The Reader and Writer have hard-coded constants regarding the
offsets and lengths of certain fields in the tar format sprinkled
all over. This makes it harder to verify that the offsets are
correct since a reviewer would need to search for them throughout
the code. Instead, all information about the layout of header
fields should be centralized in one single file. This has the
advantage of being both centralized, and also acting as a form
of documentation about the header struct format.

This method was chosen over using "encoding/binary" since that
method would cause an allocation of a header struct every time
binary.Read was called. This method causes zero allocations and
its logic is no longer than if structs were declared.

Updates #12594

Change-Id: Ic7a0565d2a2cd95d955547ace3b6dea2b57fab34
Reviewed-on: https://go-review.googlesource.com/14669
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

The GNU format does not have a prefix field, so we should make
no attempt to read it. It does however have atime and ctime fields.
Since Go previously placed incorrect values here, we liberally
read the atime and ctime fields and ignore errors so that old tar
files written by Go can at least be partially read.

This fixes half of #12594. The Writer is much harder to fix.

Updates #12594

Change-Id: Ia32845e2f262ee53366cf41dfa935f4d770c7a30
Reviewed-on: https://go-review.googlesource.com/31444
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

The proper fix for the Writer is too involved to be done in time
for Go 1.8. Instead, we do a localized fix that simply disables the
prefix encoding logic. While this will prevent some legitimate uses
of prefix, it will ensure that we don't keep outputting invalid
GNU format files that have the prefix field populated.

For headers with long filenames that could have used the prefix field,
they will be promoted to use the PAX format, which ensures that we
will still be able to encode all headers that we were able to do before.

Updates #12594
Fixes #17630
Fixes #9683

Change-Id: Ia97b524ac69865390e2ae8bb0dfb664d40a05add
Reviewed-on: https://go-review.googlesource.com/32234
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

The current logic in writeHeader attempts to encode the Header in one
format and if it discovered that it could not it would attempt to
switch to a different format mid-way through. This makes it very
hard to reason about what format will be used in the end and whether
it will even be a valid format.

Instead, we should verify from the start what formats are allowed
to encode the given input Header. If no formats are possible,
then we can return immediately, rejecting the Header.

For now, we continue on to the hairy logic in writeHeader, but
a future CL can split that logic up and specialize them for each
format now that we know what is possible.

Update #9683
Update #12594

Change-Id: I8406ea855dfcb8b478a03a7058ddf8b2b09d46dc
Reviewed-on: https://go-review.googlesource.com/54433
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Rather than going through writeHeader, which attempts to handle all formats,
implement writeGNUHeader, which only has an understanding of the GNU format.

Currently, the implementation is nearly identical to writeUSTARHeader, except:
* formatNumeric is used instead of formatOctal
* the GNU magic value is used

This is kept as a separate method since it makes more logical sense
when we add support for sparse files, long filenames, and atime/ctime fields,
which do not affect USTAR.

Updates #12594

Change-Id: I76efc0b39dc649efc22646dfc9867a7c165f34a8
Reviewed-on: https://go-review.googlesource.com/55237
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>

The logic for USTAR was disabled because a previous implementation of
Writer had a wrong understanding of the differences between USTAR and GNU,
causing the prefix field is incorrectly be populated in GNU files.

Now that this issue has been fixed, we can re-enable the logic for USTAR
path splitting, which allows Writer to use the USTAR for a wider range
of possible inputs.

Updates #9683
Updates #12594
Updates #17630

Change-Id: I9fe34e5df63f99c6dd56fee3a7e7e4d6ec3995c9
Reviewed-on: https://go-review.googlesource.com/55574
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>