bug#27871: 26.0.50; Bad handling of unmounted directory

[Philipp Stephani <p.stephani2@gmail.com>]

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Philipp <p.stephani2@gmail.com> schrieb am Sa., 29. Juli 2017 um 23:08 Uhr:

>
> Create a FUSE mount point, e.g.
>
> cd /tmp
> mkdir a a/a b
> touch a/a/a
> bindfs -f a b
>
> While bindfs(1) is running, cd to the mounted directory in a second
> shell:
>
> cd /tmp/b/a
>
> Now kill bindfs, e.g. by hitting ^C in the first shell.  The second
> shell will now be in an unmounted directory.  From that directory, start
> Emacs:
>
> $ emacs -Q -batch a
> Apparent cycle of symbolic links for (unreachable)
>
> or, with stack trace
>
> $ emacs -Q -batch -f toggle-debug-on-error a
> Debug on Error enabled globally
> ...
>   file-truename("(unreachable)" (-1) (nil))
>   file-truename("(unreachable)/a" (-1) (nil))
>   file-truename("(unreachable)" (-1) (nil))
>   file-truename("(unreachable)/a" (-1) (nil))
>   file-truename("(unreachable)" (-1) (nil))
>   file-truename("(unreachable)/a" (-1) (nil))
>   file-truename("(unreachable)" (-1) (nil))
>   file-truename("(unreachable)/a" (-1) (nil))
>   file-truename("(unreachable)" (-1) (nil))
>   file-truename("(unreachable)/a" (-1) (nil))
>   file-truename("(unreachable)" (-1) (nil))
>   file-truename("(unreachable)/a" (-1) (nil))
>   file-truename("(unreachable)" (-1) (nil))
>   file-truename("(unreachable)/a" (-1) (nil))
>   file-truename("(unreachable)" (-1) (nil))
>   file-truename("(unreachable)/a" (-1) (nil))
>   file-truename("(unreachable)" (-1) (nil))
>   file-truename("(unreachable)/a" (-1) (nil))
>   file-truename("(unreachable)" (-1) (nil))
>   file-truename("(unreachable)/a" (-1) (nil))
>   file-truename("(unreachable)" (-1) (nil))
>   file-truename("(unreachable)/a" (-1) (nil))
>   file-truename("(unreachable)" (-1) (nil))
>   file-truename("(unreachable)/a" (-1) (nil))
>   file-truename("(unreachable)" (-1) (nil))
>   file-truename("(unreachable)/a" (-1) (nil))
>   file-truename("(unreachable)" (-1) (nil))
>   file-truename("(unreachable)/a" (-1) (nil))
>   file-truename("(unreachable)/a/(unreachable)" (-1) (nil))
>   file-truename("(unreachable)/a/(unreachable)/a" (-1) (nil))
>   file-truename("(unreachable)/a/(unreachable)/a/(unreachable)" (-1) (nil))
>   file-truename("(unreachable)/a/(unreachable)/a/(unreachable)/a" (-1)
> (nil))
>
> file-truename("(unreachable)/a/(unreachable)/a/(unreachable)/a/(unreachable)"
> (-1) (nil))
>
> file-truename("(unreachable)/a/(unreachable)/a/(unreachable)/a/(unreachable)/a"
> (-1) (nil))
>
> file-truename("(unreachable)/a/(unreachable)/a/(unreachable)/a/(unreachable)/a/a")
>   find-file-noselect("(unreachable)/a/(unreachable)/a/a")
>   #f(compiled-function (displayable-buffers dir line column name)
> #<bytecode>)((nil) "(unreachable)/a/" (0) (0) "a")
>   command-line-1(("-f" "toggle-debug-on-error" "a"))
>   command-line()
>   normal-top-level()
>
> It seems like the logic for `default-directory' and/or `file-truename'
> should be improved for unmounted filesystems.
>
>
Here's a patch. With the patch, the output is

 Error getting directory: Transport endpoint is not connected
Warning (initialization): Error setting default-directory
Ignoring relative file name (a) due to nil default-directory

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From b3541b3bc936bbb8e9950d1aed14d2571e03c1fe Mon Sep 17 00:00:00 2001
From: Philipp Stephani <phst@google.com>
Date: Sat, 23 Sep 2017 12:13:39 +0200
Subject: [PATCH] Treat unreachable current directory as error

Linux prefixes an unreachable (e.g. unmounted) current directory with
the special string "(unreachable)", cf. Bug#27871.  Treat such
directories as error because they wouldn't work anyway.

* src/sysdep.c (emacs_get_current_dir_name_1): Renamed from
emacs_get_current_dir_name.
(emacs_get_current_dir_name): Check for prefix "(unreachable)".
---
 src/sysdep.c | 29 +++++++++++++++++++++++++----
 1 file changed, 25 insertions(+), 4 deletions(-)

diff --git a/src/sysdep.c b/src/sysdep.c
index 1e6e0d011b..6c76f16242 100644
--- a/src/sysdep.c
+++ b/src/sysdep.c
@@ -220,10 +220,8 @@ init_standard_fds (void)
   force_open (STDERR_FILENO, O_RDONLY);
 }
 
-/* Return the current working directory.  The result should be freed
-   with 'free'.  Return NULL on errors.  */
-char *
-emacs_get_current_dir_name (void)
+static char *
+emacs_get_current_dir_name_1 (void)
 {
 # if HAVE_GET_CURRENT_DIR_NAME && !BROKEN_GET_CURRENT_DIR_NAME
 #  ifdef HYBRID_MALLOC
@@ -283,6 +281,29 @@ emacs_get_current_dir_name (void)
   return buf;
 }
 
+/* Return the current working directory.  The result should be freed
+   with 'free'.  Return NULL on errors.  */
+char *
+emacs_get_current_dir_name (void)
+{
+  char *dir = emacs_get_current_dir_name_1 ();
+  if (dir == NULL)
+    return NULL;
+  /* On Linux, getcwd and get_current_dir_name return a string
+     starting with "(unreachable)" if the current directory doesn't
+     exist, e.g. because it was unmounted.  Treat that as an error.
+     See https://debbugs.gnu.org/cgi/bugreport.cgi?bug=27871.  */
+  const char *prefix = "(unreachable)";
+  size_t dir_len = strlen (dir);
+  size_t prefix_len = strlen (prefix);
+  if (dir_len >= prefix_len && strncmp (dir, prefix, prefix_len) == 0)
+    {
+      errno = ENOTCONN;
+      return NULL;
+    }
+  return dir;
+}
+
 \f
 /* Discard pending input on all input descriptors.  */
 
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2.14.1.821.g8fa685d3b7-goog