[PATCH v2 15/18] mingw: add support for symlinks to directories
- From
Karsten Blees via GitGitGadget <gitgitgadget@gmail.com>
- Date
- Jan 9, 2026, 20:05 UTC
- Message-ID
- <3d479fd47e68242b028e1bbfd0019dfb0ededac8.1767989115.git.gitgitgadget@gmail.com>
- In-Reply-To
- <pull.2018.v2.git.1767989115.gitgitgadget@gmail.com>
From: Karsten Blees <karsten.blees@gmail.com>
Symlinks on Windows have a flag that indicates whether the target is a file or a directory. Symlinks of wrong type simply don't work. This even affects core Win32 APIs (e.g. `DeleteFile()` refuses to delete directory symlinks).
However, `CreateFile()` with FILE_FLAG_BACKUP_SEMANTICS does work. Check the target type by first creating a tentative file symlink, opening it, and checking the type of the resulting handle. If it is a directory, recreate the symlink with the directory flag set.
It is possible to create symlinks before the target exists (or in case of symlinks to symlinks: before the target type is known). If this happens, create a tentative file symlink and postpone the directory decision: keep a list of phantom symlinks to be processed whenever a new directory is created in `mingw_mkdir()`.
Limitations: This algorithm may fail if a link target changes from file to directory or vice versa, or if the target directory is created in another process. It's the best Git can do, though.
Signed-off-by: Karsten Blees <karsten.blees@gmail.com> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> --- compat/mingw.c | 164 +++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 164 insertions(+)
diff --git a/compat/mingw.c b/compat/mingw.c index 8d366794c4..59a32e454e 100644 --- a/compat/mingw.c +++ b/compat/mingw.c @@ -296,6 +296,131 @@ int mingw_core_config(const char *var, const char *value, return 0; } +static inline int is_wdir_sep(wchar_t wchar) +{ + return wchar == L'/' || wchar == L'\\'; +} + +static const wchar_t *make_relative_to(const wchar_t *path, + const wchar_t *relative_to, wchar_t *out, + size_t size) +{ + size_t i = wcslen(relative_to), len; + + /* Is `path` already absolute? */ + if (is_wdir_sep(path[0]) || + (iswalpha(path[0]) && path[1] == L':' && is_wdir_sep(path[2]))) + return path; + + while (i > 0 && !is_wdir_sep(relative_to[i - 1])) + i--; + + /* Is `relative_to` in the current directory? */ + if (!i) + return path; + + len = wcslen(path); + if (i + len + 1 > size) { + error("Could not make '%ls' relative to '%ls' (too large)", + path, relative_to); + return NULL; + } + + memcpy(out, relative_to, i * sizeof(wchar_t)); + wcscpy(out + i, path); + return out; +} + +enum phantom_symlink_result { + PHANTOM_SYMLINK_RETRY, + PHANTOM_SYMLINK_DONE, + PHANTOM_SYMLINK_DIRECTORY +}; + +/* + * Changes a file symlink to a directory symlink if the target exists and is a + * directory. + */ +static enum phantom_symlink_result +process_phantom_symlink(const wchar_t *wtarget, const wchar_t *wlink) +{ + HANDLE hnd; + BY_HANDLE_FILE_INFORMATION fdata; + wchar_t relative[MAX_PATH]; + const wchar_t *rel; + + /* check that wlink is still a file symlink */ + if ((GetFileAttributesW(wlink) + & (FILE_ATTRIBUTE_REPARSE_POINT | FILE_ATTRIBUTE_DIRECTORY)) + != FILE_ATTRIBUTE_REPARSE_POINT) + return PHANTOM_SYMLINK_DONE; + + /* make it relative, if necessary */ + rel = make_relative_to(wtarget, wlink, relative, ARRAY_SIZE(relative)); + if (!rel) + return PHANTOM_SYMLINK_DONE; + + /* let Windows resolve the link by opening it */ + hnd = CreateFileW(rel, 0, + FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE, NULL, + OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS, NULL); + if (hnd == INVALID_HANDLE_VALUE) { + errno = err_win_to_posix(GetLastError()); + return PHANTOM_SYMLINK_RETRY; + } + + if (!GetFileInformationByHandle(hnd, &fdata)) { + errno = err_win_to_posix(GetLastError()); + CloseHandle(hnd); + return PHANTOM_SYMLINK_RETRY; + } + CloseHandle(hnd); + + /* if target exists and is a file, we're done */ + if (!