startup: v:progpath fallback: path_guess_exepath

If procfs is missing then libuv cannot find the exe path.
Fallback to path_guess_exepath(), adapted from Vim findYourself().

Closes #6734

runtime/doc/eval.txt

@@ -1749,9 +1749,7 @@ v:profiling	Normally zero.	Set to one after using ":profile start".
 		See |profiling|.
 
 					*v:progname* *progname-variable*
-v:progname	Contains the name (with path removed) with which Nvim was
-		invoked.  Allows you to do special initialisations for any
-		name you might symlink to Nvim.
+v:progname	The name by which Nvim was invoked (with path removed).
 		Read-only.
 
 					*v:progpath* *progpath-variable*

src/nvim/main.c

@@ -1223,7 +1223,8 @@ static void init_path(char *exename)
   size_t exepathlen = MAXPATHL;
   // Make v:progpath absolute.
   if (os_exepath(exepath, &exepathlen) != 0) {
-    EMSG2(e_intern2, "init_path()");
+    // Fall back to argv[0]. Missing procfs? #6734
+    path_guess_exepath(exename, exepath, sizeof(exepath));
   }
   set_vim_var_string(VV_PROGPATH, exepath, -1);
   set_vim_var_string(VV_PROGNAME, (char *)path_tail((char_u *)exename), -1);

src/nvim/os/fs.c

@@ -196,11 +196,13 @@ int os_nodetype(const char *name)
 }
 
 /// Gets the absolute path of the currently running executable.
+/// May fail if procfs is missing. #6734
+/// @see path_exepath
 ///
-/// @param[out] buffer Returns the path string.
+/// @param[out] buffer Full path to the executable.
 /// @param[in]  size   Size of `buffer`.
 ///
-/// @return `0` on success, or libuv error code on failure.
+/// @return 0 on success, or libuv error code.
 int os_exepath(char *buffer, size_t *size)
   FUNC_ATTR_NONNULL_ALL
 {

src/nvim/path.c

@@ -2237,3 +2237,50 @@ int path_is_absolute_path(const char_u *fname)
   return *fname == '/' || *fname == '~';
 #endif
 }
+
+/// Builds a full path from an invocation name `argv0`, based on heuristics.
+///
+/// @param[in]  argv0     Name by which Nvim was invoked.
+/// @param[out] buf       Guessed full path to `argv0`.
+/// @param[in]  bufsize   Size of `buf`.
+///
+/// @see os_exepath
+void path_guess_exepath(const char *argv0, char *buf, size_t bufsize)
+  FUNC_ATTR_NONNULL_ALL
+{
+  char *path = getenv("PATH");
+
+  if (path == NULL || path_is_absolute_path((char_u *)argv0)) {
+    xstrlcpy(buf, argv0, bufsize);
+  } else if (argv0[0] == '.' || strchr(argv0, PATHSEP)) {
+    // Relative to CWD.
+    if (os_dirname((char_u *)buf, MAXPATHL) != OK) {
+      buf[0] = NUL;
+    }
+    xstrlcat(buf, PATHSEPSTR, bufsize);
+    xstrlcat(buf, argv0, bufsize);
+  } else {
+    // Search $PATH for plausible location.
+    const void *iter = NULL;
+    do {
+      const char *dir;
+      size_t dir_len;
+      iter = vim_colon_env_iter(path, iter, &dir, &dir_len);
+      if (dir == NULL || dir_len == 0) {
+        break;
+      }
+      if (dir_len + 1 > sizeof(NameBuff)) {
+        continue;
+      }
+      xstrlcpy((char *)NameBuff, dir, dir_len + 1);
+      xstrlcat((char *)NameBuff, PATHSEPSTR, sizeof(NameBuff));
+      xstrlcat((char *)NameBuff, argv0, sizeof(NameBuff));
+      if (os_can_exe(NameBuff, NULL, false)) {
+        xstrlcpy(buf, (char *)NameBuff, bufsize);
+        return;
+      }
+    } while (iter != NULL);
+    // Not found in $PATH, fall back to argv0.
+    xstrlcpy(buf, argv0, bufsize);
+  }
+}

test/unit/os/env_spec.lua

@@ -13,7 +13,7 @@ require('lfs')
 
 local cimp = cimport('./src/nvim/os/os.h')
 
-describe('env function', function()
+describe('env.c', function()
   local function os_setenv(name, value, override)
     return cimp.os_setenv(to_cstr(name), to_cstr(value), override)
   end

test/unit/path_spec.lua

@@ -18,7 +18,7 @@ local cimp = cimport('./src/nvim/os/os.h', './src/nvim/path.h')
 local length = 0
 local buffer = nil
 
-describe('path function', function()
+describe('path.c', function()
   describe('path_full_dir_name', function()
     setup(function()
       lfs.mkdir('unit-test-directory')
@@ -293,6 +293,59 @@ describe('path_shorten_fname_if_possible', function()
   end)
 end)
 
+describe('path.c path_guess_exepath', function()
+  local cwd = lfs.currentdir()
+
+  for _,name in ipairs({'./nvim', '.nvim', 'foo/nvim'}) do
+    itp('"'..name..'" returns name catenated with CWD', function()
+      local bufsize = 255
+      local buf = cstr(bufsize, '')
+      cimp.path_guess_exepath(name, buf, bufsize)
+      eq(cwd..'/'..name, ffi.string(buf))
+    end)
+  end
+
+  itp('absolute path returns the name unmodified', function()
+    local name = '/foo/bar/baz'
+    local bufsize = 255
+    local buf = cstr(bufsize, '')
+    cimp.path_guess_exepath(name, buf, bufsize)
+    eq(name, ffi.string(buf))
+  end)
+
+  itp('returns the name unmodified if not found in $PATH', function()
+    local name = '23u0293_not_in_path'
+    local bufsize = 255
+    local buf = cstr(bufsize, '')
+    cimp.path_guess_exepath(name, buf, bufsize)
+    eq(name, ffi.string(buf))
+  end)
+
+  itp('does not crash if $PATH item exceeds MAXPATHL', function()
+    local orig_path_env = os.getenv('PATH')
+    local name = 'cat'  -- Some executable in $PATH.
+    local bufsize = 255
+    local buf = cstr(bufsize, '')
+    local insane_path = orig_path_env..':'..(("x/"):rep(4097))
+
+    cimp.os_setenv('PATH', insane_path, true)
+    cimp.path_guess_exepath(name, buf, bufsize)
+    eq('bin/' .. name, ffi.string(buf):sub(-#('bin/' .. name), -1))
+
+    -- Restore $PATH.
+    cimp.os_setenv('PATH', orig_path_env, true)
+  end)
+
+  itp('returns full path found in $PATH', function()
+    local name = 'cat'  -- Some executable in $PATH.
+    local bufsize = 255
+    local buf = cstr(bufsize, '')
+    cimp.path_guess_exepath(name, buf, bufsize)
+    -- Usually "/bin/cat" on unix, "/path/to/nvim/cat" on Windows.
+    eq('bin/' .. name, ffi.string(buf):sub(-#('bin/' .. name), -1))
+  end)
+end)
+
 describe('path.c', function()
   setup(function()
     lfs.mkdir('unit-test-directory');