Win32: implement readlink()

Implement readlink() by reading NTFS reparse points. Works for symlinks
and directory junctions. If symlinks are disabled, fail with ENOSYS.

Signed-off-by: Karsten Blees <blees@dcon.de>

compat/mingw.c

@@ -2,6 +2,7 @@
 #include "win32.h"
 #include <conio.h>
 #include <wchar.h>
+#include <winioctl.h>
 #include "../strbuf.h"
 #include "../run-command.h"
 #include "../cache.h"
@@ -2003,6 +2004,106 @@ int link(const char *oldpath, const char *newpath)
 	return 0;
 }
 
+#ifndef _WINNT_H
+/*
+ * The REPARSE_DATA_BUFFER structure is defined in the Windows DDK (in
+ * ntifs.h) and in MSYS1's winnt.h (which defines _WINNT_H). So define
+ * it ourselves if we are on MSYS2 (whose winnt.h defines _WINNT_).
+ */
+typedef struct _REPARSE_DATA_BUFFER {
+	DWORD  ReparseTag;
+	WORD   ReparseDataLength;
+	WORD   Reserved;
+	_ANONYMOUS_UNION union {
+		struct {
+			WORD   SubstituteNameOffset;
+			WORD   SubstituteNameLength;
+			WORD   PrintNameOffset;
+			WORD   PrintNameLength;
+			ULONG  Flags;
+			WCHAR PathBuffer[1];
+		} SymbolicLinkReparseBuffer;
+		struct {
+			WORD   SubstituteNameOffset;
+			WORD   SubstituteNameLength;
+			WORD   PrintNameOffset;
+			WORD   PrintNameLength;
+			WCHAR PathBuffer[1];
+		} MountPointReparseBuffer;
+		struct {
+			BYTE   DataBuffer[1];
+		} GenericReparseBuffer;
+	} DUMMYUNIONNAME;
+} REPARSE_DATA_BUFFER, *PREPARSE_DATA_BUFFER;
+#endif
+
+int readlink(const char *path, char *buf, size_t bufsiz)
+{
+	HANDLE handle;
+	WCHAR wpath[MAX_LONG_PATH], *wbuf;
+	REPARSE_DATA_BUFFER *b = alloca(MAXIMUM_REPARSE_DATA_BUFFER_SIZE);
+	DWORD dummy;
+	char tmpbuf[MAX_LONG_PATH];
+	int len;
+
+	/* fail if symlinks are disabled */
+	if (!has_symlinks) {
+		errno = ENOSYS;
+		return -1;
+	}
+
+	if (xutftowcs_long_path(wpath, path) < 0)
+		return -1;
+
+	/* read reparse point data */
+	handle = CreateFileW(wpath, 0,
+			FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE, NULL,
+			OPEN_EXISTING,
+			FILE_FLAG_BACKUP_SEMANTICS | FILE_FLAG_OPEN_REPARSE_POINT, NULL);
+	if (handle == INVALID_HANDLE_VALUE) {
+		errno = err_win_to_posix(GetLastError());
+		return -1;
+	}
+	if (!DeviceIoControl(handle, FSCTL_GET_REPARSE_POINT, NULL, 0, b,
+			MAXIMUM_REPARSE_DATA_BUFFER_SIZE, &dummy, NULL)) {
+		errno = err_win_to_posix(GetLastError());
+		CloseHandle(handle);
+		return -1;
+	}
+	CloseHandle(handle);
+
+	/* get target path for symlinks or mount points (aka 'junctions') */
+	switch (b->ReparseTag) {
+	case IO_REPARSE_TAG_SYMLINK:
+		wbuf = (WCHAR*) (((char*) b->SymbolicLinkReparseBuffer.PathBuffer)
+				+ b->SymbolicLinkReparseBuffer.SubstituteNameOffset);
+		*(WCHAR*) (((char*) wbuf)
+				+ b->SymbolicLinkReparseBuffer.SubstituteNameLength) = 0;
+		break;
+	case IO_REPARSE_TAG_MOUNT_POINT:
+		wbuf = (WCHAR*) (((char*) b->MountPointReparseBuffer.PathBuffer)
+				+ b->MountPointReparseBuffer.SubstituteNameOffset);
+		*(WCHAR*) (((char*) wbuf)
+				+ b->MountPointReparseBuffer.SubstituteNameLength) = 0;
+		break;
+	default:
+		errno = EINVAL;
+		return -1;
+	}
+
+	/*
+	 * Adapt to strange readlink() API: Copy up to bufsiz *bytes*, potentially
+	 * cutting off a UTF-8 sequence. Insufficient bufsize is *not* a failure
+	 * condition. There is no conversion function that produces invalid UTF-8,
+	 * so convert to a (hopefully large enough) temporary buffer, then memcpy
+	 * the requested number of bytes (including '\0' for robustness).
+	 */
+	if ((len = xwcstoutf(tmpbuf, normalize_ntpath(wbuf), MAX_LONG_PATH)) < 0)
+		return -1;
+	memcpy(buf, tmpbuf, min(bufsiz, len + 1));
+	return min(bufsiz, len);
+}
+
 pid_t waitpid(pid_t pid, int *status, int options)
 {
 	HANDLE h = OpenProcess(SYNCHRONIZE | PROCESS_QUERY_INFORMATION,

compat/mingw.h

@@ -93,8 +93,6 @@ struct itimerval {
  * trivial stubs
  */
 
-static inline int readlink(const char *path, char *buf, size_t bufsiz)
-{ errno = ENOSYS; return -1; }
 static inline int symlink(const char *oldpath, const char *newpath)
 { errno = ENOSYS; return -1; }
 static inline int fchmod(int fildes, mode_t mode)
@@ -182,6 +180,7 @@ struct passwd *getpwuid(uid_t uid);
 int setitimer(int type, struct itimerval *in, struct itimerval *out);
 int sigaction(int sig, struct sigaction *in, struct sigaction *out);
 int link(const char *oldpath, const char *newpath);
+int readlink(const char *path, char *buf, size_t bufsiz);
 
 /*
  * replacements of existing functions