real_path: resolve symlinks by hand

The current implementation of real_path uses chdir() in order to resolve
symlinks.  Unfortunately this isn't thread-safe as chdir() affects a
process as a whole and not just an individual thread.  Instead perform
the symlink resolution by hand so that the calls to chdir() can be
removed, making real_path one step closer to being reentrant.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/abspath.c b/abspath.c
index 2825de85912fc730d9a40fa66f4f83b6250a4171..cafcae0352928dae2a5951abe725b9582766cf39 100644 (file)
--- a/abspath.c
+++ b/abspath.c
@@ -11,8 +11,45 @@ int is_directory(const char *path)
        return (!stat(path, &st) && S_ISDIR(st.st_mode));
 }
 
        return (!stat(path, &st) && S_ISDIR(st.st_mode));
 }
 

+/* removes the last path component from 'path' except if 'path' is root */
+static void strip_last_component(struct strbuf *path)
+{
+       size_t offset = offset_1st_component(path->buf);
+       size_t len = path->len;
+
+       /* Find start of the last component */
+       while (offset < len && !is_dir_sep(path->buf[len - 1]))
+               len--;
+       /* Skip sequences of multiple path-separators */
+       while (offset < len && is_dir_sep(path->buf[len - 1]))
+               len--;
+
+       strbuf_setlen(path, len);
+}
+
+/* get (and remove) the next component in 'remaining' and place it in 'next' */
+static void get_next_component(struct strbuf *next, struct strbuf *remaining)
+{
+       char *start = NULL;
+       char *end = NULL;
+
+       strbuf_reset(next);
+
+       /* look for the next component */
+       /* Skip sequences of multiple path-separators */
+       for (start = remaining->buf; is_dir_sep(*start); start++)
+               ; /* nothing */
+       /* Find end of the path component */
+       for (end = start; *end && !is_dir_sep(*end); end++)
+               ; /* nothing */
+
+       strbuf_add(next, start, end - start);
+       /* remove the component from 'remaining' */
+       strbuf_remove(remaining, 0, end - remaining->buf);
+}
+

 /* We allow "recursive" symbolic links. Only within reason, though. */
 /* We allow "recursive" symbolic links. Only within reason, though. */

-#define MAXDEPTH 5
+#define MAXSYMLINKS 5

 
 /*
  * Return the real path (i.e., absolute path, with symlinks resolved
 
 /*
  * Return the real path (i.e., absolute path, with symlinks resolved


@@ -21,7 +58,6 @@ int is_directory(const char *path)
  * absolute_path().)  The return value is a pointer to a static
  * buffer.
  *
  * absolute_path().)  The return value is a pointer to a static
  * buffer.
  *

- * The input and all intermediate paths must be shorter than MAX_PATH.

  * The directory part of path (i.e., everything up to the last
  * dir_sep) must denote a valid, existing directory, but the last
  * component need not exist.  If die_on_error is set, then die with an
  * The directory part of path (i.e., everything up to the last
  * dir_sep) must denote a valid, existing directory, but the last
  * component need not exist.  If die_on_error is set, then die with an


@@ -33,22 +69,16 @@ int is_directory(const char *path)
  */
 static const char *real_path_internal(const char *path, int die_on_error)
 {
  */
 static const char *real_path_internal(const char *path, int die_on_error)
 {

-       static struct strbuf sb = STRBUF_INIT;
+       static struct strbuf resolved = STRBUF_INIT;
+       struct strbuf remaining = STRBUF_INIT;
+       struct strbuf next = STRBUF_INIT;
+       struct strbuf symlink = STRBUF_INIT;

        char *retval = NULL;
        char *retval = NULL;

-
-       /*
-        * If we have to temporarily chdir(), store the original CWD
-        * here so that we can chdir() back to it at the end of the
-        * function:
-        */
-       struct strbuf cwd = STRBUF_INIT;
-
-       int depth = MAXDEPTH;
-       char *last_elem = NULL;
+       int num_symlinks = 0;

        struct stat st;
 
        /* We've already done it */
        struct stat st;
 
        /* We've already done it */

-       if (path == sb.buf)
+       if (path == resolved.buf)

                return path;
 
        if (!*path) {
                return path;
 
        if (!*path) {


@@ -58,74 +88,112 @@ static const char *real_path_internal(const char *path, int die_on_error)
                        goto error_out;
        }
 
                        goto error_out;
        }
 

-       strbuf_reset(&sb);
-       strbuf_addstr(&sb, path);
-
-       while (depth--) {
-               if (!is_directory(sb.buf)) {
-                       char *last_slash = find_last_dir_sep(sb.buf);
-                       if (last_slash) {
-                               last_elem = xstrdup(last_slash + 1);
-                               strbuf_setlen(&sb, last_slash - sb.buf + 1);
-                       } else {
-                               last_elem = xmemdupz(sb.buf, sb.len);
-                               strbuf_reset(&sb);
-                       }
+       strbuf_reset(&resolved);
+
+       if (is_absolute_path(path)) {
+               /* absolute path; start with only root as being resolved */
+               int offset = offset_1st_component(path);
+               strbuf_add(&resolved, path, offset);
+               strbuf_addstr(&remaining, path + offset);
+       } else {
+               /* relative path; can use CWD as the initial resolved path */
+               if (strbuf_getcwd(&resolved)) {
+                       if (die_on_error)
+                               die_errno("unable to get current working directory");
+                       else
+                               goto error_out;

