cli,sdnotify: improve the handling of unix socket directories

[knz]

tldr: This patch makes it easier for users to understand when
CockroachDB has a problem using/creating unix sockets, and
what they can do to avert the situation.

Fixes #76904.

Example, before;

$ cockroach start-single-node --insecure --background
...
listen unixgram /data/home/kena/cockroach/this-is-a-very-long-directory-name-the-point-is-to-be-more-than-one-hundred-and-twenty-three-characters/sdnotify2139543236/notify.sock: bind: invalid argument

Example, after:

$ cockroach start-single-node --insecure --background
ERROR: no suitable directory found for the --background notify socket
HINT: Avoid using --background altogether (preferred), or use a
shorter directory name for --socket-dir, TMPDIR or the current directory.

Context:

CockroachDB uses unix sockets both to handle --background and to
create a postgres-compatible SQL client socket via --socket-dir. For
--background, the logic was previously hardwired to always use
os.TempDir(), i.e. either $TMPDIR or /tmp.

On Unix, socket objects are commonly limited to 128 bytes;
that's e.g. 104 path characters including final NUL on BSD, 108 on glibc.
When the actual length is larger, creating the socket fails with
a confusing error bind: invalid argument.

The problem:

Users have reported issues in the following circumstances:

• when setting their CWD/$TMPDIR to a long directory name,
  they expected --socket-dir to also influence the socket
  name used for --background. Instead, --background
  failed with a hard-to-understand error. (cli/start: sdnotify socket listen chokes if $PWD is too long #76904)

• when running tests inside a container/sandbox with a long
  prefix, they were confused that --background / --socket-dir=.
  did not work properly.

• they use macOS which has very large directory names in $TMPDIR.

To improve user experience, this patch changes the behavior
as described in the release notes below below.

Release note (cli change): CockroachDB now produces a clearer error
when the path specified via --socket-dir is too long.

Release note (cli change): When the flag --background is specified,
CockroachDB now makes 3 attempts to find a suitable directory to
create the notification socket: the value of --socket-dir if
specified, the value of $TMPDIR (or /tmp if the env var is empty),
and the current working directory. If none of these directories has a
name short enough, an explanatory error is printed.

[cockroach-teamcity]

This change is 

[jreut]

LGTM. I see mention of long macOS $TMP paths. Have these tests simply historically failed on macOS?

Reviewed 7 of 7 files at r1, 1 of 1 files at r2, 3 of 3 files at r3, all commit messages.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @bdarnell and @stevendanna)

---

pkg/cli/flags.go line 1013 at r1 (raw file):

		} else {
			socketName := ".s.PGSQL." + serverListenPort
			// On BSD, binding to a socket is limited to a path length of 104 characters

For my own education, could you link to the code or docs that support these facts?

---

pkg/cli/interactive_tests/test_socket_name.tcl line 65 at r1 (raw file):

system "$crdb sql --insecure -e 'select 1'"
stop_server $crdb
end_test

TIL we have TCL-based tests in CRDB. These are a joy to read!

[knz]

Have these tests simply historically failed on macOS?

They do when you try to run them outside of the docker sandbox.

Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @bdarnell and @stevendanna)

---

pkg/cli/flags.go line 1013 at r1 (raw file):

Previously, jreut (jordan ryan reuter) wrote…

For my own education, could you link to the code or docs that support these facts?

https://www.man7.org/linux/man-pages/man7/unix.7.html

      A UNIX domain socket address is represented in the following
       structure:

           struct sockaddr_un {
               sa_family_t sun_family;               /* AF_UNIX */
               char        sun_path[108];            /* Pathname */
           };

       The sun_family field always contains AF_UNIX.  On Linux, sun_path
       is 108 bytes in size

https://www.freebsd.org/cgi/man.cgi?query=unix&sektion=4

UNIX-domain addresses are variable-length file system pathnames of	at
     most 104 characters.  The include file <sys/un.h> defines this address:

	   struct sockaddr_un {
		   u_char  sun_len;
		   u_char  sun_family;
		   char	   sun_path[104];
	   };

[knz]

TFYR!

bors r=jreut

[craig]

Build succeeded:

• GitHub CI (Cockroach)