18 files changed, 385 insertions, 139 deletions

diff --git a/cmake/RunTests.cmake b/cmake/RunTests.cmake
index 58f19b6fa5..19aaf85d0f 100644
--- a/cmake/RunTests.cmake
+++ b/cmake/RunTests.cmake
@@ -1,6 +1,3 @@
-get_filename_component(BUSTED_DIR ${BUSTED_PRG} PATH)
-set(ENV{PATH} "${BUSTED_DIR}:$ENV{PATH}")
-
 set(ENV{VIMRUNTIME} ${WORKING_DIR}/runtime)
 set(ENV{NVIM_RPLUGIN_MANIFEST} ${WORKING_DIR}/Xtest_rplugin_manifest)
 set(ENV{XDG_CONFIG_HOME} ${WORKING_DIR}/Xtest_xdg/config)
diff --git a/runtime/autoload/health/provider.vim b/runtime/autoload/health/provider.vim
index 417426c101..61ab957829 100644
--- a/runtime/autoload/health/provider.vim
+++ b/runtime/autoload/health/provider.vim
@@ -280,7 +280,7 @@ function! s:check_python(version) abort
       let python_bin = exepath(python_bin_name)
 
       if exists('$PATH')
-        for path in split($PATH, ':')
+        for path in split($PATH, has('win32') ? ';' : ':')
           let path_bin = path.'/'.python_bin_name
           if path_bin != python_bin && index(python_multiple, path_bin) == -1
                 \ && executable(path_bin)
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 9aa60657e0..b729519d93 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -1686,11 +1686,7 @@ v:progname	Contains the name (with path removed) with which Nvim was
 		Read-only.
 
 					*v:progpath* *progpath-variable*
-v:progpath	Contains the command with which Vim was invoked, including the
-		path. To get the full path use: >
-			echo exepath(v:progpath)
-<		NOTE: This does not work when the command is a relative path
-		and the current directory has changed.
+v:progpath	Absolute path to the current running Nvim.
 		Read-only.
 
 					*v:register* *register-variable*
@@ -3104,13 +3100,10 @@ execute({command} [, {silent}])				*execute()*
 		Note: Text attributes (highlights) are not captured.
 
 exepath({expr})						*exepath()*
-		If {expr} is an executable and is either an absolute path, a
-		relative path or found in $PATH, return the full path.
-		Note that the current directory is used when {expr} starts
-		with "./", which may be a problem for Vim: >
-			echo exepath(v:progpath)
-<		If {expr} cannot be found in $PATH or is not executable then
-		an empty string is returned.
+		Returns the full path of {expr} if it is an executable and
+		given as a (partial or full) path or is found in $PATH.
+		Returns empty string otherwise.
+		If {expr} starts with "./" the |current-directory| is used.
 
 							*exists()*
 exists({expr})	The result is a Number, which is non-zero if {expr} is
diff --git a/runtime/doc/vim_diff.txt b/runtime/doc/vim_diff.txt
index 20002c1118..de93aab399 100644
--- a/runtime/doc/vim_diff.txt
+++ b/runtime/doc/vim_diff.txt
@@ -108,12 +108,13 @@ Some `CTRL-SHIFT-...` key chords are distinguished from `CTRL-...` variants
   <C-Tab>, <C-S-Tab>, <C-BS>, <C-S-BS>, <C-Enter>, <C-S-Enter>
 
 Options:
-  'inccommand' shows results while typing a |:substitute| command
+  'inccommand' shows interactive results for |:substitute|-like commands
   'statusline' supports unlimited alignment sections
   'tabline' %@Func@foo%X can call any function on mouse-click
 
 Variables:
   |v:event|
+  |v:progpath| is always absolute ("full")
   |v:windowid| is always available (for use by external UIs)
 
