>From 7bc28986ebdacbe77a43c52f36645c20b2bdf442 Mon Sep 17 00:00:00 2001
From: Mark H Weaver <address@hidden>
Date: Sun, 17 Nov 2013 02:54:31 -0500
Subject: [PATCH 6/6] Make (ice-9 popen) thread-safe.

* module/ice-9/popen.scm: Import (ice-9 threads).
  (port/pid-table): Mark as deprecated in comment.
  (port/pid-table-mutex): New variable.
  (open-pipe*): Store the pid in the port's alist.  Guard the alist
  entry instead of the port.  Lock 'port/pid-table-mutex' while mutating
  'port/pid-table'.
  (fetch-pid): Removed.
  (fetch-alist-entry): New procedure.
  (close-process-quietly): Removed.
  (close-pipe): Use 'fetch-alist-entry' instead of 'fetch-pid'.  Clear
  the cdr of the alist entry.  Improve error messages.
  (reap-pipes): Adapt to the fact that the alist entries are now guarded
  instead of the ports.  Incorporate the 'waitpid' code that was
  previously in 'close-process-quietly', but let the port finalizer
  close the port.  Clear the cdr of the alist entry.
---
 module/ice-9/popen.scm |   76 +++++++++++++++++++++++++++--------------------
 1 files changed, 44 insertions(+), 32 deletions(-)

diff --git a/module/ice-9/popen.scm b/module/ice-9/popen.scm
index f8668cd..8e43112 100644
--- a/module/ice-9/popen.scm
+++ b/module/ice-9/popen.scm
@@ -18,6 +18,7 @@
 ;;;; 
 
 (define-module (ice-9 popen)
+  :use-module (ice-9 threads)
   :export (port/pid-table open-pipe* open-pipe close-pipe open-input-pipe
 	   open-output-pipe open-input-output-pipe))
 
@@ -40,7 +41,10 @@
 (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 +60,19 @@ 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)))
+            (alist-entry (cons 'popen-pid pid)))
+
+        ;; Store the alist-entry in the guardian instead of the port,
+        ;; so that we can still call 'waitpid' even if 'close-port'
+        ;; is called (which clears the port entry).
+        (pipe-guardian alist-entry)
+        (%set-port-alist! port (cons alist-entry (%port-alist port)))
+
+        ;; 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 +83,46 @@ 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-alist-entry port)
+  (assq 'popen-pid (%port-alist port)))
 
 (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"))
+  (let* ((alist-entry (fetch-alist-entry p))
+         (pid (cdr alist-entry)))
+    ;; set the cdr to #f so that the reaper won't wait on this pid
+    ;; again, and to detect repeated calls to 'close-pipe'.
+    (set-cdr! alist-entry #f)
+    (unless alist-entry
+      (error "close-pipe: port not created by (ice-9 popen)"))
+    (unless pid
+      (error "close-pipe: pid has already been cleared"))
     (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 ((alist-entry (pipe-guardian)))
+      (when alist-entry
+        (let ((pid (cdr alist-entry)))
+          ;; 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 alist-entry)  ; not ready for collection
+                      (set-cdr! alist-entry #f)))) ; avoid calling waitpid again
+              (lambda args #f))))
         (loop)))))
 
 (add-hook! after-gc-hook reap-pipes)
-- 
1.7.5.4