xsane/xsane-0.999-signal-handling.patch

From 54a700c45c81f1ffff5f3dfcecb3929143e31d48 Mon Sep 17 00:00:00 2001
From: Nils Philippsen <nils@redhat.com>
Date: Wed, 19 Mar 2014 13:23:16 +0100
Subject: [PATCH] patch: signal-handling

Squashed commit of the following:

commit 2b629e1d9c60dcf26defe6d05b515611874db270
Author: Nils Philippsen <nils@redhat.com>
Date:   Thu Mar 13 13:38:12 2014 +0100

    separate signal handlers in top and bottom half

    This is to avoid race-conditions occurring when a signal is received
    while the signal handler is not yet finished. It also avoids calling
    non-reentrant functions from a signal handler. The top half (the real
    signal handler) just writes a character into a pipe which gets picked up
    and serviced by the bottom half from the normal event loop, this
    serializes things and makes use of non-reentrant functions safe.
---
 src/xsane.c | 146 +++++++++++++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 131 insertions(+), 15 deletions(-)

diff --git a/src/xsane.c b/src/xsane.c
index eee76ff..02b4da4 100644
--- a/src/xsane.c
+++ b/src/xsane.c
@@ -47,6 +47,7 @@
 #endif
 
 #include <sys/wait.h>
+#include <glib-unix.h>
 
 #include <stdarg.h>
 
@@ -121,6 +122,7 @@ static const Preferences_medium_t pref_default_medium[]=
 
 int DBG_LEVEL = 0;
 static guint xsane_resolution_timer = 0;
+static int xsane_signal_pipe[2];
 
 /* ---------------------------------------------------------------------------------------------------------------------- */
 
@@ -161,8 +163,11 @@ void xsane_pref_save(void);
 static int xsane_pref_restore(void);
 static void xsane_pref_save_media(void);
 static void xsane_pref_restore_media(void);
-static RETSIGTYPE xsane_quit_handler(int signal);
-static RETSIGTYPE xsane_sigchld_handler(int signal);
+static RETSIGTYPE xsane_signal_handler_top_half(int signal);
+static gboolean xsane_signal_handler_bottom_half(gint fd,
+                                                 GIOCondition condition,
+                                                 gpointer user_data);
+static void xsane_sigchld_handler(void);
 static void xsane_quit(void);
 static void xsane_exit(void);
 static gint xsane_standard_option_win_delete(GtkWidget *widget, gpointer data);
@@ -2296,16 +2301,119 @@ static void xsane_pref_restore_media(void)
 
 /* ---------------------------------------------------------------------------------------------------------------------- */
 
-static RETSIGTYPE xsane_quit_handler(int signal)
+static RETSIGTYPE xsane_signal_handler_top_half(int signal)
 {
-  DBG(DBG_proc, "xsane_quit_handler\n");
+  const char *msg_func = "xsane_signal_handler_top_half(): ";
+  const char *msg_short_write = "Short write() while processing signal.\n";
+  const char *msg_err = "Error during write().\n";
+  char sig_char;
+  ssize_t written;
+  int errno_saved = errno;
 