 Commands:
diff --git a/src/nvim/buffer.c b/src/nvim/buffer.c
index 26dbbe8bb5..868e842ecd 100644
--- a/src/nvim/buffer.c
+++ b/src/nvim/buffer.c
@@ -4086,10 +4086,8 @@ void fname_expand(buf_T *buf, char_u **ffname, char_u **sfname)
 
 #ifdef WIN32
   if (!buf->b_p_bin) {
-    char_u  *rfname;
-
     // If the file name is a shortcut file, use the file it links to.
-    rfname = os_resolve_shortcut(*ffname);
+    char_u *rfname = (char_u *)os_resolve_shortcut(*ffname);
     if (rfname != NULL) {
       xfree(*ffname);
       *ffname = rfname;
diff --git a/src/nvim/eval.c b/src/nvim/eval.c
index 9c041ca790..1b4539ddd3 100644
--- a/src/nvim/eval.c
+++ b/src/nvim/eval.c
@@ -14137,11 +14137,9 @@ static void f_resolve(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   p = get_tv_string(&argvars[0]);
 #ifdef WIN32
   {
-    char_u  *v = NULL;
-
-    v = os_resolve_shortcut(p);
+    char *v = os_resolve_shortcut(p);
     if (v != NULL) {
-      rettv->vval.v_string = v;
+      rettv->vval.v_string = (char_u *)v;
     } else {
       rettv->vval.v_string = vim_strsave(p);
     }
diff --git a/src/nvim/main.c b/src/nvim/main.c
index c7a60d07c1..6194e5f948 100644
--- a/src/nvim/main.c
+++ b/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)
diff --git a/src/nvim/mbyte.c b/src/nvim/mbyte.c
index c855d68605..6a87a63b8c 100644
--- a/src/nvim/mbyte.c
+++ b/src/nvim/mbyte.c
@@ -1517,7 +1517,7 @@ int utf16_to_utf8(const WCHAR *strw, char **str)
                               0,
                               strw,
                               -1,
-                              (LPSTR *)pos,
+                              pos,
                               utf8_len,
                               NULL,
                               NULL);
diff --git a/src/nvim/os/env.c b/src/nvim/os/env.c
index 747a34d8ce..5a3c1ef2c8 100644
--- a/src/nvim/os/env.c
+++ b/src/nvim/os/env.c
@@ -9,6 +9,7 @@
 #include "nvim/vim.h"
 #include "nvim/ascii.h"
 #include "nvim/charset.h"
+#include "nvim/fileio.h"
 #include "nvim/os/os.h"
 #include "nvim/memory.h"
 #include "nvim/message.h"
@@ -18,6 +19,10 @@
 #include "nvim/ex_getln.h"
 #include "nvim/version.h"
 
