On Windows platforms, we need some logic to emulate symlink(3P) defined
by POSIX. As unprivileged symlinks on Windows are a rather new feature,
our current implementation is comparatively new and still has some
rough edges in special cases.

One such case is relative symlinks. While relative symlinks to files in
the same directory work as expected, libgit2 currently fails to create
reltaive symlinks pointing into other directories. This is due to the
fact that we forgot to translate the Unix-style target path to
Windows-style. Most importantly, we are currently not converting
directory separators from "/" to "\".

Fix the issue by calling `git_win32_path_canonicalize` on the target.
Add a test that verifies our ability to create such relative links
across directories.

netops: handle intact query parameters in service_suffix removal

Some servers leave the query parameters intact in the
Location header when responding with a redirect.
The service_suffix removal check as written assumed
that the server removed them.
Handle both cases.

Along with PR #5325, this fixes #5321.

There are two new tests. The first already passed;
the second previously failed.

Refactor packfile code to use zstream abstraction

While we do have a `git_zstream` abstraction that encapsulates all the
calls to zlib as well as its error handling, we do not use it in our
pack file code. Refactor it to make the code a lot easier to understand.

While we do have a zstream abstraction that encapsulates all the calls
to zlib as well as its error handling, we do not use it in our pack file
code. Refactor it to make the code a lot easier to understand.

Fix git_submodule_sync with relative url

When setting up relative URLs for a submodule, then we resolve it to
the actual location and write that into ".git/config" instead of
writing the relative value. We do not yet have a test to nail down this
behaviour, which is now being added by this commit.

The submodule code has grown out-of-date regarding its coding style.
Update `git_submodule_reload` and `git_submodule_sync` to more closely
resemble what the rest of our code base uses.

git_submodule_sync should resolve submodule before writing to .git/config
to have the same behavior as git_submodule_init, which does the right thing.

http: avoid generating double slashes in url

Correct typo in name of referenced parameter

Signed-off-by: Remy Suen <remy.suen@gmail.com>

Prior to this change, given a remote url with a trailing slash,
such as http://localhost/a/, service requests would contain a
double slash: http://localhost/a//info/refs?service=git-receive-pack.
Detect and prevent that.

Updates #5321

patch_parse: fix undefined behaviour due to arithmetic on NULL pointers

smart_pkt: fix overflow resulting in OOB read/write of one byte

branch: clarify documentation around branches

Doing arithmetic with NULL pointers is undefined behaviour in the C
standard. We do so regardless when parsing patches, as we happily add a
potential prefix length to prefixed paths. While this works out just
fine as the prefix length is always equal to zero in these cases, thus
resulting in another NULL pointer, it still is undefined behaviour and
was pointed out to us by OSSfuzz.

Fix the issue by checking whether paths are NULL, avoiding the
arithmetic if they are.

When parsing OK packets, we copy any information after the initial "ok "
prefix into the resulting packet. As newlines act as packet boundaries,
we also strip the trailing newline if there is any. We do not check
whether there is any data left after the initial "ok " prefix though,
which leads to a pointer overflow in that case as `len == 0`:

	if (line[len - 1] == '\n')
		--len;

This out-of-bounds read is a rather useless gadget, as we can only
deduce whether at some offset there is a newline character. In case
there accidentally is one, we overflow `len` to `SIZE_MAX` and then
write a NUL byte into an array indexed by it:

	pkt->ref[len] = '\0';

Again, this doesn't seem like something that's possible to be exploited
in any meaningful way, but it may surely lead to inconsistencies or DoS.

Fix the issue by checking whether there is any trailing data after the
packet prefix.

As git_reference__name will reallocate storage to account for longer
names (it's actually allocator-dependent), it will cause all existing
pointers to the old object to become dangling, as they now point to
freed memory.

Fix the issue by renaming to a more descriptive name, and pass a pointer
to the actual reference that can safely be invalidated if the realloc
succeeds.

examples: checkout: implement guess heuristic for remote branches

Minor doc improvements

attr: Update definition of binary macro

Security fixes for master

Windows/DOS only supports drive letters that are alpha characters A-Z.
However, you can `subst` any one-character as a drive letter, including
numbers or even emoji.  Test that we can identify emoji as drive
letters.

Users may want to turn off core.protectNTFS, perhaps to import (and then
repair) a broken tree.  Ensure that core.protectNTFS=false is honored.

Enable core.protectNTFS by default everywhere and in every codepath, not
just on checkout.

Test that when we enable core.protectNTFS that we cannot add
platform-specific invalid paths to the index.

The name of the `add_invalid_filename` function suggests that we
_want_ to add an invalid filename.  Rename the function to show that
we expect to _fail_ to add the invalid filename.

Ensure that the new protection around .git::$INDEX_ALLOCATION rules are
enabled for using the treebuilder when core.protectNTFS is set.

Ensure that the new protection around .git::$INDEX_ALLOCATION rules are
enabled for adding to the index when core.protectNTFS is set.

The name of the `write_invalid_filename` function suggests that we
_want_ to write an invalid filename.  Rename the function to show that
we expect to _fail_ to write the invalid filename.

The function `only_spaces_and_dots` used to detect the end of the
filename on win32.  Now we look at spaces and dots _before_ the end of
the string _or_ a `:` character, which would signify a win32 alternate
data stream.

Thus, rename the function `ntfs_end_of_filename` to indicate that it
detects the (virtual) end of a filename, that any further characters
would be elided to the given path.

We just safe-guarded `.git` against NTFS Alternate Data Stream-related
attack vectors, and now it is time to do the same for `.gitmodules`.

Note: In the added regression test, we refrain from verifying all kinds
of variations between short names and NTFS Alternate Data Streams: as
the new code disallows _all_ Alternate Data Streams of `.gitmodules`, it
is enough to test one in order to know that all of them are guarded
against.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

A little-known feature of NTFS is that it offers to store metadata in
so-called "Alternate Data Streams" (inspired by Apple's "resource
forks") that are copied together with the file they are associated with.
These Alternate Data Streams can be accessed via `<file name>:<stream
name>:<stream type>`.

Directories, too, have Alternate Data Streams, and they even have a
default stream type `$INDEX_ALLOCATION`. Which means that `abc/` and
`abc::$INDEX_ALLOCATION/` are actually equivalent.

This is of course another attack vector on the Git directory that we
definitely want to prevent.

On Windows, we already do this incidentally, by disallowing colons in
file/directory names.

While it looks as if files'/directories' Alternate Data Streams are not
accessible in the Windows Subsystem for Linux, and neither via
CIFS/SMB-mounted network shares in Linux, it _is_ possible to access
them on SMB-mounted network shares on macOS.

Therefore, let's go the extra mile and prevent this particular attack
_everywhere_. To keep things simple, let's just disallow *any* Alternate
Data Stream of `.git`.

This is libgit2's variant of CVE-2019-1352.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>