Proposal: creation of a similar SymlinkFile() func, like BasicFile() but also takes a linkname as a parameter.

If this is reasonable, should all Symlink() instances be moved over to SymlinkFile()?
https://cs.chromium.org/search/?q=isolated.Symlink%5C(+file:%5Einfra/go/src/go%5C.chromium%5C.org/luci/+package:%5Echromium$&type=cs

The problem is isolatedFile() and its calling function archive(), it needs to:

If the destination is outside the tree:
  If destination is a file:
    save the symlink destination as a file as the original symlink
  else if a directory:
    recurse in the directory as if it had been there original, instead of a symlink
else:
  all the destination as if it had been specified as an input

Please clarify what is happening on the level of the File struct.
In your algorithm above, what File objects are being made?


More visually, right now a symlink entry in an isolated in the UI
looks like
---------------------------------------------------------------
|    file      | digest | mode | size |  symlink   | filetype |
---------------------------------------------------------------
| linkname.txt |        |      |      | source.txt |          |
---------------------------------------------------------------
It has no digest, mode, or size. Should it have these fields?
Moreover, what determines whether 'linkname.txt' is downloadable?

If the destination is outside the tree:
  If destination is a file:
    File is a file
  else if a directory:
    File is not added at all, since the directory is walked into.
else:
  File is a symlink

The following revision refers to this bug:
  https://chromium.googlesource.com/infra/luci/luci-go.git/+/f0368c7871cdb4400c990eecc8774a3fbcf0e1f4

commit f0368c7871cdb4400c990eecc8774a3fbcf0e1f4
Author: Joshua Seaton <joshuaseaton@google.com>
Date: Thu Jan 10 19:45:16 2019

[isolated] Follow symbolic links in files

Bug: 917604
Test: The following was done
```
mkdir a
echo 'foo' > a/source
mkdir b
ln -s ../a/source b/link
isolated archive -I https://isolateserver.appspot.com -namespace default-gzip -files $CWD/b:link
```
At HEAD: https://isolateserver.appspot.com/browse?namespace=default-gzip&digest=dcdb41730f584e42fc101c11bd00ad7dfc5a10cd
With this change: https://isolateserver.appspot.com/browse?namespace=default-gzip&digest=f54ad05e5f19414626c74218e808edcef11088dc

Change-Id: Iad484b29e1ac9edfdb3f2cfb3c9f2adff8506ba9
Reviewed-on: https://chromium-review.googlesource.com/c/1394248
Commit-Queue: Marc-Antoine Ruel <maruel@chromium.org>
Reviewed-by: Marc-Antoine Ruel <maruel@chromium.org>

[modify] https://crrev.com/f0368c7871cdb4400c990eecc8774a3fbcf0e1f4/client/isolated/archive.go
[modify] https://crrev.com/f0368c7871cdb4400c990eecc8774a3fbcf0e1f4/client/isolated/archive_test.go

The following revision refers to this bug:
  https://chromium.googlesource.com/infra/luci/luci-go.git/+/ef5a719a5a06ca80bb59ffea625670f4397142a9

commit ef5a719a5a06ca80bb59ffea625670f4397142a9
Author: Joshua Seaton <joshuaseaton@google.com>
Date: Tue Jan 15 03:31:35 2019

[archiver] Make PushDirectory tests table-driven

This will make future modifications to PushDirectory easier to test.

Bug: 917604
Change-Id: Ia3d14209ec5caaa4cb18a5655716789f341235fd
Reviewed-on: https://chromium-review.googlesource.com/c/1404260
Commit-Queue: Joshua Seaton <joshuaseaton@google.com>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>

[modify] https://crrev.com/ef5a719a5a06ca80bb59ffea625670f4397142a9/client/archiver/directory_test.go

The following revision refers to this bug:
  https://chromium.googlesource.com/infra/luci/luci-go.git/+/5c78afc7db0efe3c70843bee7c3fd991ef29326c

commit 5c78afc7db0efe3c70843bee7c3fd991ef29326c
Author: Joshua Seaton <joshuaseaton@google.com>
Date: Tue Jan 15 03:37:45 2019

[isolated] Follow symbolic links in directories.

Bug: 917604
Test: The following was done
```
mkdir a
echo 'foo' > a/srcf
mkdir a/srcdir
echo 'bar' > a/srcdir/f
mkdir b
echo 'baz' > b/intreesrc
ln -s ../a/srcf b/linkf
ln -s ../a/srcdir b/linkdir
ln -s intreesrc b/intreelink

isolated archive -I https://isolateserver.appspot.com -namespace default-gzip -dirs $CWD:b
```
At HEAD: https://isolateserver.appspot.com/browse?namespace=default-gzip&digest=f5f3a5991eab03d049d8eda6ed49ef393c46e7ec
With this change: https://isolateserver.appspot.com/browse?namespace=default-gzip&digest=711b1dd90aa45ccb53fc5ebec86c2d37b40c914a

Change-Id: I4b11c9c079f95e43e5c5f5b80ebeaa84b5af7307
Reviewed-on: https://chromium-review.googlesource.com/c/1390262
Commit-Queue: Joshua Seaton <joshuaseaton@google.com>
Reviewed-by: Marc-Antoine Ruel <maruel@chromium.org>

[modify] https://crrev.com/5c78afc7db0efe3c70843bee7c3fd991ef29326c/client/archiver/directory.go
[modify] https://crrev.com/5c78afc7db0efe3c70843bee7c3fd991ef29326c/client/archiver/directory_test.go
[modify] https://crrev.com/5c78afc7db0efe3c70843bee7c3fd991ef29326c/client/internal/common/filesystem_view.go