+#ifdef WIN32
+#include "nvim/mbyte.h"  // for utf8_to_utf16, utf16_to_utf8
+#endif
+
 #ifdef HAVE__NSGETENVIRON
 #include <crt_externs.h>
 #endif
@@ -45,7 +50,21 @@ bool os_env_exists(const char *name)
 int os_setenv(const char *name, const char *value, int overwrite)
   FUNC_ATTR_NONNULL_ALL
 {
-#ifdef HAVE_SETENV
+#ifdef WIN32
+  size_t envbuflen = strlen(name) + strlen(value) + 2;
+  char *envbuf = xmalloc(envbuflen);
+  snprintf(envbuf, envbuflen, "%s=%s", name, value);
+
+  WCHAR *p;
+  utf8_to_utf16(envbuf, &p);
+  xfree(envbuf);
+  if (p == NULL) {
+    return -1;
+  }
+  _wputenv(p);
+  xfree(p);  // Unlike Unix systems, we can free the string for _wputenv().
+  return 0;
+#elif defined(HAVE_SETENV)
   return setenv(name, value, overwrite);
 #elif defined(HAVE_PUTENV_S)
   if (!overwrite && os_getenv(name) != NULL) {
@@ -831,3 +850,43 @@ 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;
+}
diff --git a/src/nvim/os/fs.c b/src/nvim/os/fs.c
index 30e08ac129..097c672887 100644
--- a/src/nvim/os/fs.c
+++ b/src/nvim/os/fs.c
@@ -219,49 +219,86 @@ int os_exepath(char *buffer, size_t *size)
 bool os_can_exe(const char_u *name, char_u **abspath, bool use_path)
   FUNC_ATTR_NONNULL_ARG(1)
 {
-  // when use_path is false or if it's an absolute or relative path don't
-  // need to use $PATH.
-  if (!use_path || path_is_absolute_path(name)
-      || (name[0] == '.'
-          && (name[1] == '/'
-              || (name[1] == '.' && name[2] == '/')))) {
-    // There must be a path separator, files in the current directory
-    // can't be executed
-    if (gettail_dir(name) != name && is_executable(name)) {
+  bool no_path = !use_path || path_is_absolute_path(name);
+#ifndef WIN32
+  // If the filename is "qualified" (relative or absolute) do not check $PATH.
+  no_path |= (name[0] == '.'
+              && (name[1] == '/' || (name[1] == '.' && name[2] == '/')));
+#endif
+
+  if (no_path) {
+#ifdef WIN32
+    const char *pathext = os_getenv("PATHEXT");
+    if (!pathext) {
+      pathext = ".com;.exe;.bat;.cmd";
+    }
+    bool ok = is_executable((char *)name) || is_executable_ext((char *)name,
+                                                               pathext);
+#else
+    // Must have path separator, cannot execute files in the current directory.
+    bool ok = gettail_dir(name) != name && is_executable((char *)name);
+#endif
+    if (ok) {
       if (abspath != NULL) {
         *abspath = save_absolute_path(name);
       }
-
       return true;
     }
-
     return false;
   }
 
   return is_executable_in_path(name, abspath);
 }
 
-// Return true if "name" is an executable file, false if not or it doesn't
-// exist.
-static bool is_executable(const char_u *name)
+/// Returns true if `name` is an executable file.
+static bool is_executable(const char *name)
   FUNC_ATTR_NONNULL_ALL
 {
-  int32_t mode = os_getperm(name);
+  int32_t mode = os_getperm((char_u *)name);
 
   if (mode < 0) {
     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));
 #else
   return (S_ISREG(mode) && (S_IXUSR & mode));
 #endif
+}
 
+#ifdef WIN32
+/// Appends file extensions from `pathext` to `name` and returns true if any
+/// such combination is executable.
+static bool is_executable_ext(char *name, const char *pathext)
+  FUNC_ATTR_NONNULL_ALL
+{
+  xstrlcpy((char *)NameBuff, name, sizeof(NameBuff));
+  char *buf_end = xstrchrnul((char *)NameBuff, '\0');
+  for (const char *ext = pathext; *ext; ext++) {
+    // Skip the extension if there is no suffix after a '.'.
+    if (ext[0] == '.' && (ext[1] == '\0' || ext[1] == ENV_SEPCHAR)) {
+      ext++;
+      continue;
+    }
+
+    const char *ext_end = xstrchrnul(ext, ENV_SEPCHAR);
+    STRLCPY(buf_end, ext, ext_end - ext + 1);
+
+    if (is_executable((char *)NameBuff)) {
+      return true;
+    }
+
+    if (*ext_end != ENV_SEPCHAR) {
+      break;
+    }
+    ext = ext_end;
+  }
   return false;
 }
+#endif
 
 /// Checks if a file is inside the `$PATH` and is executable.
 ///
@@ -272,89 +309,69 @@ static bool is_executable(const char_u *name)
 static bool is_executable_in_path(const char_u *name, char_u **abspath)
   FUNC_ATTR_NONNULL_ARG(1)
 {
-  const char *path = os_getenv("PATH");
-  if (path == NULL) {
+  const char *path_env = os_getenv("PATH");
+  if (path_env == NULL) {
     return false;
   }
 
