win: Append process dir to $PATH

This allows executables to be found by :!, system(), and executable() if
they live next to ("sibling" to) nvim.exe. This is what gvim on Windows
does, and also matches the behavior of Win32 SearchPath().

c4a249a736/src/os_win32.c (L354-L370)

src/nvim/main.c

@@ -238,9 +238,7 @@ int main(int argc, char **argv)
   // Check if we have an interactive window.
   check_and_set_isatty(&params);
 
-  // Get the name with which Nvim was invoked, with and without path.
-  set_vim_var_string(VV_PROGPATH, argv[0], -1);
-  set_vim_var_string(VV_PROGNAME, (char *) path_tail((char_u *) argv[0]), -1);
+  init_path(argv[0]);
 
   event_init();
   /*
@@ -1194,9 +1192,27 @@ static void check_and_set_isatty(mparm_T *paramp)
   paramp->err_isatty = os_isatty(fileno(stderr));
   TIME_MSG("window checked");
 }
-/*
- * Get filename from command line, given that there is one.
- */
+
+// Sets v:progname and v:progpath. Also modifies $PATH on Windows.
+static void init_path(char *exename)
+{
+  char exepath[MAXPATHL] = { 0 };
+  size_t exepathlen = MAXPATHL;
+  // Make v:progpath absolute.
+  if (os_exepath(exepath, &exepathlen) != 0) {
+    EMSG2(e_intern2, "init_path()");
+  }
+  set_vim_var_string(VV_PROGPATH, exepath, -1);
+  set_vim_var_string(VV_PROGNAME, (char *)path_tail((char_u *)exename), -1);
+
+#ifdef WIN32
+  // Append the process start directory to $PATH, so that ":!foo" finds tools
+  // shipped with Windows package. This also mimics SearchPath().
+  os_setenv_append_path(exepath);
+#endif
+}
+
+/// Get filename from command line, if any.
 static char_u *get_fname(mparm_T *parmp, char_u *cwd)
 {
 #if !defined(UNIX)

src/nvim/os/env.c

@@ -831,3 +831,43 @@ char_u *get_env_name(expand_T *xp, int idx)
   return NULL;
 }
 
+/// Appends the head of `fname` to $PATH and sets it in the environment.
+///
+/// @param fname  Full path whose parent directory will be appended to $PATH.
+///
+/// @return true if `path` was appended-to
+bool os_setenv_append_path(const char *fname)
+  FUNC_ATTR_NONNULL_ALL
+{
+#ifdef WIN32
+// 8191 (plus NUL) is considered the practical maximum.
+# define MAX_ENVPATHLEN 8192
+#else
+// No prescribed maximum on unix.
+# define MAX_ENVPATHLEN INT_MAX
+#endif
+  if (!path_is_absolute_path((char_u *)fname)) {
+    EMSG2(_(e_intern2), "os_setenv_append_path()");
+    return false;
+  }
+  const char *tail = (char *)path_tail_with_sep((char_u *)fname);
+  const char *dir = (char *)vim_strnsave((char_u *)fname,
+                                         (size_t)(tail - fname));
+  const char *path = os_getenv("PATH");
+  const size_t pathlen = path ? strlen(path) : 0;
+  const size_t newlen = pathlen + strlen(dir) + 2;
+  if (newlen < MAX_ENVPATHLEN) {
+    char *temp = xmalloc(newlen);
+    if (pathlen == 0) {
+      temp[0] = NUL;
+    } else {
+      xstrlcpy(temp, path, newlen);
+      xstrlcat(temp, ENV_SEPSTR, newlen);
+    }
+    xstrlcat(temp, dir, newlen);
+    os_setenv("PATH", temp, 1);
+    xfree(temp);
+    return true;
+  }
+  return false;
+}

src/nvim/os/fs.c

@@ -224,7 +224,7 @@ bool os_can_exe(const char_u *name, char_u **abspath, bool use_path)
   if (!use_path || path_is_absolute_path(name)
       || (name[0] == '.'
           && (name[1] == '/' || (name[1] == '.' && name[2] == '/')))) {
-#if WIN32
+#ifdef WIN32
     bool ok = is_executable(name);
 #else
     // Must have path separator, cannot execute files in the current directory.
@@ -255,7 +255,7 @@ static bool is_executable(const char_u *name)
     return false;
   }
 
-#if WIN32
+#ifdef WIN32
   // Windows does not have exec bit; just check if the file exists and is not
   // a directory.
   return (S_ISREG(mode));
@@ -281,7 +281,7 @@ static bool is_executable_in_path(const char_u *name, char_u **abspath)
 #ifdef WIN32
   // Prepend ".;" to $PATH.
   size_t pathlen = strlen(path_env);
-  char *path = memcpy(xmallocz(pathlen + 3), ".;", 2);
+  char *path = memcpy(xmallocz(pathlen + 3), "." ENV_SEPSTR, 2);
   memcpy(path + 2, path_env, pathlen);
 #else
   char *path = xstrdup(path_env);
@@ -1027,7 +1027,7 @@ shortcut_end:
 int os_translate_sys_error(int sys_errno) {
 #ifdef HAVE_UV_TRANSLATE_SYS_ERROR
   return uv_translate_sys_error(sys_errno);
-#elif WIN32
+#elif defined(WIN32)
   // TODO(equalsraf): libuv does not yet expose uv_translate_sys_error()
   // in its public API, include a version here until it can be used.
   // See https://github.com/libuv/libuv/issues/79

src/nvim/os/unix_defs.h

@@ -16,7 +16,8 @@
 // Special wildcards that need to be handled by the shell.
 #define SPECIAL_WILDCHAR "`'{"
 
-// Separator character for environment variables.
+// Character that separates entries in $PATH.
 #define ENV_SEPCHAR ':'
+#define ENV_SEPSTR  ":"
 
 #endif  // NVIM_OS_UNIX_DEFS_H

src/nvim/os/win_defs.h

@@ -20,8 +20,9 @@
 
 #define FNAME_ILLEGAL "\"*?><|"
 
-// Separator character for environment variables.
+// Character that separates entries in $PATH.
 #define ENV_SEPCHAR ';'
+#define ENV_SEPSTR  ";"
 
 #define USE_CRNL