(fdata.dwFileAttributes & FILE_ATTRIBUTE_DIRECTORY)) + return PHANTOM_SYMLINK_DONE; + + /* otherwise recreate the symlink with directory flag */ + if (DeleteFileW(wlink) && CreateSymbolicLinkW(wlink, wtarget, 1)) + return PHANTOM_SYMLINK_DIRECTORY; + + errno = err_win_to_posix(GetLastError()); + return PHANTOM_SYMLINK_RETRY; +} + +/* keep track of newly created symlinks to non-existing targets */ +struct phantom_symlink_info { + struct phantom_symlink_info *next; + wchar_t *wlink; + wchar_t *wtarget; +}; + +static struct phantom_symlink_info *phantom_symlinks = NULL; +static CRITICAL_SECTION phantom_symlinks_cs; + +static void process_phantom_symlinks(void) +{ + struct phantom_symlink_info *current, **psi; + EnterCriticalSection(&phantom_symlinks_cs); + /* process phantom symlinks list */ + psi = &phantom_symlinks; + while ((current = *psi)) { + enum phantom_symlink_result result = process_phantom_symlink( + current->wtarget, current->wlink); + if (result == PHANTOM_SYMLINK_RETRY) { + psi = ¤t->next; + } else { + /* symlink was processed, remove from list */ + *psi = current->next; + free(current); + /* if symlink was a directory, start over */ + if (result == PHANTOM_SYMLINK_DIRECTORY) + psi = &phantom_symlinks; + } + } + LeaveCriticalSection(&phantom_symlinks_cs); +} + /* Normalizes NT paths as returned by some low-level APIs. */ static wchar_t *normalize_ntpath(wchar_t *wbuf) { @@ -479,6 +604,8 @@ int mingw_mkdir(const char *path, int mode UNUSED) if (xutftowcs_path(wpath, path) < 0) return -1; ret = _wmkdir(wpath); + if (!ret) + process_phantom_symlinks(); if (!ret && needs_hiding(path)) return set_hidden_flag(wpath, 1); return ret; @@ -2723,6 +2850,42 @@ int symlink(const char *target, const char *link) errno = err_win_to_posix(GetLastError()); return -1; } + + /* convert to directory symlink if target exists */ + switch (process_phantom_symlink(wtarget, wlink)) { + case PHANTOM_SYMLINK_RETRY: { + /* if target doesn't exist, add to phantom symlinks list */ + wchar_t wfullpath[MAX_PATH]; + struct phantom_symlink_info *psi; + + /* convert to absolute path to be independent of cwd */ + len = GetFullPathNameW(wlink, MAX_PATH, wfullpath, NULL); + if (!len || len >= MAX_PATH) { + errno = err_win_to_posix(GetLastError()); + return -1; + } + + /* over-allocate and fill phantom_symlink_info structure */ + psi = xmalloc(sizeof(struct phantom_symlink_info) + + sizeof(wchar_t) * (len + wcslen(wtarget) + 2)); + psi->wlink = (wchar_t *)(psi + 1); + wcscpy(psi->wlink, wfullpath); + psi->wtarget = psi->wlink + len + 1; + wcscpy(psi->wtarget, wtarget); + + EnterCriticalSection(&phantom_symlinks_cs); + psi->next = phantom_symlinks; + phantom_symlinks = psi; + LeaveCriticalSection(&phantom_symlinks_cs); + break; + } + case PHANTOM_SYMLINK_DIRECTORY: + /* if we created a dir symlink, process other phantom symlinks */ + process_phantom_symlinks(); + break; + default: + break; + } return 0; } @@ -3424,6 +3587,7 @@ int wmain(int argc, const wchar_t **wargv) /* initialize critical section for waitpid pinfo_t list */ InitializeCriticalSection(&pinfo_cs); + InitializeCriticalSection(&phantom_symlinks_cs); /* set up default file mode and file modes for stdin/out/err */ _fmode = _O_BINARY;
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