                }
                }

+               strbuf_addstr(&remaining, path);
+       }

 
 

-               if (sb.len) {
-                       if (!cwd.len && strbuf_getcwd(&cwd)) {
+       /* Iterate over the remaining path components */
+       while (remaining.len > 0) {
+               get_next_component(&next, &remaining);
+
+               if (next.len == 0) {
+                       continue; /* empty component */
+               } else if (next.len == 1 && !strcmp(next.buf, ".")) {
+                       continue; /* '.' component */
+               } else if (next.len == 2 && !strcmp(next.buf, "..")) {
+                       /* '..' component; strip the last path component */
+                       strip_last_component(&resolved);
+                       continue;
+               }
+
+               /* append the next component and resolve resultant path */
+               if (!is_dir_sep(resolved.buf[resolved.len - 1]))
+                       strbuf_addch(&resolved, '/');
+               strbuf_addbuf(&resolved, &next);
+
+               if (lstat(resolved.buf, &st)) {
+                       /* error out unless this was the last component */
+                       if (errno != ENOENT || remaining.len) {

                                if (die_on_error)
                                if (die_on_error)

-                                       die_errno("Could not get current working directory");
+                                       die_errno("Invalid path '%s'",
+                                                 resolved.buf);

                                else
                                        goto error_out;
                        }
                                else
                                        goto error_out;
                        }

+               } else if (S_ISLNK(st.st_mode)) {
+                       ssize_t len;
+                       strbuf_reset(&symlink);

 
 

-                       if (chdir(sb.buf)) {
+                       if (num_symlinks++ > MAXSYMLINKS) {

                                if (die_on_error)
                                if (die_on_error)

-                                       die_errno("Could not switch to '%s'",
-                                                 sb.buf);
+                                       die("More than %d nested symlinks "
+                                           "on path '%s'", MAXSYMLINKS, path);

                                else
                                        goto error_out;
                        }
                                else
                                        goto error_out;
                        }

-               }
-               if (strbuf_getcwd(&sb)) {
-                       if (die_on_error)
-                               die_errno("Could not get current working directory");
-                       else
-                               goto error_out;
-               }
-
-               if (last_elem) {
-                       if (sb.len && !is_dir_sep(sb.buf[sb.len - 1]))
-                               strbuf_addch(&sb, '/');
-                       strbuf_addstr(&sb, last_elem);
-                       free(last_elem);
-                       last_elem = NULL;
-               }

 
 

-               if (!lstat(sb.buf, &st) && S_ISLNK(st.st_mode)) {
-                       struct strbuf next_sb = STRBUF_INIT;
-                       ssize_t len = strbuf_readlink(&next_sb, sb.buf, 0);
+                       len = strbuf_readlink(&symlink, resolved.buf,
+                                             st.st_size);

                        if (len < 0) {
                                if (die_on_error)
                                        die_errno("Invalid symlink '%s'",
                        if (len < 0) {
                                if (die_on_error)
                                        die_errno("Invalid symlink '%s'",

-                                                 sb.buf);
+                                                 resolved.buf);

                                else
                                        goto error_out;
                        }
                                else
                                        goto error_out;
                        }

-                       strbuf_swap(&sb, &next_sb);
-                       strbuf_release(&next_sb);
-               } else
-                       break;
+
+                       if (is_absolute_path(symlink.buf)) {
+                               /* absolute symlink; set resolved to root */
+                               int offset = offset_1st_component(symlink.buf);
+                               strbuf_reset(&resolved);
+                               strbuf_add(&resolved, symlink.buf, offset);
+                               strbuf_remove(&symlink, 0, offset);
+                       } else {
+                               /*
+                                * relative symlink
+                                * strip off the last component since it will
+                                * be replaced with the contents of the symlink
+                                */
+                               strip_last_component(&resolved);
+                       }
+
+                       /*
+                        * if there are still remaining components to resolve
+                        * then append them to symlink
+                        */
+                       if (remaining.len) {
+                               strbuf_addch(&symlink, '/');
+                               strbuf_addbuf(&symlink, &remaining);
+                       }
+
+                       /*
+                        * use the symlink as the remaining components that
+                        * need to be resloved
+                        */
+                       strbuf_swap(&symlink, &remaining);
+               }

        }
 
        }
 

-       retval = sb.buf;
+       retval = resolved.buf;
+

 error_out:
 error_out:

-       free(last_elem);
-       if (cwd.len && chdir(cwd.buf))
-               die_errno("Could not change back to '%s'", cwd.buf);
-       strbuf_release(&cwd);
+       strbuf_release(&remaining);
+       strbuf_release(&next);
+       strbuf_release(&symlink);

 
        return retval;
 }
 
        return retval;
 }