-  size_t buf_len = STRLEN(name) + STRLEN(path) + 2;
+#ifdef WIN32
+  // Prepend ".;" to $PATH.
+  size_t pathlen = strlen(path_env);
+  char *path = memcpy(xmallocz(pathlen + 3), "." ENV_SEPSTR, 2);
+  memcpy(path + 2, path_env, pathlen);
+#else
+  char *path = xstrdup(path_env);
+#endif
+
+  size_t buf_len = STRLEN(name) + strlen(path) + 2;
 
 #ifdef WIN32
   const char *pathext = os_getenv("PATHEXT");
   if (!pathext) {
     pathext = ".com;.exe;.bat;.cmd";
   }
-
-  buf_len += STRLEN(pathext);
+  buf_len += strlen(pathext);
 #endif
 
-  char_u *buf = xmalloc(buf_len);
+  char *buf = xmalloc(buf_len);
 
   // Walk through all entries in $PATH to check if "name" exists there and
   // is an executable file.
+  char *p = path;
+  bool rv = false;
   for (;; ) {
-    const char *e = xstrchrnul(path, ENV_SEPCHAR);
+    char *e = xstrchrnul(p, ENV_SEPCHAR);
 
-    // Glue together the given directory from $PATH with name and save into
-    // buf.
-    STRLCPY(buf, path, e - path + 1);
-    append_path((char *) buf, (const char *) name, buf_len);
-
-    if (is_executable(buf)) {
-      // Check if the caller asked for a copy of the path.
-      if (abspath != NULL) {
-        *abspath = save_absolute_path(buf);
-      }
-
-      xfree(buf);
-
-      return true;
-    }
+    // Combine the $PATH segment with `name`.
+    STRLCPY(buf, p, e - p + 1);
+    append_path(buf, (char *)name, buf_len);
 
 #ifdef WIN32
-    // Try appending file extensions from $PATHEXT to the name.
-    char *buf_end = xstrchrnul((char *)buf, '\0');
-    for (const char *ext = pathext; *ext; ext++) {
-      // Skip the extension if there is no suffix after a '.'.
-      if (ext[0] == '.' && (ext[1] == '\0' || ext[1] == ';')) {
-        *ext++;
-
-        continue;
-      }
-
-      const char *ext_end = xstrchrnul(ext, ENV_SEPCHAR);
-      STRLCPY(buf_end, ext, ext_end - ext + 1);
-
-      if (is_executable(buf)) {
-        // Check if the caller asked for a copy of the path.
-        if (abspath != NULL) {
-          *abspath = save_absolute_path(buf);
-        }
-
-        xfree(buf);
-
-        return true;
+    bool ok = is_executable(buf) || is_executable_ext(buf, pathext);
+#else
+    bool ok = is_executable(buf);
+#endif
+    if (ok) {
+      if (abspath != NULL) {  // Caller asked for a copy of the path.
+        *abspath = save_absolute_path((char_u *)buf);
       }
 
-      if (*ext_end != ENV_SEPCHAR) {
-        break;
-      }
-      ext = ext_end;
+      rv = true;
+      goto end;
     }
-#endif
 
     if (*e != ENV_SEPCHAR) {
       // End of $PATH without finding any executable called name.
-      xfree(buf);
-      return false;
+      goto end;
     }
 
-    path = e + 1;
+    p = e + 1;
   }
 
-  // We should never get to this point.
-  assert(false);
-  return false;
+end:
+  xfree(buf);
+  xfree(path);
+  return rv;
 }
 
