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Diffstat (limited to 'src/force-link.c')
| -rw-r--r-- | src/force-link.c | 187 |
1 files changed, 187 insertions, 0 deletions
diff --git a/src/force-link.c b/src/force-link.c new file mode 100644 index 000000000..e0db07574 --- /dev/null +++ b/src/force-link.c @@ -0,0 +1,187 @@ +/* Implement ln -f "atomically" + + Copyright 2017 Free Software Foundation, Inc. + + This program is free software: you can redistribute it and/or modify + it under the terms of the GNU General Public License as published by + the Free Software Foundation, either version 3 of the License, or + (at your option) any later version. + + This program is distributed in the hope that it will be useful, + but WITHOUT ANY WARRANTY; without even the implied warranty of + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the + GNU General Public License for more details. + + You should have received a copy of the GNU General Public License + along with this program. If not, see <http://www.gnu.org/licenses/>. */ + +/* Written by Paul Eggert. */ + +/* A naive "ln -f A B" unlinks B and then links A to B. This module + instead links A to a randomly-named temporary T in B's directory, + and then renames T to B. This approach has a window with a + randomly-named temporary, which is safer for many applications than + a window where B does not exist. */ + +#include <config.h> + +#include "force-link.h" + +#include <dirname.h> +#include <tempname.h> + +#include <errno.h> +#include <stdlib.h> +#include <string.h> +#include <unistd.h> + +/* A basename pattern suitable for a temporary file. It should work + even on file systems like FAT that support only short names. + "Cu" is short for "Coreutils" or for "Changeable unstable", + take your pick.... */ + +static char const simple_pattern[] = "CuXXXXXX"; +enum { x_suffix_len = sizeof "XXXXXX" - 1 }; + +/* A size for smallish buffers containing file names. Longer file + names can use malloc. */ + +enum { smallsize = 256 }; + +/* Return a template for a file in the same directory as DSTNAME. + Use BUF if the template fits, otherwise use malloc and return NULL + (setting errno) if unsuccessful. */ + +static char * +samedir_template (char const *dstname, char buf[smallsize]) +{ + ptrdiff_t dstdirlen = last_component (dstname) - dstname; + size_t dsttmpsize = dstdirlen + sizeof simple_pattern; + char *dsttmp; + if (dsttmpsize <= smallsize) + dsttmp = buf; + else + { + dsttmp = malloc (dsttmpsize); + if (!dsttmp) + return dsttmp; + } + strcpy (mempcpy (dsttmp, dstname, dstdirlen), simple_pattern); + return dsttmp; +} + + +/* Auxiliaries for force_linkat. */ + +struct link_arg +{ + int srcdir; + char const *srcname; + int dstdir; + int flags; +}; + +static int +try_link (char *dest, void *arg) +{ + struct link_arg *a = arg; + return linkat (a->srcdir, a->srcname, a->dstdir, dest, a->flags); +} + +/* Hard-link directory SRCDIR's file SRCNAME to directory DSTDIR's + file DSTNAME, using linkat-style FLAGS to control the linking. + If FORCE and DSTNAME already exists, replace it atomically. Return + 1 if successful and DSTNAME already existed, + 0 if successful and DSTNAME did not already exist, and + -1 (setting errno) on failure. */ +int +force_linkat (int srcdir, char const *srcname, + int dstdir, char const *dstname, int flags, bool force) +{ + int r = linkat (srcdir, srcname, dstdir, dstname, flags); + if (!force || r == 0 || errno != EEXIST) + return r; + + char buf[smallsize]; + char *dsttmp = samedir_template (dstname, buf); + if (! dsttmp) + return -1; + struct link_arg arg = { srcdir, srcname, dstdir, flags }; + int err; + + if (try_tempname_len (dsttmp, 0, &arg, try_link, x_suffix_len) != 0) + err = errno; + else + { + err = renameat (dstdir, dsttmp, dstdir, dstname) == 0 ? 0 : errno; + /* Unlink DSTTMP even if renameat succeeded, in case DSTTMP + and DSTNAME were already the same hard link and renameat + was a no-op. */ + unlinkat (dstdir, dsttmp, 0); + } + + if (dsttmp != buf) + free (dsttmp); + if (!err) + return 1; + errno = err; + return -1; +} + + +/* Auxiliaries for force_symlinkat. */ + +struct symlink_arg +{ + char const *srcname; + int dstdir; +}; + +static int +try_symlink (char *dest, void *arg) +{ + struct symlink_arg *a = arg; + return symlinkat (a->srcname, a->dstdir, dest); +} + +/* Create a symlink containing SRCNAME in directory DSTDIR's file DSTNAME. + If FORCE and DSTNAME already exists, replace it atomically. Return + 1 if successful and DSTNAME already existed, + 0 if successful and DSTNAME did not already exist, and + -1 (setting errno) on failure. */ +int +force_symlinkat (char const *srcname, int dstdir, char const *dstname, + bool force) +{ + int r = symlinkat (srcname, dstdir, dstname); + if (!force || r == 0 || errno != EEXIST) + return r; + + char buf[smallsize]; + char *dsttmp = samedir_template (dstname, buf); + if (!dsttmp) + return -1; + struct symlink_arg arg = { srcname, dstdir }; + int err; + + if (try_tempname_len (dsttmp, 0, &arg, try_symlink, x_suffix_len) != 0) + err = errno; + else if (renameat (dstdir, dsttmp, dstdir, dstname) != 0) + { + err = errno; + unlinkat (dstdir, dsttmp, 0); + } + else + { + /* Don't worry about renameat being a no-op, since DSTTMP is + newly created. */ + err = 0; + } + + if (dsttmp != buf) + free (dsttmp); + if (!err) + return 1; + errno = err; + return -1; +} |