This is one of those "how did this ever work?" bugs.
The current build failures are happening because
a fault comes up while executing on m->curg on a
system-created thread using an m obtained from needm,
but TLS is set to m->g0, not m->curg. On fault,
sigtramp starts executing, assumes r10 (g) might be
incorrect, reloads it from TLS, and gets m->g0, not
m->curg. Then sighandler dutifully pushes a call to
sigpanic onto the stack and returns to it.
We're now executing on the m->curg stack but with
g=m->g0. Sigpanic does a stack split check, sees that
the SP is not in range (50% chance depending on relative
ordering of m->g0's and m->curg's stacks), and then
calls morestack. Morestack sees that g=m->g0 and
crashes the program.

The fix is to replace every change of g in asm_arm.s
with a call to a function that both updates g and
saves the updated g to TLS.

Why did it start happening? That's unclear.
Unfortunately there were other bugs in the initial
checkin that mask exactly which of a sequence of
CLs started the behavior where sigpanic would end
up tripping the stack split.

Fixes arm build.
Fixes #8675.

LGTM=iant
R=golang-codereviews, iant
CC=dave, golang-codereviews, khr, minux, r
https://golang.org/cl/135570043

Maybe will help us understand Solaris build failure.

TBR=aram
CC=golang-codereviews
https://golang.org/cl/139290043

Fixes arm build.

TBR=khr
CC=golang-codereviews
https://golang.org/cl/132700043

The Unix make.bash builds cmd/dist from *.c.
make.bat apparently does not.

TBR=r
CC=golang-codereviews
https://golang.org/cl/137280043

After the three pending CLs listed below, there will be no more .goc files.

134580043	runtime: move stubs.goc code into runtime.c
133670043	runtime: fix windows syscalls for copying stacks
141180043	runtime: eliminate Go -> C -> block paths for Solaris

LGTM=bradfitz
R=golang-codereviews, bradfitz, dave
CC=golang-codereviews, iant, r
https://golang.org/cl/132680043

Update #8675
Fixes nacl/amd64p32 build.

TBR=khr
CC=golang-codereviews
https://golang.org/cl/141140045

Syscall and everything it calls must be nosplit:
we cannot split a stack once Syscall has been invoked,
because we don't know which of its arguments are
pointers.

LGTM=khr, r, alex.brainman
R=dvyukov, iant, khr, r, bradfitz, alex.brainman
CC=golang-codereviews
https://golang.org/cl/133670043

LGTM=aram, r
R=golang-codereviews, aram, r
CC=golang-codereviews, iant, khr
https://golang.org/cl/141180043

Increase NOSPLIT reservation from 192 to 384 bytes.
The problem is that the non-Unix systems (Solaris and Windows)
just can't make system calls in a small amount of space,
and then worse they do things that are complex enough
to warrant calling runtime.throw on failure.
We don't have time to rewrite the code to use less stack.

I'm not happy about this, but it's still a small amount.

The good news is that we're doing this to get to only
using copying stacks for stack growth. Once that is true,
we can drop the default stack size from 8k to 4k, which
should more than make up for the bytes we're losing here.

LGTM=r
R=iant, r, bradfitz, aram.h
CC=golang-codereviews
https://golang.org/cl/140350043

LGTM=bradfitz
R=adg, bradfitz, minux
CC=golang-codereviews
https://golang.org/cl/140310045

This will keep the go command from trying to build it
when the cmd/ tree is no longer a special case.

Also update doc.go to refer to the correct location.
(It was incorrect even before this CL.)

LGTM=r
R=iant, r
CC=golang-codereviews
https://golang.org/cl/134560043

This will withstand the src/pkg/ to src/ move.

LGTM=r
R=iant, r
CC=golang-codereviews
https://golang.org/cl/134530044

Now that the calling conventions are the same,
there's no danger to using plain C for these.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=dvyukov, golang-codereviews, iant, khr, r
https://golang.org/cl/134580043

This was supposed to be in CL 135490044
but got lost in a transfer from machine to machine.