 /// Opens or creates a file and returns a non-negative integer representing
@@ -407,11 +424,11 @@ ptrdiff_t os_read(const int fd, bool *ret_eof, char *const ret_buf,
   size_t read_bytes = 0;
   bool did_try_to_free = false;
   while (read_bytes != size) {
+    assert(size >= read_bytes);
     const ptrdiff_t cur_read_bytes = read(fd, ret_buf + read_bytes,
                                           size - read_bytes);
     if (cur_read_bytes > 0) {
       read_bytes += (size_t)cur_read_bytes;
-      assert(read_bytes <= size);
     }
     if (cur_read_bytes < 0) {
       const int error = os_translate_sys_error(errno);
@@ -510,6 +527,7 @@ ptrdiff_t os_write(const int fd, const char *const buf, const size_t size)
   }
   size_t written_bytes = 0;
   while (written_bytes != size) {
+    assert(size >= written_bytes);
     const ptrdiff_t cur_written_bytes = write(fd, buf + written_bytes,
                                               size - written_bytes);
     if (cur_written_bytes > 0) {
@@ -932,12 +950,12 @@ bool os_fileid_equal_fileinfo(const FileID *file_id,
 /// When "fname" is the name of a shortcut (*.lnk) resolve the file it points
 /// to and return that name in allocated memory.
 /// Otherwise NULL is returned.
-char_u * os_resolve_shortcut(char_u *fname)
+char *os_resolve_shortcut(char_u *fname)
 {
   HRESULT hr;
   IPersistFile *ppf = NULL;
   OLECHAR wsz[MAX_PATH];
-  char_u *rfname = NULL;
+  char *rfname = NULL;
   int len;
   IShellLinkW *pslw = NULL;
   WIN32_FIND_DATAW ffdw;
@@ -1019,7 +1037,7 @@ 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
diff --git a/src/nvim/os/unix_defs.h b/src/nvim/os/unix_defs.h
index 690a39c3cd..c98aa88bfa 100644
--- a/src/nvim/os/unix_defs.h
+++ b/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
diff --git a/src/nvim/os/win_defs.h b/src/nvim/os/win_defs.h
index 6a29f86e79..8de896c490 100644
--- a/src/nvim/os/win_defs.h
+++ b/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
 
diff --git a/src/nvim/path.c b/src/nvim/path.c
index 374d72ddd3..ea06fb8dde 100644
--- a/src/nvim/path.c
+++ b/src/nvim/path.c
@@ -995,12 +995,10 @@ static void uniquefy_paths(garray_T *gap, char_u *pattern)
     ga_remove_duplicate_strings(gap);
 }
 
-/*
- * Return the end of the directory name, on the first path
- * separator:
- * "/path/file", "/path/dir/", "/path//dir", "/file"
- *	 ^	       ^	     ^	      ^
- */
+/// Return the end of the directory name, on the first path
+/// separator:
+/// "/path/file", "/path/dir/", "/path//dir", "/file"
+///       ^             ^             ^        ^
 char_u *gettail_dir(const char_u *fname)
 {
   const char_u      *dir_end = fname;
@@ -2131,17 +2129,12 @@ int append_path(char *path, const char *to_append, size_t max_len)
   size_t current_length = strlen(path);
   size_t to_append_length = strlen(to_append);
 
-  // Do not append empty strings.
-  if (to_append_length == 0) {
-    return OK;
-  }
-
-  // Do not append a dot.
-  if (STRCMP(to_append, ".") == 0) {
+  // Do not append empty string or a dot.
+  if (to_append_length == 0 || strcmp(to_append, ".") == 0) {
     return OK;
   }
 
-  // Glue both paths with a slash.
+  // Combine the path segments, separated by a slash.
   if (current_length > 0 && !vim_ispathsep_nocolon(path[current_length-1])) {
     current_length += 1;  // Count the trailing slash.
 
