+  uv_mutex_unlock(&mutex);
+  if (ret_value == 0 && homedir_size < MAXPATHL) {
+    return homedir_buf;
+  }
+  ELOG("uv_os_homedir() failed %d: %s", ret_value, os_strerror(ret_value));
+  homedir_buf[0] = NUL;
+  return NULL;
+}
+
 #if defined(EXITFREE)
 
 void free_homedir(void)

test/functional/eval/environ_spec.lua

@@ -3,6 +3,11 @@ local clear = helpers.clear
 local eq = helpers.eq
 local environ = helpers.funcs.environ
 local exists = helpers.funcs.exists
+local system = helpers.funcs.system
+local nvim_prog = helpers.nvim_prog
+local command = helpers.command
+local eval = helpers.eval
+local setenv = helpers.funcs.setenv
 
 describe('environment variables', function()
   it('environ() handles empty env variable', function()
@@ -17,3 +22,59 @@ describe('environment variables', function()
     eq(0, exists('$DOES_NOT_EXIST'))
   end)
 end)
+
+describe('empty $HOME', function()
+  local original_home = os.getenv('HOME')
+
+  -- recover $HOME after each test
+  after_each(function()
+    if original_home ~= nil then
+      setenv('HOME', original_home)
+    end
+    os.remove('test_empty_home')
+    os.remove('./~')
+  end)
+
+  local function tilde_in_cwd()
+    -- get files in cwd
+    command("let test_empty_home_cwd_files = split(globpath('.', '*'), '\n')")
+    -- get the index of the file named '~'
+    command('let test_empty_home_tilde_index = index(test_empty_home_cwd_files, "./~")')
+    return eval('test_empty_home_tilde_index') ~= -1
+  end
+
+  local function write_and_test_tilde()
+    system({nvim_prog, '-u', 'NONE', '-i', 'NONE', '--headless',
+                                          '-c', 'write test_empty_home', '+q'})
+    eq(false, tilde_in_cwd())
+  end
+
+  it("'~' folder not created in cwd if $HOME and related env not defined", function()
+    command("unlet $HOME")
+    write_and_test_tilde()
+
+    command("let $HOMEDRIVE='C:'")
+    command("let $USERPROFILE='C:\\'")
+    write_and_test_tilde()
+
+    command("unlet $HOMEDRIVE")
+    write_and_test_tilde()
+
+    command("unlet $USERPROFILE")
+    write_and_test_tilde()
+
+    command("let $HOME='%USERPROFILE%'")
+    command("let $USERPROFILE='C:\\'")
+    write_and_test_tilde()
+  end)
+
+  it("'~' folder not created in cwd if writing a file with invalid $HOME", function()
+    setenv('HOME', '/path/does/not/exist')
+    write_and_test_tilde()
+  end)
+
+  it("'~' folder not created in cwd if writing a file with $HOME=''", function()
+    command("let $HOME=''")
+    write_and_test_tilde()
+  end)
+end)