getenv: return NULL if empty #2574

Making an environment variable empty can be a way of unsetting it for
platforms that don't support unsetenv(). In most cases, we treat empty
variables as having been unset. For all others, use os_env_exists().

10 files changed, 63 insertions, 56 deletions

diff --git a/src/nvim/diff.c b/src/nvim/diff.c
index 7c1a9de623..6e2b3056e4 100644
--- a/src/nvim/diff.c
+++ b/src/nvim/diff.c
@@ -808,7 +808,7 @@ static void diff_file(char_u *tmp_orig, char_u *tmp_new, char_u *tmp_diff)
 
     /* We don't want $DIFF_OPTIONS to get in the way. */
     if (os_getenv("DIFF_OPTIONS")) {
-      vim_setenv("DIFF_OPTIONS", "");
+      os_unsetenv("DIFF_OPTIONS");
     }
 
     /* Build the diff command and execute it.  Always use -a, binary
diff --git a/src/nvim/ex_cmds.c b/src/nvim/ex_cmds.c
index 824cc125f4..b4adef9235 100644
--- a/src/nvim/ex_cmds.c
+++ b/src/nvim/ex_cmds.c
@@ -1658,9 +1658,9 @@ static char_u *viminfo_filename(char_u *file)
       file = use_viminfo;
     else if ((file = find_viminfo_parameter('n')) == NULL || *file == NUL) {
 #ifdef VIMINFO_FILE2
-      /* don't use $HOME when not defined (turned into "c:/"!). */
-      if (os_getenv((char_u *)"HOME") == NULL) {
-        /* don't use $VIM when not available. */
+      // don't use $HOME when not defined (turned into "c:/"!).
+      if (!os_env_exists("HOME")) {
+        // don't use $VIM when not available.
         expand_env((char_u *)"$VIM", NameBuff, MAXPATHL);
         if (STRCMP("$VIM", NameBuff) != 0)          /* $VIM was expanded */
           file = (char_u *)VIMINFO_FILE2;
diff --git a/src/nvim/ex_cmds2.c b/src/nvim/ex_cmds2.c
index 3fe4acc471..5c1f487d5e 100644
--- a/src/nvim/ex_cmds2.c
+++ b/src/nvim/ex_cmds2.c
@@ -3025,10 +3025,11 @@ char *get_mess_lang(void)
 #  endif
 # else
   p = os_getenv("LC_ALL");
-  if (p == NULL || *p == NUL) {
+  if (p == NULL) {
     p = os_getenv("LC_MESSAGES");
-    if (p == NULL || *p == NUL)
+    if (p == NULL) {
       p = os_getenv("LANG");
+    }
   }
 # endif
   return p;
@@ -3044,15 +3045,17 @@ static char_u *get_mess_env(void)
   char_u      *p;
 