TBR=khr
R=khr
CC=golang-codereviews
https://golang.org/cl/135560043

LGTM=dave, rsc
R=khr, dvyukov, dave, gobot, rsc
CC=golang-codereviews, rsc
https://golang.org/cl/133220044

The gp->panicwrap adjustment is just fatally flawed.
Now that there is a Panic.argp field, update that instead.
That can be done on entry only, so that unwinding doesn't
need to worry about undoing anything. The wrappers
emit a few more instructions in the prologue but everything
else in the system gets much simpler.

It also fixes (without trying) a broken test I never checked in.

Fixes #7491.

LGTM=khr
R=khr
CC=dvyukov, golang-codereviews, iant, r
https://golang.org/cl/135490044

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/136260043

testSchedLocal* tests need to malloc now because their
stack frames are too big to fit on the G0 stack.

LGTM=iant
R=golang-codereviews, iant, khr
CC=golang-codereviews
https://golang.org/cl/133660043

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/141150043

LGTM=rsc
R=rsc, khr
CC=golang-codereviews
https://golang.org/cl/132660043

I forgot to clear this before submitting.

TBR=khr
CC=golang-codereviews
https://golang.org/cl/132640044

newstackcall creates a new stack segment, and we want to
be able to throw away all that code.

LGTM=khr
R=khr, iant
CC=dvyukov, golang-codereviews, r
https://golang.org/cl/139270043

If there is doubt about passing arguments correctly
(as there is in this test), there should be doubt about
getting the results back intact too. Using 0 and 1
(especially 0 for success) makes it easy to get a PASS
accidentally when the return value is not actually
being propagated. Use less common values.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews, r
https://golang.org/cl/141110043

We cannot let a real panic start there, because there is C code
on the stack, and worse, there is an assembly frame with a
saved copy of the registers and we have no idea which ones
are pointers.

Instead, detect the nil ptr load/store and return out of the C
and assembly into a stub that will start the call to sigpanic.

Fixes GOARM=5 build.

LGTM=iant
R=golang-codereviews, iant
CC=dave, golang-codereviews, minux, r
https://golang.org/cl/138130043

Minor changes to make logic clearer.
Observed while working on the conversion.

LGTM=iant, dvyukov
R=dvyukov, iant
CC=golang-codereviews
https://golang.org/cl/140250043

Update #8662

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/138120043

created panic1.go just so diffs were available.
After this CL is in, I'd like to move panic.go -> defer.go
and panic1.go -> panic.go.

LGTM=rsc
R=rsc, khr
CC=golang-codereviews
https://golang.org/cl/133530045

sigprof and setcpuprofilerate coordinate the enabling/disabling
of the handler using a Mutex. This has always been a bit dodgy:
setcpuprofilerate must be careful to turn off signals before acquiring
the lock to avoid a deadlock.

Now the lock implementations use onM, and onM isn't okay on the
signal stack. We know how to make it okay, but it's more work than
is probably worth doing.

Since this is super-dodgy anyway, replace the lock with a simple
cas loop. It is only contended if setcpuprofilerate is being called,
and that doesn't happen frequently enough to care about the
raw speed or about using futexes/semaphores.

TBR to fix freebsd/amd64 and dragonfly/amd64 builds.
Happy to make changes in a follow-up CL.

TBR=dvyukov
CC=golang-codereviews
https://golang.org/cl/141080044

The general kernel system call interface
takes 6 arguments: R0, R1, R2, R3, R4, R5.

Syscall is for calls that only need 3.
The amd64 and 386 versions zero the extra arg registers,
but the arm version does not.

func utimensat calls Syscall with 3 arguments.
The kernel expects a 4th argument.
That turns out to be whatever is in R3 at the time of the call.
CL 137160043 changed various pieces of code and apparently
changed the value left in R3 at the time of utimensat's Syscall.
This causes the kernel to return EINVAL.

Change linux/arm Syscall to zero R3, R4, R5, so that calls will
behave deterministically, even if they pass too few arguments.

