win: avoid duplicate separators in $PATH #12869

Seems like redundant env var separators (";" on Windows) in $PATH can
cause weird behavior. From #7377:

> After some time, system(['win32yank', '-o']) and system('win32yank -o')
> start returning different results: specifically first returns an
> empty string.
>
> 1. $PATH weirdly contains double semicolon followed by path to the
>    “installation directory” (unpacked directory from archive).
> 2. If I run `let $PATH=substitute($PATH, ';;', ';', 'g')` the problem is fixed.

close #7377
ref 224f99b85d

src/nvim/os/env.c

@@ -1176,8 +1176,10 @@ bool os_setenv_append_path(const char *fname)
       temp[0] = NUL;
     } else {
       xstrlcpy(temp, path, newlen);
+      if (ENV_SEPCHAR != path[pathlen - 1]) {
         xstrlcat(temp, ENV_SEPSTR, newlen);
       }
+    }
     xstrlcat(temp, os_buf, newlen);
     os_setenv("PATH", temp, 1);
     xfree(temp);

test/unit/os/env_spec.lua

@@ -78,15 +78,22 @@ describe('env.c', function()
   end)
 
   describe('os_setenv_append_path', function()
-    itp('appends /foo/bar to $PATH', function()
+    itp('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(true, cimp.os_setenv_append_path(to_cstr('/foo/bar/baz.exe')))
       eq(original_path..':/foo/bar', os.getenv('PATH'))
     end)
 
+    itp('avoids redundant separator when appending to $PATH #7377', function()
+      os_setenv('PATH', '/a/b/c:', true)
+      eq(true, cimp.os_setenv_append_path(to_cstr('/foo/bar/baz.exe')))
+      -- Must not have duplicate separators. #7377
+      eq('/a/b/c:/foo/bar', os.getenv('PATH'))
+    end)
+
     itp('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(false, cimp.os_setenv_append_path(to_cstr('foo/bar/baz.exe')))
       eq(original_path, os.getenv('PATH'))
     end)
   end)