   p = (char_u *)os_getenv("LC_ALL");
-  if (p == NULL || *p == NUL) {
+  if (p == NULL) {
     p = (char_u *)os_getenv("LC_MESSAGES");
-    if (p == NULL || *p == NUL) {
+    if (p == NULL) {
       p = (char_u *)os_getenv("LANG");
-      if (p != NULL && ascii_isdigit(*p))
+      if (p != NULL && ascii_isdigit(*p)) {
         p = NULL;                       /* ignore something like "1043" */
+      }
 # ifdef HAVE_GET_LOCALE_VAL
-      if (p == NULL || *p == NUL)
+      if (p == NULL) {
         p = (char_u *)get_locale_val(LC_CTYPE);
+      }
 # endif
     }
   }
diff --git a/src/nvim/main.c b/src/nvim/main.c
index fc72039b5f..ddc39caa6f 100644
--- a/src/nvim/main.c
+++ b/src/nvim/main.c
@@ -1889,8 +1889,8 @@ static void main_start_gui(void)
 ///         OK otherwise.
 static int process_env(char *env, bool is_viminit)
 {
-  char *initstr = (char *)os_getenv(env);
-  if (initstr != NULL && *initstr != NUL) {
+  const char *initstr = os_getenv(env);
+  if (initstr != NULL) {
     if (is_viminit) {
       vimrc_found(NULL, NULL);
     }
@@ -1900,7 +1900,7 @@ static int process_env(char *env, bool is_viminit)
     sourcing_lnum = 0;
     scid_T save_sid = current_SID;
     current_SID = SID_ENV;
-    do_cmdline_cmd(initstr);
+    do_cmdline_cmd((char *)initstr);
     sourcing_name = save_sourcing_name;
     sourcing_lnum = save_sourcing_lnum;
     current_SID = save_sid;;
diff --git a/src/nvim/mbyte.c b/src/nvim/mbyte.c
index b6e08ab277..368ae8e773 100644
--- a/src/nvim/mbyte.c
+++ b/src/nvim/mbyte.c
@@ -3398,22 +3398,29 @@ static int enc_alias_search(char_u *name)
  */
 char_u * enc_locale(void)
 {
-  char        *s;
-  char        *p;
   int i;
   char buf[50];
+
+  const char *s;
 # ifdef HAVE_NL_LANGINFO_CODESET
-  if ((s = nl_langinfo(CODESET)) == NULL || *s == NUL)
+  if (!(s = nl_langinfo(CODESET)) || *s == NUL)
 # endif
+  {
 #  if defined(HAVE_LOCALE_H) || defined(X_LOCALE)
-    if ((s = setlocale(LC_CTYPE, NULL)) == NULL || *s == NUL)
+    if (!(s = setlocale(LC_CTYPE, NULL)) || *s == NUL)
 #  endif
-      if ((s = (char *)os_getenv("LC_ALL")) == NULL || *s == NUL)
-        if ((s = (char *)os_getenv("LC_CTYPE")) == NULL || *s == NUL)
-          s = (char *)os_getenv("LANG");
+    {
+      if ((s = os_getenv("LC_ALL"))) {
+        if ((s = os_getenv("LC_CTYPE"))) {
+          s = os_getenv("LANG");
+        }
+      }
+    }
+  }
 
-  if (s == NULL || *s == NUL)
+  if (!s) {
     return NULL;
+  }
 
   /* The most generic locale format is:
    * language[_territory][.codeset][@modifier][+special][,[sponsor][_revision]]
@@ -3423,11 +3430,12 @@ char_u * enc_locale(void)
    * Exception: "ja_JP.EUC" == "euc-jp", "zh_CN.EUC" = "euc-cn",
    * "ko_KR.EUC" == "euc-kr"
    */
-  if ((p = (char *)vim_strchr((char_u *)s, '.')) != NULL) {
-    if (p > s + 2 && STRNICMP(p + 1, "EUC", 3) == 0
+  const char *p = (char *)vim_strchr((char_u *)s, '.');
+  if (p != NULL) {
+    if (p > s + 2 && !STRNICMP(p + 1, "EUC", 3)
         && !isalnum((int)p[4]) && p[4] != '-' && p[-3] == '_') {
       /* copy "XY.EUC" to "euc-XY" to buf[10] */
-      STRCPY(buf + 10, "euc-");
+      strcpy(buf + 10, "euc-");
       buf[14] = p[-2];
       buf[15] = p[-1];
       buf[16] = 0;
diff --git a/src/nvim/message.c b/src/nvim/message.c
index 9bb92f8928..5b4c90cc8f 100644
--- a/src/nvim/message.c
+++ b/src/nvim/message.c
@@ -743,7 +743,7 @@ void ex_messages(exarg_T *eap)
   msg_hist_off = TRUE;
 
   s = os_getenv("LANG");
-  if (s != NULL && *s != NUL)
+  if (s)
     msg_attr((char_u *)
         _("Messages maintainer: Bram Moolenaar <Bram@vim.org>"),
         hl_attr(HLF_T));
diff --git a/src/nvim/msgpack_rpc/server.c b/src/nvim/msgpack_rpc/server.c
index ab1b04d73f..388b5a04cf 100644
--- a/src/nvim/msgpack_rpc/server.c
+++ b/src/nvim/msgpack_rpc/server.c
@@ -16,6 +16,7 @@
 #include "nvim/log.h"
 #include "nvim/tempfile.h"
 #include "nvim/path.h"
+#include "nvim/strings.h"
 
