Add UUCP locking

Add documentation


git-svn-id: file:///home/lennart/svn/public/bidilink/trunk@7 9cde1c1d-e4d0-0310-8a68-bf217395ea82

1 files changed, 150 insertions, 0 deletions

diff --git a/src/client-tty.c b/src/client-tty.c
index 5f31249..59ddec5 100644
--- a/src/client-tty.c
+++ b/src/client-tty.c
@@ -30,17 +30,165 @@
 #include <string.h>
 #include <errno.h>
 #include <assert.h>
+#include <limits.h>
+#include <sys/stat.h>
+#include <inttypes.h>
+#include <sys/types.h>
+#include <signal.h>
+#include <pwd.h>
 
 #include "client-tty.h"
 
+#define LOCKDIR "/var/lock"
+
+static const char *lockfile(const char *dev) {
+    static char lockfile[PATH_MAX];
+    char *p = strrchr(dev, '/');
+    if (p)
+        p++;
+    else
+        p = (char*) dev;
+    
+    snprintf(lockfile, sizeof(lockfile), "%s/LCK..%s", LOCKDIR, p);
+    return lockfile;
+}
+
+static const char *tempfile(const char *path) {
+    static char t[PATH_MAX];
+    snprintf(t, sizeof(t), "%s.tmp.%lu", path, (unsigned long) getpid());
+    return t;
+}
+
+static int device_lock(const char *dev) {
+    struct stat st;
+    int fd;
+    const char *path, *temp;
+    char buf[100];
+    char uidbuf[32];
+
+    if (stat(LOCKDIR, &st) != 0 || !S_ISDIR(st.st_mode)) {
+        fprintf(stderr, "Failed to lock device, directory "LOCKDIR" not existent.\n");
+        return -1;
+    }
+
+    path = lockfile(dev);
+    temp = tempfile(path);
+
+    for (;;) {
+        mode_t u;
+        struct passwd* pw;
+        char *username;
+        
+        if ((fd = open(path, O_RDONLY)) < 0) {
+            if (errno != ENOENT) {
+                fprintf(stderr, "Failed to open lock file: %s\n", strerror(errno));
+                return -1;
+            }
+        }
+        
+        if (fd >= 0) {
+            ssize_t n;
+
+            n = read(fd, buf, sizeof(buf) - 1);
+            close(fd);
+
+            if (n < 0) {
+                close(fd);
+                fprintf(stderr, "Failed to read from lock file: %s\n", strerror(errno));
+                return -1;
+            }
+            
+            if (n > 0) {
+                pid_t pid;
+                
+                if (n == 4)
+                    pid = (pid_t) *((uint32_t*) buf);
+                else {
+                    unsigned long v;
+                    buf[n] = 0;
+                    sscanf(buf, "%lu", &v);
+                    pid = (pid_t) v;
+                }
+                
+                if (pid > 0) {
+                    if (kill(pid, 0) < 0 && errno == ESRCH) {
+                        fprintf(stderr, "Lockfile is stale. Overriding it.\n");
+                        /* Yes, here is a race condition */
+                        unlink(path);
+                    } else
+                        return 1;
+                }
+            }
+        }
+
+        u = umask(0033);
+        fd = open(temp, O_WRONLY | O_CREAT | O_EXCL, 0666);
+        umask(u);
+        
+        if (fd < 0) {
+            fprintf(stderr, "Failed to create temporary lock file: %s\n", strerror(errno));
+            return -1;
+        }
+
+        if ((pw = getpwuid(getuid())))
+            username = pw->pw_name;
+        else
+            snprintf(username = uidbuf, sizeof(uidbuf), "%lu", (unsigned long) getuid());
+
+        snprintf(buf, sizeof(buf), "%10lu %s %.20s\n", (unsigned long) getpid(), "bidilink", username);
+        if (write(fd, buf, strlen(buf)) < 0) {
+            fprintf(stderr, "Failed to write to temporary lock file: %s\n", strerror(errno));
+            close(fd);
+            return -1;
+        }
+
+        close(fd);
+
+        if (link(temp, path) < 0) {
+            if (errno == EEXIST)
+                continue;
+
+            fprintf(stderr, "Failed to link temporary lock file: %s\n", strerror(errno));
+        }
+
+        unlink(temp);
+
+        return 0;
+    }
+}
+
+static int device_unlock(const char *dev) {
+    if (unlink(lockfile(dev)) < 0) {
+        fprintf(stderr, "Failed to remove lock file: %s\n", strerror(errno));
+        return -1;
+    }
+
+    return 0;
+}
+
+static void destruct(struct stream *s) {
+    assert(s && s->user);
+
+    device_unlock((char*) s->user);
+    free(s->user);
+}
+
 struct stream* stream_client_tty(const char *args) {
     struct stream *s = NULL;
     struct termios ts;
     int fd = -1;
+    int r;
     
     s = malloc(sizeof(struct stream));
     assert(s);
     memset(s, 0, sizeof(struct stream));
+
+    if ((r = device_lock(args)) != 0) {
+        if (r > 0)
+            fprintf(stderr, "Device locked.\n");
+        goto fail;
+    }
+
     
     if ((fd = open(args, O_RDWR|O_NOCTTY)) < 0) {
         fprintf(stderr, "open('%s', O_RDWR|O_NOCTTY): %s\n", args, strerror(errno));
@@ -67,6 +215,8 @@ struct stream* stream_client_tty(const char *args) {
         goto fail;
     }
 
+    s->user = strdup(args);
+    s->destruct = destruct;
     s->input_fd = s->output_fd = fd;
     return s;