daemon: Make --monitor process change its process title.

When --monitor is used, administrators sometimes become confused about the
presence of two copies of each process.  This commit attempts to clarify
the situation by making the monitoring process change its process name, as
seen in /proc/$pid/cmdline and in "ps", to clearly indicate what is going
on.

CC: Dan Wendlandt <dan@nicira.com>

lib/command-line.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2008, 2009 Nicira Networks.
+ * Copyright (c) 2008, 2009, 2010 Nicira Networks.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -20,7 +20,6 @@
 #include <limits.h>
 #include <stdlib.h>
 #include "util.h"
-#include "vlog.h"
 
 /* Given the GNU-style long options in 'options', returns a string that may be
  * passed to getopt() with the corresponding short options.  The caller is
@@ -87,3 +86,111 @@ run_command(int argc, char *argv[], const struct command commands[])
 
     ovs_fatal(0, "unknown command '%s'; use --help for help", argv[0]);
 }
+
+/* Process title. */
+
+#ifdef __linux__
+static char *argv_start;       /* Start of command-line arguments in memory. */
+static size_t argv_size;       /* Number of bytes of command-line arguments. */
+static char *saved_proctitle;  /* Saved command-line arguments. */
+
+/* Prepares the process so that proctitle_set() can later succeed.
+ *
+ * This modifies the argv[] array so that it no longer points into the memory
+ * that it originally does.  Later, proctitle_set() might overwrite that
+ * memory.  That means that this function should be called before anything else
+ * that accesses the process's argv[] array.  Ideally, it should be called
+ * before anything else, period, at the very beginning of program
+ * execution.  */
+void
+proctitle_init(int argc, char **argv)
+{
+    int i;
+
+    if (!argc || !argv[0]) {
+        /* This situation should never occur, but... */
+        return;
+    }
+
+    /* Specialized version of first loop iteration below. */
+    argv_start = argv[0];
+    argv_size = strlen(argv[0]) + 1;
+    argv[0] = xstrdup(argv[0]);
+
+    for (i = 1; i < argc; i++) {
+        size_t size = strlen(argv[i]) + 1;
+
+        /* Add (argv[i], strlen(argv[i])+1) to (argv_start, argv_size). */
+        if (argv[i] + size == argv_start) {
+            /* Arguments grow downward in memory. */
+            argv_start -= size;
+            argv_size += size;
+        } else if (argv[i] == argv_start + argv_size) {
+            /* Arguments grow upward in memory. */
+            argv_size += size;
+        } else {
+            /* Arguments not contiguous.  (Is this really Linux?) */
+        }
+
+        /* Copy out the old argument so we can reuse the space. */
+        argv[i] = xstrdup(argv[i]);
+    }
+}
+
+/* Changes the name of the process, as shown by "ps", to 'format', which is
+ * formatted as if by printf(). */
+void
+proctitle_set(const char *format, ...)
+{
+    va_list args;
+    int n;
+
+    if (!argv_start || argv_size < 8) {
+        return;
+    }
+
+    if (!saved_proctitle) {
+        saved_proctitle = xmemdup(argv_start, argv_size);
+    }
+
+    va_start(args, format);
+    n = vsnprintf(argv_start, argv_size, format, args);
+    if (n >= argv_size) {
+        /* The name is too long, so add an ellipsis at the end. */
+        strcpy(&argv_start[argv_size - 4], "...");
+    } else {
+        /* Fill the extra space with null bytes, so that trailing bytes don't
+         * show up in the command line. */
+        memset(&argv_start[n], '\0', argv_size - n);
+    }
+    va_end(args);
+}
+
+/* Restores the process's original command line, as seen by "ps". */
+void
+proctitle_restore(void)
+{
+    if (saved_proctitle) {
+        memcpy(argv_start, saved_proctitle, argv_size);
+        free(saved_proctitle);
+        saved_proctitle = NULL;
+    }
+}
+#else  /* !__linux__ */
+/* Stubs that don't do anything on non-Linux systems. */
+
+void
+proctitle_init(int argc UNUSED, char **argv UNUSED)
+{
+}
+
+void
+proctitle_set(const char *format UNUSED, ...)
+{
+}
+
+void
+proctitle_restore(void)
+{
+}
+#endif  /* !__linux__ */