 #define MAX_CONNECTIONS 32
 #define ADDRESS_MAX_SIZE 256
@@ -59,7 +60,7 @@ bool server_init(void)
 
   bool must_free = false;
   const char *listen_address = os_getenv(LISTEN_ADDRESS_ENV_VAR);
-  if (listen_address == NULL || *listen_address == NUL) {
+  if (listen_address == NULL) {
     must_free = true;
     listen_address = (char *)vim_tempname();
   }
@@ -216,7 +217,7 @@ int server_start(const char *endpoint)
 
   // Update $NVIM_LISTEN_ADDRESS, if not set.
   const char *listen_address = os_getenv(LISTEN_ADDRESS_ENV_VAR);
-  if (listen_address == NULL || *listen_address == NUL) {
+  if (listen_address == NULL) {
     os_setenv(LISTEN_ADDRESS_ENV_VAR, addr, 1);
   }
 
diff --git a/src/nvim/option.c b/src/nvim/option.c
index b3789e0e5b..6fb661b6e2 100644
--- a/src/nvim/option.c
+++ b/src/nvim/option.c
@@ -1727,7 +1727,7 @@ void set_init_1(void)
   p_cp = FALSE;
 
   /* Use POSIX compatibility when $VIM_POSIX is set. */
-  if (os_getenv("VIM_POSIX") != NULL) {
+  if (os_env_exists("VIM_POSIX")) {
     set_string_default("cpo", (char_u *)CPO_ALL);
     set_string_default("shm", (char_u *)"A");
   }
@@ -1736,8 +1736,7 @@ void set_init_1(void)
    * Find default value for 'shell' option.
    * Don't use it if it is empty.
    */
-  if (((p = (char_u *)os_getenv("SHELL")) != NULL && *p != NUL)
-      )
+  if ((p = (char_u *)os_getenv("SHELL")) != NULL)
     set_string_default("sh", p);
 
   /*
@@ -1926,7 +1925,7 @@ void set_init_1(void)
    * NOTE: mlterm's author is being asked to 'set' a variable
    *       instead of an environment variable due to inheritance.
    */
-  if (os_getenv("MLTERM") != NULL)
+  if (os_env_exists("MLTERM"))
     set_option_value((char_u *)"tbidi", 1L, NULL, 0);
 
   /* Parse default for 'fillchars'. */
diff --git a/src/nvim/os/env.c b/src/nvim/os/env.c
index fd115ab2b3..7be8a868bd 100644
--- a/src/nvim/os/env.c
+++ b/src/nvim/os/env.c
@@ -27,12 +27,24 @@
 #include <sys/utsname.h>
 #endif
 
+/// Like getenv(), but returns NULL if the variable is empty.
 const char *os_getenv(const char *name)
+  FUNC_ATTR_NONNULL_ALL
 {
-  return getenv(name);
+  const char *e = getenv(name);
+  return e == NULL || *e == NUL ? NULL : e;
+}
+
+/// Returns `true` if the environment variable, `name`, has been defined
+/// (even if empty).
+bool os_env_exists(const char *name)
+  FUNC_ATTR_NONNULL_ALL
+{
+  return getenv(name) != NULL;
 }
 
 int os_setenv(const char *name, const char *value, int overwrite)
+  FUNC_ATTR_NONNULL_ALL
 {
   return setenv(name, value, overwrite);
 }
@@ -123,17 +135,11 @@ static char_u   *homedir = NULL;
 
 void init_homedir(void)
 {
-  char_u  *var;
-
   /* In case we are called a second time (when 'encoding' changes). */
   xfree(homedir);
   homedir = NULL;
 