@@ -2150,7 +2143,7 @@ int append_path(char *path, const char *to_append, size_t max_len)
       return FAIL;
     }
 
-    STRCAT(path, PATHSEPSTR);
+    xstrlcat(path, PATHSEPSTR, max_len);
   }
 
   // +1 for the NUL at the end.
@@ -2158,7 +2151,7 @@ int append_path(char *path, const char *to_append, size_t max_len)
     return FAIL;
   }
 
-  STRCAT(path, to_append);
+  xstrlcat(path, to_append, max_len);
   return OK;
 }
 
diff --git a/test/functional/eval/executable_spec.lua b/test/functional/eval/executable_spec.lua
new file mode 100644
index 0000000000..c931b47221
--- /dev/null
+++ b/test/functional/eval/executable_spec.lua
@@ -0,0 +1,156 @@
+local helpers = require('test.functional.helpers')(after_each)
+local eq, clear, call, iswin, write_file =
+  helpers.eq, helpers.clear, helpers.call, helpers.iswin, helpers.write_file
+
+describe('executable()', function()
+  before_each(clear)
+
+  it('returns 1 for commands in $PATH', function()
+    local exe = iswin() and 'ping' or 'ls'
+    eq(1, call('executable', exe))
+  end)
+
+  it('returns 0 for non-existent files', function()
+    eq(0, call('executable', 'no_such_file_exists_209ufq23f'))
+  end)
+
+  it('sibling to nvim binary', function()
+    -- Some executable in build/bin/, *not* in $PATH nor CWD.
+    local sibling_exe = 'printargs-test'
+    -- Windows: siblings are in Nvim's "pseudo-$PATH".
+    local expected = iswin() and 1 or 0
+    if iswin() then
+      -- $PATH on AppVeyor CI might be oversized, redefine it to a minimal one.
+      clear({env={PATH=[[C:\Windows\system32;C:\Windows]]}})
+      eq('arg1=lemon;arg2=sky;arg3=tree;',
+         call('system', sibling_exe..' lemon sky tree'))
+    end
+    local is_executable = call('executable', sibling_exe)
+    if iswin() and is_executable ~= expected then
+      pending('XXX: sometimes fails on AppVeyor')
+    end
+  end)
+
+  describe('exec-bit', function()
+    setup(function()
+      clear()
+      write_file('Xtest_not_executable', 'non-executable file')
+      write_file('Xtest_executable', 'executable file (exec-bit set)')
+      if not iswin() then  -- N/A for Windows.
+        call('system', {'chmod', '-x', 'Xtest_not_executable'})
+        call('system', {'chmod', '+x', 'Xtest_executable'})
+      end
+    end)
+
+    teardown(function()
+      os.remove('Xtest_not_executable')
+      os.remove('Xtest_executable')
+    end)
+
+    it('not set', function()
+      local expected = iswin() and 1 or 0
+      eq(expected, call('executable', 'Xtest_not_executable'))
+      eq(expected, call('executable', './Xtest_not_executable'))
+    end)
+
+    it('set, unqualified and not in $PATH', function()
+      local expected = iswin() and 1 or 0
+      eq(expected, call('executable', 'Xtest_executable'))
+    end)
+
+    it('set, qualified as a path', function()
+      eq(1, call('executable', './Xtest_executable'))
+    end)
+  end)
+end)
+
+describe('executable() (Windows)', function()
+  if not iswin() then return end  -- N/A for Unix.
+
+  local exts = {'bat', 'exe', 'com', 'cmd'}
+  setup(function()
+    for _, ext in ipairs(exts) do
+      write_file('test_executable_'..ext..'.'..ext, '')
+    end
+    write_file('test_executable_zzz.zzz', '')
+  end)
+
+  teardown(function()
+    for _, ext in ipairs(exts) do
+      os.remove('test_executable_'..ext..'.'..ext)
+    end
+    os.remove('test_executable_zzz.zzz')
+  end)
+
+  it('tries default extensions on a filename if $PATHEXT is empty', function()
+    -- Empty $PATHEXT defaults to ".com;.exe;.bat;.cmd".
