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[RFC PATCH 2/2] _AS_ENSURE_STANDARD_FDS: use lsof and fstat when possibl
From: |
Zack Weinberg |
Subject: |
[RFC PATCH 2/2] _AS_ENSURE_STANDARD_FDS: use lsof and fstat when possible |
Date: |
Thu, 27 Aug 2020 11:39:33 -0400 |
This follow-up patch adds additional ways of detecting whether fds 0,
1, 2 are closed, using the ‘lsof’ and ‘fstat’ utilities. The former
is not a standard component of any OS, but is very widely installed
and can produce machine-parseable output; the latter ships with many
BSD variants but its output is in a fixed format not designed for
machine parsing. Both of them may fail due to privilege restrictions.
The biggest problem with using these is that we have to run a program
and inspect its output. I tried capturing the output using command
substitution, but that is often implemented using a pipe from the
child process to the parent shell; if fd 0 (for instance) started out
closed in the parent, the pipe is likely to occupy that fd number
right when lsof/fstat is inspecting the parent’s file table, causing a
false report that fd 0 is open. The only alternatives I am aware of
are temporary files or named pipes, both of which involve creating
directory entries somewhere...and we can’t assume we have mktemp(1).
This patch uses a file in the current working directory, but that
breaks all of the tests that assume ‘configure --help’ will not write
to the current working directory (which seems like a reasonable
promise for us to make, tbh).
Better ideas solicited.
* lib/m4sugar/m4sh.m4 (_AS_ENSURE_STANDARD_FDS): If /proc/<pid>/fdinfo
is not available, try to use lsof or fstat.
---
lib/m4sugar/m4sh.m4 | 66 ++++++++++++++++++++++++++++++++++++++-------
1 file changed, 57 insertions(+), 9 deletions(-)
diff --git a/lib/m4sugar/m4sh.m4 b/lib/m4sugar/m4sh.m4
index 3b1e9015..6e145956 100644
--- a/lib/m4sugar/m4sh.m4
+++ b/lib/m4sugar/m4sh.m4
@@ -359,16 +359,64 @@ if test -d /proc/$$/fdinfo; then
done
else
- # Shell redirection operations can only tell whether an fd is open,
- # not whether it is readable or writable, so this is a last resort.
- # `exec >&n` fails in POSIX sh when fd N is closed, but succeeds
- # regardless of whether fd N is open in some old shells, e.g. Solaris
- # /bin/sh. We can live with that; at least it never fails when fd N
- # is *open*.
- if (exec 3>&0) 2>/dev/null; then :; else exec 0</dev/null; fi
- if (exec 3>&1) 2>/dev/null; then :; else exec 1>/dev/null; fi
- if (exec 3>&2) ; then :; else exec 2>/dev/null; fi
+ # If lsof is available, we can ask it to print just the information
+ # we want, in a format that's easy to parse.
+ # We can't run this command inside backquotes; with some shells that
+ # may cause the parent shell to have a pipe active on fd 0 right when
+ # lsof scans its file table.
+ # lsof may exist but not actually work, e.g. if it's restricted to root.
+ : > as_fd_status.$$
+ if command -v lsof > /dev/null 2>&1; then
+ lsof -a -p $$ -d 0,1,2 -F fa > as_fd_status.$$ 2> /dev/null || :
+ fi
+ as_fd_status=`cat as_fd_status.$$`
+ if test -n "$as_fd_status"; then
+ case "$as_fd_status" in *f0?a[ru]*) ;; *) exec 0</dev/null;; esac
+ case "$as_fd_status" in *f1?a[wu]*) ;; *) exec 1>/dev/null;; esac
+ case "$as_fd_status" in *f2?a[wu]*) ;; *) exec 2>/dev/null;; esac
+
+ else
+ # BSD fstat can also print the information we want. The same concerns
+ # re backquotes and privilege restrictions as above apply.
+ if command -v fstat > /dev/null 2>&1; then
+ fstat -p $$ -n > as_fd_status.$$ 2> /dev/null || :
+ fi
+
+ # fstat's output is a fixed space-separated set of columns, not
+ # designed for machine parsing:
+ # USER CMD PID FD DEV INUM MODE SZ|DV R/W.
+ # Pipes and sockets are formatted differently from regular files
+ # and device nodes, with an unpredictable number of columns in
+ # between FD and R/W. We just ignore everything after the fd
+ # and before the last few characters on the line.
+ # If there are unexpected spaces in the USER or CMD columns, the
+ # sed program will output nothing and we will go on to the next
+ # case.
+ # The first sed 's' command has hard tabs in its arguments.
+ as_fd_status="`sed -ne '[
+ s/[ ][ ]*/ /g
+ s/ $//
+ s/^[^ ]* [^ ]* [0-9]* \([012]\)\**.* \([rw][rw]*\)$/f\1,\2,/p
+ ]' as_fd_status.$$`"
+ if test -n "$as_fd_status"; then
+ case "$as_fd_status" in *f0,r,*|*f0,rw,*) ;; *) exec 0</dev/null;; esac
+ case "$as_fd_status" in *f1,w,*|*f1,rw,*) ;; *) exec 1>/dev/null;; esac
+ case "$as_fd_status" in *f2,w,*|*f2,rw,*) ;; *) exec 2>/dev/null;; esac
+
+ else
+ # Shell redirection operations can only tell whether an fd is open,
+ # not whether it is readable or writable, so this is a last resort.
+ # `exec >&n` fails in POSIX sh when fd N is closed, but succeeds
+ # regardless of whether fd N is open in some old shells, e.g. Solaris
+ # /bin/sh. We can live with that; at least it never fails when fd N
+ # is *open*.
+ if (exec 3>&0) 2>/dev/null; then :; else exec 0</dev/null; fi
+ if (exec 3>&1) 2>/dev/null; then :; else exec 1>/dev/null; fi
+ if (exec 3>&2) ; then :; else exec 2>/dev/null; fi
+ fi
+ fi
+ rm -f as_fd_status.$$
fi
])
--
2.28.0