lockfile.c

   1 /*
   2  * Copyright (c) 2005, Junio C Hamano
   3  */
   4
   5 #include "cache.h"
   6 #include "lockfile.h"
   7
   8 /*
   9  * path = absolute or relative path name
  10  *
  11  * Remove the last path name element from path (leaving the preceding
  12  * "/", if any).  If path is empty or the root directory ("/"), set
  13  * path to the empty string.
  14  */
  15 static void trim_last_path_component(struct strbuf *path)
  16 {
  17         int i = path->len;
  18
  19         /* back up past trailing slashes, if any */
  20         while (i && path->buf[i - 1] == '/')
  21                 i--;
  22
  23         /*
  24          * then go backwards until a slash, or the beginning of the
  25          * string
  26          */
  27         while (i && path->buf[i - 1] != '/')
  28                 i--;
  29
  30         strbuf_setlen(path, i);
  31 }
  32
  33
  34 /* We allow "recursive" symbolic links. Only within reason, though */
  35 #define MAXDEPTH 5
  36
  37 /*
  38  * path contains a path that might be a symlink.
  39  *
  40  * If path is a symlink, attempt to overwrite it with a path to the
  41  * real file or directory (which may or may not exist), following a
  42  * chain of symlinks if necessary.  Otherwise, leave path unmodified.
  43  *
  44  * This is a best-effort routine.  If an error occurs, path will
  45  * either be left unmodified or will name a different symlink in a
  46  * symlink chain that started with the original path.
  47  */
  48 static void resolve_symlink(struct strbuf *path)
  49 {
  50         int depth = MAXDEPTH;
  51         static struct strbuf link = STRBUF_INIT;
  52
  53         while (depth--) {
  54                 if (strbuf_readlink(&link, path->buf, path->len) < 0)
  55                         break;
  56
  57                 if (is_absolute_path(link.buf))
  58                         /* absolute path simply replaces p */
  59                         strbuf_reset(path);
  60                 else
  61                         /*
  62                          * link is a relative path, so replace the
  63                          * last element of p with it.
  64                          */
  65                         trim_last_path_component(path);
  66
  67                 strbuf_addbuf(path, &link);
  68         }
  69         strbuf_reset(&link);
  70 }
  71
  72 /* Make sure errno contains a meaningful value on error */
  73 static int lock_file(struct lock_file *lk, const char *path, int flags)
  74 {
  75         int fd;
  76         struct strbuf filename = STRBUF_INIT;
  77
  78         strbuf_addstr(&filename, path);
  79         if (!(flags & LOCK_NO_DEREF))
  80                 resolve_symlink(&filename);
  81
  82         strbuf_addstr(&filename, LOCK_SUFFIX);
  83         fd = create_tempfile(&lk->tempfile, filename.buf);
  84         strbuf_release(&filename);
  85         return fd;
  86 }
  87
  88 /*
  89  * Constants defining the gaps between attempts to lock a file. The
  90  * first backoff period is approximately INITIAL_BACKOFF_MS
  91  * milliseconds. The longest backoff period is approximately
  92  * (BACKOFF_MAX_MULTIPLIER * INITIAL_BACKOFF_MS) milliseconds.
  93  */
  94 #define INITIAL_BACKOFF_MS 1L
  95 #define BACKOFF_MAX_MULTIPLIER 1000
  96
  97 /*
  98  * Try locking path, retrying with quadratic backoff for at least
  99  * timeout_ms milliseconds. If timeout_ms is 0, try locking the file
 100  * exactly once. If timeout_ms is -1, try indefinitely.
