Make (ice-9 popen) thread-safe.

Fixes <http://bugs.gnu.org/15683>.
Reported by David Pirotte <david@altosw.be>.

* module/ice-9/popen.scm: Import (ice-9 threads) and (srfi srfi-9).
  (<pipe-info>): New record type.
  (port/pid-table): Mark as deprecated in comment.
  (port/pid-table-mutex): New variable.
  (open-pipe*): Store the pid in the pipe-info record, and store the
  pipe-info as a port property.  Guard the pipe-info instead of the
  port.  Lock 'port/pid-table-mutex' while mutating 'port/pid-table'.
  (fetch-pid): Removed.
  (fetch-pipe-info): New procedure.
  (close-process-quietly): Removed.
  (close-pipe): Use 'fetch-pipe-info' instead of 'fetch-pid'.  Clear
  the pid from the pipe-info.  Improve error messages.
  (reap-pipes): Adapt to the fact that the pipe-info is now guarded
  instead of the port.  Incorporate the 'waitpid' code that was
  previously in 'close-process-quietly', but let the port finalizer
  close the port.  Clear the pid from the pipe-info.

module/ice-9/popen.scm

@@ -1,6 +1,7 @@
 ;; popen emulation, for non-stdio based ports.
 
-;;;; Copyright (C) 1998, 1999, 2000, 2001, 2003, 2006, 2010, 2011, 2012, 2013 Free Software Foundation, Inc.
+;;;; Copyright (C) 1998, 1999, 2000, 2001, 2003, 2006, 2010, 2011, 2012,
+;;;;   2013 Free Software Foundation, Inc.
 ;;;; 
 ;;;; This library is free software; you can redistribute it and/or
 ;;;; modify it under the terms of the GNU Lesser General Public
@@ -18,6 +19,8 @@
 ;;;; 
 
 (define-module (ice-9 popen)
+  :use-module (ice-9 threads)
+  :use-module (srfi srfi-9)
   :export (port/pid-table open-pipe* open-pipe close-pipe open-input-pipe
 	   open-output-pipe open-input-output-pipe))
 
@@ -25,6 +28,11 @@
   (load-extension (string-append "libguile-" (effective-version))
                   "scm_init_popen"))
 
+(define-record-type <pipe-info>
+  (make-pipe-info pid)
+  pipe-info?
+  (pid pipe-info-pid set-pipe-info-pid!))
+
 (define (make-rw-port read-port write-port)
   (make-soft-port
    (vector
@@ -40,7 +48,10 @@
 (define pipe-guardian (make-guardian))
 
 ;; a weak hash-table to store the process ids.
+;; XXX use of this table is deprecated.  It is no longer used here, and
+;; is populated for backward compatibility only (since it is exported).
 (define port/pid-table (make-weak-key-hash-table 31))
+(define port/pid-table-mutex (make-mutex))
 
 (define (open-pipe* mode command . args)
   "Executes the program @var{command} with optional arguments
@@ -56,9 +67,19 @@ port to the process is created: it should be the value of
                            (make-rw-port read-port write-port))
                       read-port
                       write-port
-                      (%make-void-port mode))))
-        (pipe-guardian port)
-        (hashq-set! port/pid-table port pid)
+                      (%make-void-port mode)))
+            (pipe-info (make-pipe-info pid)))
+
+        ;; Guard the pipe-info instead of the port, so that we can still
+        ;; call 'waitpid' even if 'close-port' is called (which clears
+        ;; the port entry).
+        (pipe-guardian pipe-info)
+        (%set-port-property! port 'popen-pipe-info pipe-info)
+
+        ;; XXX populate port/pid-table for backward compatibility.
+        (with-mutex port/pid-table-mutex
+          (hashq-set! port/pid-table port pid))
+
         port))))
 
 (define (open-pipe command mode)
@@ -69,48 +90,45 @@ port to the process is created: it should be the value of
 @code{OPEN_READ}, @code{OPEN_WRITE} or @code{OPEN_BOTH}."
   (open-pipe* mode "/bin/sh" "-c" command))
 
-(define (fetch-pid port)
-  (let ((pid (hashq-ref port/pid-table port)))
-    (hashq-remove! port/pid-table port)
-    pid))
+(define (fetch-pipe-info port)
+  (%port-property port 'popen-pipe-info))
 
 (define (close-process port pid)
   (close-port port)
   (cdr (waitpid pid)))
 
-;; for the background cleanup handler: just clean up without reporting
-;; errors.  also avoids blocking the process: if the child isn't ready
-;; to be collected, puts it back into the guardian's live list so it
-;; can be tried again the next time the cleanup runs.
-(define (close-process-quietly port pid)
-  (catch 'system-error
-	 (lambda ()
-	   (close-port port))
-	 (lambda args #f))
-  (catch 'system-error
-	 (lambda ()
-	   (let ((pid/status (waitpid pid WNOHANG)))
-             (when (zero? (car pid/status))
-               ;; not ready for collection
-               (pipe-guardian port)
-               (hashq-set! port/pid-table port pid))))
-	 (lambda args #f)))
-
 (define (close-pipe p)
   "Closes the pipe created by @code{open-pipe}, then waits for the process
 to terminate and returns its status value, @xref{Processes, waitpid}, for
 information on how to interpret this value."
-  (let ((pid (fetch-pid p)))
-    (unless pid (error "close-pipe: pipe not in table"))
-    (close-process p pid)))
+  (let ((pipe-info (fetch-pipe-info p)))
+    (unless pipe-info
+      (error "close-pipe: port not created by (ice-9 popen)"))
+    (let ((pid (pipe-info-pid pipe-info)))
+      (unless pid
+        (error "close-pipe: pid has already been cleared"))
+      ;; clear the pid to avoid repeated calls to 'waitpid'.
+      (set-pipe-info-pid! pipe-info #f)
+      (close-process p pid))))
 
 (define (reap-pipes)
   (let loop ()
-    (let ((p (pipe-guardian)))
-      (when p
-        ;; maybe removed already by close-pipe.
-        (let ((pid (fetch-pid p)))
-          (when pid (close-process-quietly p pid)))
+    (let ((pipe-info (pipe-guardian)))
+      (when pipe-info
+        (let ((pid (pipe-info-pid pipe-info)))
+          ;; maybe 'close-pipe' was already called.
+          (when pid
+            ;; clean up without reporting errors.  also avoids blocking
+            ;; the process: if the child isn't ready to be collected,
+            ;; puts it back into the guardian's live list so it can be
+            ;; tried again the next time the cleanup runs.
+            (catch 'system-error
+              (lambda ()
+                (let ((pid/status (waitpid pid WNOHANG)))
+                  (if (zero? (car pid/status))
+                      (pipe-guardian pipe-info) ; not ready for collection
+                      (set-pipe-info-pid! pipe-info #f))))
+              (lambda args #f))))
         (loop)))))
 
 (add-hook! after-gc-hook reap-pipes)