Some x/tools tests use too much memory (virtual or real) or other
resources to run on "small" builders. Originally only linux-arm
was classified as small. This change addes plan9-arm to the list,
since it normally runs on Raspberry Pi boards with 1GB of RAM.

Partial workaround for golang/go#38772

Change-Id: I93307af10cccf7b1e26d57b9af914c85cf676d43
Reviewed-on: https://go-review.googlesource.com/c/tools/+/232478
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

TestVerifyUnified in internal/lsp/diff/difftest requires specific
behaviour of the 'diff' command which is known to be satisfied by
GNU diff. The plan9 'diff' command has no '-u' option, and the
illumos 'diff -u' produces output in a different format. Checking
specifically for the GNU version in the HasTool function ensures
the expected behaviour, and otherwise causes the test to be skipped.

Updates golang/go#38772

Change-Id: I5493fa8cfc48a112dc0b7356618c62d3ccb0366f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/232479
Reviewed-by: Ian Cottrell <iancottrell@google.com>
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

The Plan 9 runtime startup was enabling notes (like Unix signals)
before the gsignal stack was allocated. This left a small window
of time where an interrupt (eg by the parent killing a subprocess
quickly after exec) would cause a null pointer dereference in
sigtramp. This would leave the interrupted process suspended in
'broken' state instead of exiting. We've observed this on the
builders, where it can make a test time out waiting for the broken
process to terminate.

Updates #38772

Change-Id: I54584069fd3109595f06c78724c1f6419e028aab
Reviewed-on: https://go-review.googlesource.com/c/go/+/234397
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David du Colombier <0intro@gmail.com>

Windows does actually support symlinks, but older versions of
Windows only support symlinks when running as an administrator.
Newer versions of Windows support symlinks for all users.

Instead of skipping based on GOOS, first try the Symlink operation.
If it succeeds, we can proceed with the test; otherwise, we can try to
write a regular file to determine whether the problem was the symlink
operation itself or the destination path.

For golang/go#38772

Change-Id: Idaa9592011473de7f514b889859e420a84db6d01
Reviewed-on: https://go-review.googlesource.com/c/tools/+/234537
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>

In os.Getenv and os.Setenv, instead of directly reading and writing the
Plan 9 environment device (which may be shared with other processes),
use a local copy of environment variables cached at the start of
execution. This gives the same semantics for Getenv and Setenv as on
other operating systems which don't share the environment, making it
more likely that Go programs (for example the build tests) will be
portable to Plan 9.

This doesn't preclude writing non-portable Plan 9 Go programs which make
use of the shared environment semantics (for example to have a command
which exports variable definitions to the parent shell). To do this, use
  ioutil.ReadFile("/env/"+key) and
  ioutil.WriteFile("/env/"+key, value, 0666)
in place of os.Getenv(key) and os.Setenv(key, value) respectively.

Note that CL 5599054 previously added env cacheing, citing efficiency
as the reason. However it made the cache write-through, with Setenv
changing the shared environment as well as the cache (so not consistent
with Posix semantics), and Clearenv breaking the sharing of the
environment between the calling thread and other threads (leading to
unpredictable behaviour). Because of these inconsistencies (#8849),
CL 158970045 removed the cacheing again.

This CL restores cacheing but without write-through. The local cache is
initialised at start of execution, manipulated by the standard functions
in syscall/env_unix.go to ensure the same semantics, and exported only
when exec'ing a new program.

Fixes #34971
Fixes #25234
Fixes #19388
Updates #38772

Change-Id: I2dd15516d27414afaf99ea382f0e00be37a570c3
Reviewed-on: https://go-review.googlesource.com/c/go/+/236520
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Fazlul Shahriar <fshahriar@gmail.com>
Reviewed-by: David du Colombier <0intro@gmail.com>

…d links

Also make Export create all parent directories before all files.
If the files are symlinks to directories, the target directory must
exist before the symlink is created on Windows. Otherwise, the symlink
will be created with the wrong type (and thus broken).

Fixes golang/go#46503
Updates golang/go#38772
Updates golang/go#39183

Change-Id: I17864ed66e0464e0ed1f56c55751b384b8c59484
Reviewed-on: https://go-review.googlesource.com/c/tools/+/234112
Trust: Bryan C. Mills <bcmills@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Ian Cottrell <iancottrell@google.com>

…d links

Also make Export create all parent directories before all files.
If the files are symlinks to directories, the target directory must
exist before the symlink is created on Windows. Otherwise, the symlink
will be created with the wrong type (and thus broken).

Fixes golang/go#46503
Updates golang/go#38772
Updates golang/go#39183

Change-Id: I17864ed66e0464e0ed1f56c55751b384b8c59484
Reviewed-on: https://go-review.googlesource.com/c/tools/+/234112
Trust: Bryan C. Mills <bcmills@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Ian Cottrell <iancottrell@google.com>