+    clear({env={PATHEXT=''}})
+    for _,ext in ipairs(exts) do
+      eq(1, call('executable', 'test_executable_'..ext))
+    end
+    eq(0, call('executable', 'test_executable_zzz'))
+  end)
+
+  it('tries default extensions on a filepath if $PATHEXT is empty', function()
+    -- Empty $PATHEXT defaults to ".com;.exe;.bat;.cmd".
+    clear({env={PATHEXT=''}})
+    for _,ext in ipairs(exts) do
+      eq(1, call('executable', '.\\test_executable_'..ext))
+    end
+    eq(0, call('executable', '.\\test_executable_zzz'))
+  end)
+
+  it('full path with extension', function()
+    -- Some executable we can expect in the test env.
+    local exe = 'printargs-test'
+    local exedir = helpers.eval("fnamemodify(v:progpath, ':h')")
+    local exepath = exedir..'/'..exe..'.exe'
+    eq(1, call('executable', exepath))
+    eq('arg1=lemon;arg2=sky;arg3=tree;',
+       call('system', exepath..' lemon sky tree'))
+  end)
+
+  it('full path without extension', function()
+    -- Some executable we can expect in the test env.
+    local exe = 'printargs-test'
+    local exedir = helpers.eval("fnamemodify(v:progpath, ':h')")
+    local exepath = exedir..'/'..exe
+    eq('arg1=lemon;arg2=sky;arg3=tree;',
+       call('system', exepath..' lemon sky tree'))
+     eq(1, call('executable', exepath))
+  end)
+
+  it('respects $PATHEXT when trying extensions on a filename', function()
+    clear({env={PATHEXT='.zzz'}})
+    for _,ext in ipairs(exts) do
+      eq(0, call('executable', 'test_executable_'..ext))
+    end
+    eq(1, call('executable', 'test_executable_zzz'))
+  end)
+
+  it('respects $PATHEXT when trying extensions on a filepath', function()
+    clear({env={PATHEXT='.zzz'}})
+    for _,ext in ipairs(exts) do
+      eq(0, call('executable', '.\\test_executable_'..ext))
+    end
+    eq(1, call('executable', '.\\test_executable_zzz'))
+  end)
+
+  it('returns 1 for any existing filename', function()
+    clear({env={PATHEXT=''}})
+    for _,ext in ipairs(exts) do
+      eq(1, call('executable', 'test_executable_'..ext..'.'..ext))
+    end
+    eq(1, call('executable', 'test_executable_zzz.zzz'))
+  end)
+
+  it('returns 1 for any existing path (backslashes)', function()
+    clear({env={PATHEXT=''}})
+    for _,ext in ipairs(exts) do
+      eq(1, call('executable', '.\\test_executable_'..ext..'.'..ext))
+      eq(1, call('executable', './test_executable_'..ext..'.'..ext))
+    end
+    eq(1, call('executable', '.\\test_executable_zzz.zzz'))
+    eq(1, call('executable', './test_executable_zzz.zzz'))
+  end)
+end)
diff --git a/test/functional/helpers.lua b/test/functional/helpers.lua
index 880cb6546c..ca59c0dd2e 100644
--- a/test/functional/helpers.lua
+++ b/test/functional/helpers.lua
@@ -290,7 +290,9 @@ local function clear(...)
         'NVIM_LOG_FILE',
         'NVIM_RPLUGIN_MANIFEST',
       }) do
-        env_tbl[k] = os.getenv(k)
+        if not env_tbl[k] then
+          env_tbl[k] = os.getenv(k)
+        end
       end
       env = {}
       for k, v in pairs(env_tbl) do
diff --git a/test/functional/options/defaults_spec.lua b/test/functional/options/defaults_spec.lua
index caeca5e4e2..f3328886b5 100644
--- a/test/functional/options/defaults_spec.lua
+++ b/test/functional/options/defaults_spec.lua
@@ -96,6 +96,10 @@ describe('startup defaults', function()
       eq(meths.get_option('runtimepath'), meths.get_option('packpath'))
     end)
   end)
+
+  it('v:progpath is set to the absolute path', function()
+    eq(eval("fnamemodify(v:progpath, ':p')"), eval('v:progpath'))
+  end)
 end)
 