-  xsane_quit();
+  switch (signal)
+  {
+    case SIGTERM:
+      sig_char = 't';
+      break;
+    case SIGINT:
+      sig_char = 'i';
+      break;
+    case SIGHUP:
+      sig_char = 'h';
+      break;
+    case SIGCHLD:
+      sig_char = 'c';
+      break;
+    default:
+      sig_char = '?';
+      break;
+  }
+
+  if ((written = write(xsane_signal_pipe[1], &sig_char, 1)) <= 0)
+  {
+    /* At this point, all bets are off. Salvage what we can. */
+
+    const char *msg = (written == 0) ? msg_short_write : msg_err;
+
+    if ((write(STDERR_FILENO, msg_func, strlen(msg_func)) < 0) ||
+        (write(STDERR_FILENO, msg, strlen(msg)) < 0))
+    {
+      /* This is really a no-op, but at this point it doesn't really matter
+       * anymore if the writes succeeded or not. */
+      goto bail_out;
+    }
+
+bail_out:
+    /* Ignore SIGCHLD errors, zombie processes don't hurt that much. */
+    if (signal != SIGCHLD)
+    {
+      struct SIGACTION act;
+      memset(&act, 0, sizeof(act));
+      act.sa_handler = SIG_DFL;
+      sigaction(signal, &act, NULL);
+      raise(signal);
+    }
+  }
+
+  errno = errno_saved;
+}
+
+static gboolean xsane_signal_handler_bottom_half(gint fd,
+                                                 GIOCondition condition,
+                                                 gpointer user_data)
+{
+  char sig_char;
+  ssize_t readlen;
+
+  DBG(DBG_proc, "xsane_signal_handler_bottom_half\n");
+
+  while ((readlen = read(xsane_signal_pipe[0], &sig_char, 1)) != 0)
+  {
+    if (readlen < 0)
+    {
+      if (errno == EINTR)
+      {
+        /* if interrupted by signal, just repeat reading */
+        continue;
+      }
+      else
+      {
+        break;
+      }
+    }
+
+    switch (sig_char)
+    {
+      case 't':
+      case 'i':
+      case 'h':
+        xsane_quit();
+        break;
+      case 'c':
+        xsane_sigchld_handler();
+        break;
+      default:
+        DBG(DBG_error,
+            "Don't know how to cope with character-encoded signal: '%c'\n",
+            sig_char);
+        break;
+    }
+  }
+
+  /* previous invocation might have read more than it should, so ignore
+   * EGAIN/EWOULDBLOCK */
+  if (readlen < 0 && errno != EAGAIN && errno != EWOULDBLOCK)
+  {
+    DBG(DBG_error, "Error while reading from pipe: %d '%s'\n", errno,
+        strerror(errno));
+  }
+
+  return TRUE;
 }
 
 /* ---------------------------------------------------------------------------------------------------------------------- */
 
-static RETSIGTYPE xsane_sigchld_handler(int signal)
+static void xsane_sigchld_handler(void)
 {
  int status;
  XsaneChildprocess **childprocess_listptr = &xsane.childprocess_list;
@@ -6062,18 +6170,26 @@ void xsane_interface(int argc, char **argv)
     }
   }
 
+  if (!g_unix_open_pipe(xsane_signal_pipe, FD_CLOEXEC, NULL) ||
+      fcntl(xsane_signal_pipe[0], F_SETFL, O_NONBLOCK, 1) ||
+      fcntl(xsane_signal_pipe[1], F_SETFL, O_NONBLOCK, 1) ||
+      !g_unix_fd_add(xsane_signal_pipe[0], G_IO_IN | G_IO_PRI,
+                     xsane_signal_handler_bottom_half, NULL))
+  {
+    DBG(DBG_error,
+        "Couldn't create signal handling pipe, set flags on it or install\n"
+        "bottom half of handler.\n");
+    exit(1);
+  }
+
   /* define SIGTERM, SIGINT, SIGHUP-handler to make sure that e.g. all temporary files are deleted */
   /* when xsane gets such a signal */
   memset(&act, 0, sizeof(act));
-  act.sa_handler = xsane_quit_handler;
-  sigaction(SIGTERM, &act, 0);
-  sigaction(SIGINT,  &act, 0);
-  sigaction(SIGHUP,  &act, 0);
-
-  /* add a signal handler that cleans up zombie child processes */
-  memset(&act, 0, sizeof(act));
-  act.sa_handler = xsane_sigchld_handler;
-  sigaction(SIGCHLD, &act, 0);
+  act.sa_handler = xsane_signal_handler_top_half;
+  sigaction(SIGTERM, &act, NULL);
+  sigaction(SIGINT,  &act, NULL);
+  sigaction(SIGHUP,  &act, NULL);
+  sigaction(SIGCHLD, &act, NULL);
 
   gtk_main();
   sane_exit();
-- 
1.8.5.3