-  var = (char_u *)os_getenv("HOME");
-
-  if (var != NULL && *var == NUL)       /* empty is same as not set */
-    var = NULL;
-
+  char_u *var = (char_u *)os_getenv("HOME");
 
   if (var != NULL) {
 #ifdef UNIX
@@ -417,11 +423,6 @@ static char *remove_tail(char *p, char *pend, char *name)
 char *vim_getenv(const char *name)
 {
   const char *kos_env_path = os_getenv(name);
-  if (kos_env_path != NULL
-      && *kos_env_path == NUL) {  // empty is the same as not set
-    kos_env_path = NULL;
-  }
-
   if (kos_env_path != NULL) {
     return xstrdup(kos_env_path);
   }
@@ -440,10 +441,6 @@ char *vim_getenv(const char *name)
 #endif
       ) {
     kos_env_path = os_getenv("VIM");
-    if (kos_env_path != NULL
-        && *kos_env_path == NUL) {  // empty is the same as not set
-      kos_env_path = NULL;
-    }
     if (kos_env_path != NULL) {
       vim_path = vim_version_dir(kos_env_path);
       if (vim_path == NULL) {
@@ -533,8 +530,6 @@ void home_replace(buf_T *buf, char_u *src, char_u *dst, int dstlen, bool one)
 {
   size_t dirlen = 0, envlen = 0;
   size_t len;
-  char_u      *homedir_env, *homedir_env_orig;
-  char_u      *p;
 
   if (src == NULL) {
     *dst = NUL;
@@ -556,12 +551,11 @@ void home_replace(buf_T *buf, char_u *src, char_u *dst, int dstlen, bool one)
   if (homedir != NULL)
     dirlen = STRLEN(homedir);
 
-  homedir_env_orig = homedir_env = (char_u *)os_getenv("HOME");
-  /* Empty is the same as not set. */
-  if (homedir_env != NULL && *homedir_env == NUL)
-    homedir_env = NULL;
+  char_u *homedir_env = (char_u *)os_getenv("HOME");
+  bool must_free = false;
 
   if (homedir_env != NULL && vim_strchr(homedir_env, '~') != NULL) {
+    must_free = true;
     size_t usedlen = 0;
     size_t flen = STRLEN(homedir_env);
     char_u *fbuf = NULL;
@@ -587,7 +581,7 @@ void home_replace(buf_T *buf, char_u *src, char_u *dst, int dstlen, bool one)
      * as "~er/bla" (which would seem to indicate the file "bla" in user
      * er's home directory)).
      */
-    p = homedir;
+    char_u *p = homedir;
     len = dirlen;
     for (;; ) {
       if (   len
@@ -623,8 +617,9 @@ void home_replace(buf_T *buf, char_u *src, char_u *dst, int dstlen, bool one)
 
   *dst = NUL;
 
-  if (homedir_env != homedir_env_orig)
+  if (must_free) {
     xfree(homedir_env);
+  }
 }
 
 /// Like home_replace, store the replaced string in allocated memory.
diff --git a/src/nvim/tui/tui.c b/src/nvim/tui/tui.c
index 1a8e4523b7..7df1c4f381 100644
--- a/src/nvim/tui/tui.c
+++ b/src/nvim/tui/tui.c
@@ -15,6 +15,7 @@
 #include "nvim/api/private/helpers.h"
 #include "nvim/os/event.h"
 #include "nvim/tui/tui.h"
+#include "nvim/strings.h"
 
 // Space reserved in the output buffer to restore the cursor to normal when
 // flushing. No existing terminal will require 32 bytes to do that.