 describe('XDG-based defaults', function()
diff --git a/test/unit/os/env_spec.lua b/test/unit/os/env_spec.lua
index 64bbaaa8c2..3c2cc164c9 100644
--- a/test/unit/os/env_spec.lua
+++ b/test/unit/os/env_spec.lua
@@ -14,15 +14,15 @@ local cimp = cimport('./src/nvim/os/os.h')
 
 describe('env function', function()
   local function os_setenv(name, value, override)
-    return cimp.os_setenv((to_cstr(name)), (to_cstr(value)), override)
+    return cimp.os_setenv(to_cstr(name), to_cstr(value), override)
   end
 
   local function os_unsetenv(name, _, _)
-    return cimp.os_unsetenv((to_cstr(name)))
+    return cimp.os_unsetenv(to_cstr(name))
   end
 
   local function os_getenv(name)
-    local rval = cimp.os_getenv((to_cstr(name)))
+    local rval = cimp.os_getenv(to_cstr(name))
     if rval ~= NULL then
       return ffi.string(rval)
     else
@@ -52,6 +52,20 @@ describe('env function', function()
     end)
   end)
 
+  describe('os_setenv_append_path', function()
+    it('appends /foo/bar to $PATH', function()
+      local original_path = os.getenv('PATH')
+      eq(true, cimp.os_setenv_append_path(to_cstr('/foo/bar/baz')))
+      eq(original_path..':/foo/bar', os.getenv('PATH'))
+    end)
+
+    it('returns false if `fname` is not absolute', function()
+      local original_path = os.getenv('PATH')
+      eq(false, cimp.os_setenv_append_path(to_cstr('foo/bar/baz')))
+      eq(original_path, os.getenv('PATH'))
+    end)
+  end)
+
   describe('os_getenv', function()
     it('reads an env variable', function()
       local name = 'NEOVIM_UNIT_TEST_GETENV_1N'
diff --git a/test/unit/os/fs_spec.lua b/test/unit/os/fs_spec.lua
index 5d889d6e33..516fb5a7d1 100644
--- a/test/unit/os/fs_spec.lua
+++ b/test/unit/os/fs_spec.lua
@@ -23,7 +23,7 @@ cimport('./src/nvim/os/shell.h')
 cimport('./src/nvim/option_defs.h')
 cimport('./src/nvim/main.h')
 cimport('./src/nvim/fileio.h')
-local fs = cimport('./src/nvim/os/os.h')
+local fs = cimport('./src/nvim/os/os.h', './src/nvim/path.h')
 cppimport('sys/stat.h')
 cppimport('fcntl.h')
 cppimport('uv-errno.h')
@@ -77,11 +77,9 @@ describe('fs function', function()
     lfs.link('test.file', 'unit-test-directory/test_link.file', true)
 
     lfs.link('non_existing_file.file', 'unit-test-directory/test_broken_link.file', true)
-    -- Since the tests are executed, they are called by an executable. We use
-    -- that executable for several asserts.
+    -- The tests are invoked with an absolute path to `busted` executable.
     absolute_executable = arg[0]
-    -- Split absolute_executable into a directory and the actual file name for
-    -- later usage.
+    -- Split the absolute_executable path into a directory and filename.
     directory, executable_name = string.match(absolute_executable, '^(.*)/(.*)$')
   end)
 
@@ -194,11 +192,8 @@ describe('fs function', function()
     end)
 
     it('returns the absolute path when given an executable inside $PATH', function()
-      -- Since executable_name does not start with "./", the path will be
-      -- selected from $PATH. Make sure the ends match, ignore the directories.
-      local _, busted = string.match(absolute_executable, '^(.*)/(.*)$')
-      local _, name = string.match(exe(executable_name), '^(.*)/(.*)$')
-      eq(busted, name)
+      local fullpath = exe('ls')
+      eq(1, fs.path_is_absolute_path(to_cstr(fullpath)))
     end)
 
     it('returns the absolute path when given an executable relative to the current dir', function()