Arguably, utimensat could be fixed too, but the predictable
zeroing is certainly worth doing, and once done utimensat's
use of Syscall is fine.

Fixes arm build.

TBR=bradfitz
CC=golang-codereviews
https://golang.org/cl/141080043

I did this just to clean things up, but it will be important
when we drop the pkg directory later.

LGTM=bradfitz
R=r, bradfitz
CC=golang-codereviews
https://golang.org/cl/132600043

Behavior before this CL:

1. If onM is called on a g0 stack, it just calls the given function.

2. If onM is called on a gsignal stack, it calls badonm.

3. If onM is called on a curg stack, it switches to the g0 stack
and then calls the function.

In cases 1 and 2, if the program then crashes (and badonm always does),
we want to see what called onM, but the traceback stops at onM.
In case 3, the traceback must stop at onM, because the g0
stack we are renting really does stop at onM.

The current code stops the traceback at onM to handle 3,
at the cost of making 1 and 2 crash with incomplete traces.

Change traceback to scan past onM but in case 3 make it look
like on the rented g0 stack, onM was called from mstart.
The traceback already knows that mstart is a top-of-stack function.

Alternate fix at CL 132610043 but I think this one is cleaner.
This CL makes 3 the exception, while that CL makes 1 and 2 the exception.

Submitting TBR to try to get better stack traces out of the
freebsd/amd64 builder, but happy to make changes in a
followup CL.

TBR=khr
R=khr
CC=golang-codereviews
https://golang.org/cl/133620043

The old change worked fine in my client, but my client
must not have been in a completely clean state.

TBR=r
CC=golang-codereviews
https://golang.org/cl/138100043

1) cmd/dist was copying textflag.h to the build include directory,
but only after compiling package runtime. So other packages could
use it, just not runtime. Copy earlier, so that runtime can use it too.

2) We decided for android that anything marked linux is also included
in the build. The generated linux-specific files in cmd/dist must therefore
have explicit +build !android tags, or else you can't have simultaneous
linux/arm and android/arm builds in a single client. The tag was already
there for at least one file, but it was missing from many others.

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/134500043

sysAlloc is the only mem function called from Go.

LGTM=iant, khr
R=golang-codereviews, khr, 0intro, iant
CC=dvyukov, golang-codereviews, r
https://golang.org/cl/139210043

Mostly NOSPLIT additions.
Had to rewrite atomic_arm.c in Go because it calls lock,
and lock is too complex.

With this CL, I find no Go -> C calls that can split the stack
on any system except Solaris and Windows.

Solaris and Windows need more work and will be done separately.

LGTM=iant, dave
R=golang-codereviews, bradfitz, iant, dave
CC=dvyukov, golang-codereviews, khr, r
https://golang.org/cl/137160043

The -nocgo builder failed because it has cgo disabled
as well as no USER environment variable:
http://build.golang.org/log/2250abb82f5022b72a12997b8ff89fcdeff094c9

# Checking API compatibility.
Error getting current user: user: Current not implemented on linux/amd64
exit status 1

Don't require the environment variable here.

LGTM=minux
R=dave, adg, minux
CC=golang-codereviews
https://golang.org/cl/140290043

(a b string, ok bool) is not a valid signature

Fixes #8656.

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://golang.org/cl/137140043

Convert no-op race functions.
Everything else is tiny and gets NOSPLITs.

After this, all that is left on darwin is sysAlloc, panic, and gothrow (all pending).
There may be system-specific calls in other builds.

LGTM=iant
R=golang-codereviews, iant
CC=dvyukov, golang-codereviews, khr, r
https://golang.org/cl/140240044

LGTM=khr, rsc
R=golang-codereviews, khr, rsc
CC=golang-codereviews
https://golang.org/cl/131670043

When this code was written, there was no way for Go to
reuse the C function and enum values. Now there is.

LGTM=bradfitz
R=rlh, bradfitz
CC=dvyukov, golang-codereviews, iant, khr, r
https://golang.org/cl/139150045