poll-loop: Make wakeup logging more portable and easier to understand.

Until now, when the poll_loop module's log level was turned up to "debug",
it would log a backtrace of the call stack for the event that caused poll()
to wake up in poll_block().  This was pretty useful from time to time to
find out why ovs-vswitchd was using more CPU than expected, because we
could find out what was causing it to wake up.

But there were some issues.  One is simply that the backtrace was printed
as a series of hexadecimal numbers, so GDB or another debugger was needed
to translate it into human-readable format.  Compiler optimizations meant
that even the human-readable backtrace wasn't, in my experience, as helpful
as it could have been.  And, of course, one needed to have the binary to
interpret the backtrace.  When the backtrace couldn't be interpreted or
wasn't meaningful, there was essentially nothing to fall back on.

This commit changes the way that "debug" logging for poll_block() wakeups
works.  Instead of logging a backtrace, it logs the source code file name
and line number of the call to a poll_loop function, using __FILE__ and
__LINE__.  This is by itself much more meaningful than a sequence of
hexadecimal numbers, since no additional interpretation is necessary.  It
can be useful even if the Open vSwitch version is only approximately known.

In addition to the file and line, this commit adds, for wakeups caused by
file descriptors, information about the file descriptor itself: what kind
of file it is (regular file, directory, socket, etc.), the name of the file
(on Linux only), and the local and remote endpoints for socket file
descriptors.

Here are a few examples of the new output format:

932-ms timeout at ../ofproto/in-band.c:507
[POLLIN] on fd 20 (192.168.0.20:35388<->192.168.0.3:6633) at ../lib/stream-fd.c:149
[POLLIN] on fd 7 (FIFO pipe:[48049]) at ../lib/fatal-signal.c:168

lib/socket-util.c

@@ -19,6 +19,7 @@
 #include <arpa/inet.h>
 #include <errno.h>
 #include <fcntl.h>
+#include <net/if.h>
 #include <netdb.h>
 #include <poll.h>
 #include <stddef.h>
@@ -30,9 +31,18 @@
 #include <sys/stat.h>
 #include <sys/un.h>
 #include <unistd.h>
+#include "dynamic-string.h"
 #include "fatal-signal.h"
+#include "packets.h"
 #include "util.h"
 #include "vlog.h"
+#if AF_PACKET && __linux__
+#include <linux/if_packet.h>
+#endif
+#ifdef HAVE_NETLINK
+#include "netlink-protocol.h"
+#include "netlink-socket.h"
+#endif
 
 VLOG_DEFINE_THIS_MODULE(socket_util);
 
