cygwin: Use native Win32 API for stat

lstat/stat functions in Cygwin are very slow, because they try to emulate
some *nix things that Git does not actually need. This patch adds Win32
specific implementation of these functions for Cygwin.

This implementation handles most situation directly but in some rare cases
it falls back on the implementation provided for Cygwin. This is necessary
for two reasons:

- Cygwin has its own file hierarchy, so absolute paths used in Cygwin is
  not suitable to be used Win32 API. cygwin_conv_to_win32_path can not be
  used because it automatically dereference Cygwin symbol links, also it
  causes extra syscall. Fortunately Git rarely use absolute paths, so we
  always use Cygwin implementation for absolute paths.

- Support of symbol links. Cygwin stores symbol links as ordinary using
  one of two possible formats. Therefore, the fast implementation falls
  back to Cygwin functions if it detects potential use of symbol links.

The speed of this implementation should be the same as mingw_lstat for
common cases, but it is considerable slower when the specified file name
does not exist.

Despite all efforts to make the fast implementation as robust as possible,
it may not work well for some very rare situations. I am aware only one
situation: use Cygwin mount to bind unrelated paths inside repository
together.  Therefore, the core.ignoreCygwinFSTricks configuration option is
provided, which controls whether native or Cygwin version of stat is used.

Signed-off-by: Dmitry Potapov <dpotapov@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

2 files changed, 136 insertions, 0 deletions

diff --git a/compat/cygwin.c b/compat/cygwin.c
new file mode 100644
index 000000000..423ff20b0
--- /dev/null
+++ b/compat/cygwin.c
@@ -0,0 +1,127 @@
+#define WIN32_LEAN_AND_MEAN
+#include "../git-compat-util.h"
+#include "win32.h"
+#include "../cache.h" /* to read configuration */
+
+static inline void filetime_to_timespec(const FILETIME *ft, struct timespec *ts)
+{
+	long long winTime = ((long long)ft->dwHighDateTime << 32) +
+			ft->dwLowDateTime;
+	winTime -= 116444736000000000LL; /* Windows to Unix Epoch conversion */
+	/* convert 100-nsecond interval to seconds and nanoseconds */
+	ts->tv_sec = (time_t)(winTime/10000000);
+	ts->tv_nsec = (long)(winTime - ts->tv_sec*10000000LL) * 100;
+}
+
+#define size_to_blocks(s) (((s)+511)/512)
+
+/* do_stat is a common implementation for cygwin_lstat and cygwin_stat.
+ *
+ * To simplify its logic, in the case of cygwin symlinks, this implementation
+ * falls back to the cygwin version of stat/lstat, which is provided as the
+ * last argument.
+ */
+static int do_stat(const char *file_name, struct stat *buf, stat_fn_t cygstat)
+{
+	WIN32_FILE_ATTRIBUTE_DATA fdata;
+
+	if (file_name[0] == '/')
+		return cygstat (file_name, buf);
+
+	if (!(errno = get_file_attr(file_name, &fdata))) {
+		/*
+		 * If the system attribute is set and it is not a directory then
+		 * it could be a symbol link created in the nowinsymlinks mode.
+		 * Normally, Cygwin works in the winsymlinks mode, so this situation
+		 * is very unlikely. For the sake of simplicity of our code, let's
+		 * Cygwin to handle it.
+		 */
+		if ((fdata.dwFileAttributes & FILE_ATTRIBUTE_SYSTEM) &&
+		    !(fdata.dwFileAttributes & FILE_ATTRIBUTE_DIRECTORY))
+			return cygstat(file_name, buf);
+
+		/* fill out the stat structure */
+		buf->st_dev = buf->st_rdev = 0; /* not used by Git */
+		buf->st_ino = 0;
+		buf->st_mode = file_attr_to_st_mode(fdata.dwFileAttributes);
+		buf->st_nlink = 1;
+		buf->st_uid = buf->st_gid = 0;
+#ifdef __CYGWIN_USE_BIG_TYPES__
+		buf->st_size = ((_off64_t)fdata.nFileSizeHigh << 32) +
+			fdata.nFileSizeLow;
+#else
+		buf->st_size = (off_t)fdata.nFileSizeLow;
+#endif
+		buf->st_blocks = size_to_blocks(buf->st_size);
+		filetime_to_timespec(&fdata.ftLastAccessTime, &buf->st_atim);
+		filetime_to_timespec(&fdata.ftLastWriteTime, &buf->st_mtim);
+		filetime_to_timespec(&fdata.ftCreationTime, &buf->st_ctim);
+		return 0;
+	} else if (errno == ENOENT) {
+		/*
+		 * In the winsymlinks mode (which is the default), Cygwin
+		 * emulates symbol links using Windows shortcut files. These
+		 * files are formed by adding .lnk extension. So, if we have
+		 * not found the specified file name, it could be that it is
+		 * a symbol link. Let's Cygwin to deal with that.
+		 */
+		return cygstat(file_name, buf);
+	}
+	return -1;
+}
+
+/* We provide our own lstat/stat functions, since the provided Cygwin versions
+ * of these functions are too slow. These stat functions are tailored for Git's
+ * usage, and therefore they are not meant to be complete and correct emulation
+ * of lstat/stat functionality.
+ */
+static int cygwin_lstat(const char *path, struct stat *buf)
+{
+	return do_stat(path, buf, lstat);
+}
+
+static int cygwin_stat(const char *path, struct stat *buf)
+{
+	return do_stat(path, buf, stat);
+}
+
+
+/*
+ * At start up, we are trying to determine whether Win32 API or cygwin stat
+ * functions should be used. The choice is determined by core.ignorecygwinfstricks.
+ * Reading this option is not always possible immediately as git_dir may be
+ * not be set yet. So until it is set, use cygwin lstat/stat functions.
+ */
+static int native_stat = 1;
+
+static int git_cygwin_config(const char *var, const char *value, void *cb)
+{
+	if (!strcmp(var, "core.ignorecygwinfstricks"))
+		native_stat = git_config_bool(var, value);
+	return 0;
+}
+
+static int init_stat(void)
+{
+	if (have_git_dir()) {
+		git_config(git_cygwin_config, NULL);
+		cygwin_stat_fn = native_stat ? cygwin_stat : stat;
+		cygwin_lstat_fn = native_stat ? cygwin_lstat : lstat;
+		return 0;
+	}
+	return 1;
+}
+
+static int cygwin_stat_stub(const char *file_name, struct stat *buf)
+{
+	return (init_stat() ? stat : *cygwin_stat_fn)(file_name, buf);
+}
+
+static int cygwin_lstat_stub(const char *file_name, struct stat *buf)
+{
+	return (init_stat() ? lstat : *cygwin_lstat_fn)(file_name, buf);
+}
+
+stat_fn_t cygwin_stat_fn = cygwin_stat_stub;
+stat_fn_t cygwin_lstat_fn = cygwin_lstat_stub;
+
diff --git a/compat/cygwin.h b/compat/cygwin.h
new file mode 100644
index 000000000..a3229f5b4
--- /dev/null
+++ b/compat/cygwin.h
@@ -0,0 +1,9 @@
+#include <sys/types.h>
+#include <sys/stat.h>
+
+typedef int (*stat_fn_t)(const char*, struct stat*);
+extern stat_fn_t cygwin_stat_fn;
+extern stat_fn_t cygwin_lstat_fn;
+
+#define stat(path, buf) (*cygwin_stat_fn)(path, buf)
+#define lstat(path, buf) (*cygwin_lstat_fn)(path, buf)