 101  */
 102 static int lock_file_timeout(struct lock_file *lk, const char *path,
 103                              int flags, long timeout_ms)
 104 {
 105         int n = 1;
 106         int multiplier = 1;
 107         long remaining_ms = 0;
 108         static int random_initialized = 0;
 109
 110         if (timeout_ms == 0)
 111                 return lock_file(lk, path, flags);
 112
 113         if (!random_initialized) {
 114                 srand((unsigned int)getpid());
 115                 random_initialized = 1;
 116         }
 117
 118         if (timeout_ms > 0)
 119                 remaining_ms = timeout_ms;
 120
 121         while (1) {
 122                 long backoff_ms, wait_ms;
 123                 int fd;
 124
 125                 fd = lock_file(lk, path, flags);
 126
 127                 if (fd >= 0)
 128                         return fd; /* success */
 129                 else if (errno != EEXIST)
 130                         return -1; /* failure other than lock held */
 131                 else if (timeout_ms > 0 && remaining_ms <= 0)
 132                         return -1; /* failure due to timeout */
 133
 134                 backoff_ms = multiplier * INITIAL_BACKOFF_MS;
 135                 /* back off for between 0.75*backoff_ms and 1.25*backoff_ms */
 136                 wait_ms = (750 + rand() % 500) * backoff_ms / 1000;
 137                 sleep_millisec(wait_ms);
 138                 remaining_ms -= wait_ms;
 139
 140                 /* Recursion: (n+1)^2 = n^2 + 2n + 1 */
 141                 multiplier += 2*n + 1;
 142                 if (multiplier > BACKOFF_MAX_MULTIPLIER)
 143                         multiplier = BACKOFF_MAX_MULTIPLIER;
 144                 else
 145                         n++;
 146         }
 147 }
 148
 149 void unable_to_lock_message(const char *path, int err, struct strbuf *buf)
 150 {
 151         if (err == EEXIST) {
 152                 strbuf_addf(buf, "Unable to create '%s.lock': %s.\n\n"
 153                     "If no other git process is currently running, this probably means a\n"
 154                     "git process crashed in this repository earlier. Make sure no other git\n"
 155                     "process is running and remove the file manually to continue.",
 156                             absolute_path(path), strerror(err));
 157         } else
 158                 strbuf_addf(buf, "Unable to create '%s.lock': %s",
 159                             absolute_path(path), strerror(err));
 160 }
 161
 162 NORETURN void unable_to_lock_die(const char *path, int err)
 163 {
 164         struct strbuf buf = STRBUF_INIT;
 165
 166         unable_to_lock_message(path, err, &buf);
 167         die("%s", buf.buf);
 168 }
 169
 170 /* This should return a meaningful errno on failure */
 171 int hold_lock_file_for_update_timeout(struct lock_file *lk, const char *path,
 172                                       int flags, long timeout_ms)
 173 {
 174         int fd = lock_file_timeout(lk, path, flags, timeout_ms);
 175         if (fd < 0 && (flags & LOCK_DIE_ON_ERROR))
 176                 unable_to_lock_die(path, errno);
 177         return fd;
 178 }
 179
 180 char *get_locked_file_path(struct lock_file *lk)
 181 {
 182         struct strbuf ret = STRBUF_INIT;
 183
 184         strbuf_addstr(&ret, get_tempfile_path(&lk->tempfile));
 185         if (ret.len <= LOCK_SUFFIX_LEN ||
 186             strcmp(ret.buf + ret.len - LOCK_SUFFIX_LEN, LOCK_SUFFIX))
 187                 die("BUG: get_locked_file_path() called for malformed lock object");
 188         /* remove ".lock": */
 189         strbuf_setlen(&ret, ret.len - LOCK_SUFFIX_LEN);
 190         return strbuf_detach(&ret, NULL);
 191 }
 192
 193 int commit_lock_file(struct lock_file *lk)
 194 {
 195         char *result_path = get_locked_file_path(lk);
 196
 197         if (commit_lock_file_to(lk, result_path)) {
 198                 int save_errno = errno;
 199                 free(result_path);
 200                 errno = save_errno;
 201                 return -1;
 202         }
 203         free(result_path);
 204         return 0;
 205 }