@@ -784,3 +794,148 @@ xpipe(int fds[2])
         VLOG_FATAL("failed to create pipe (%s)", strerror(errno));
     }
 }
+
+static int
+getsockopt_int(int fd, int level, int optname, int *valuep)
+{
+    socklen_t len = sizeof *valuep;
+
+    return (getsockopt(fd, level, optname, valuep, &len) ? errno
+            : len == sizeof *valuep ? 0
+            : EINVAL);
+}
+
+static void
+describe_sockaddr(struct ds *string, int fd,
+                  int (*getaddr)(int, struct sockaddr *, socklen_t *))
+{
+    struct sockaddr_storage ss;
+    socklen_t len = sizeof ss;
+
+    if (!getaddr(fd, (struct sockaddr *) &ss, &len)) {
+        if (ss.ss_family == AF_INET) {
+            struct sockaddr_in sin;
+
+            memcpy(&sin, &ss, sizeof sin);
+            ds_put_format(string, IP_FMT":%"PRIu16,
+                          IP_ARGS(&sin.sin_addr.s_addr), ntohs(sin.sin_port));
+        } else if (ss.ss_family == AF_UNIX) {
+            struct sockaddr_un sun;
+            const char *null;
+            size_t maxlen;
+
+            memcpy(&sun, &ss, sizeof sun);
+            maxlen = len - offsetof(struct sockaddr_un, sun_path);
+            null = memchr(sun.sun_path, '\0', maxlen);
+            ds_put_buffer(string, sun.sun_path,
+                          null ? null - sun.sun_path : maxlen);
+        }
+#ifdef HAVE_NETLINK
+        else if (ss.ss_family == AF_NETLINK) {
+            int protocol;
+
+/* SO_PROTOCOL was introduced in 2.6.32.  Support it regardless of the version
+ * of the Linux kernel headers in use at build time. */
+#ifndef SO_PROTOCOL
+#define SO_PROTOCOL 38
+#endif
+
+            if (!getsockopt_int(fd, SOL_SOCKET, SO_PROTOCOL, &protocol)) {
+                switch (protocol) {
+                case NETLINK_ROUTE:
+                    ds_put_cstr(string, "NETLINK_ROUTE");
+                    break;
+
+                case NETLINK_GENERIC:
+                    ds_put_cstr(string, "NETLINK_GENERIC");
+                    break;
+
+                default:
+                    ds_put_format(string, "AF_NETLINK family %d", protocol);
+                    break;
+                }
+            } else {
+                ds_put_cstr(string, "AF_NETLINK");
+            }
+        }
+#endif
+#if AF_PACKET && __linux__
+        else if (ss.ss_family == AF_PACKET) {
+            struct sockaddr_ll sll;
+
+            memcpy(&sll, &ss, sizeof sll);
+            ds_put_cstr(string, "AF_PACKET");
+            if (sll.sll_ifindex) {
+                char name[IFNAMSIZ];
+
+                if (if_indextoname(sll.sll_ifindex, name)) {
+                    ds_put_format(string, "(%s)", name);
+                } else {
+                    ds_put_format(string, "(ifindex=%d)", sll.sll_ifindex);
+                }
+            }
+            if (sll.sll_protocol) {
+                ds_put_format(string, "(protocol=0x%"PRIu16")",
+                              ntohs(sll.sll_protocol));
+            }
+        }
+#endif
+        else if (ss.ss_family == AF_UNSPEC) {
+            ds_put_cstr(string, "AF_UNSPEC");
+        } else {
+            ds_put_format(string, "AF_%d", (int) ss.ss_family);
+        }
+    }
+}
+
+
+#ifdef __linux__
+static void
+put_fd_filename(struct ds *string, int fd)
+{
+    char buf[1024];
+    char *linkname;
+    int n;
+
+    linkname = xasprintf("/proc/self/fd/%d", fd);
+    n = readlink(linkname, buf, sizeof buf);
+    if (n > 0) {
+        ds_put_char(string, ' ');
+        ds_put_buffer(string, buf, n);
+        if (n > sizeof buf) {
+            ds_put_cstr(string, "...");
+        }
+    }
+    free(linkname);
+}
+#endif
+
+/* Returns a malloc()'d string describing 'fd', for use in logging. */
+char *
+describe_fd(int fd)
+{
+    struct ds string;
+    struct stat s;
+
+    ds_init(&string);
+    if (fstat(fd, &s)) {
+        ds_put_format(&string, "fstat failed (%s)", strerror(errno));
+    } else if (S_ISSOCK(s.st_mode)) {
+        describe_sockaddr(&string, fd, getsockname);
+        ds_put_cstr(&string, "<->");
+        describe_sockaddr(&string, fd, getpeername);
+    } else {
+        ds_put_cstr(&string, (isatty(fd) ? "tty"
+                              : S_ISDIR(s.st_mode) ? "directory"
+                              : S_ISCHR(s.st_mode) ? "character device"
+                              : S_ISBLK(s.st_mode) ? "block device"
+                              : S_ISREG(s.st_mode) ? "file"
+                              : S_ISFIFO(s.st_mode) ? "FIFO"
+                              : S_ISLNK(s.st_mode) ? "symbolic link"
+                              : "unknown"));
+#ifdef __linux__
+        put_fd_filename(&string, fd);
+#endif
+    }
+    return ds_steal_cstr(&string);
+}