+----------------------------------------------------------------------+
| Hello, if you are reading this and run macOS, please test this code: |
|                                                                      |
| $ GO111MODULE=on go get golang.org/dl/gotip@latest                   |
| $ gotip download                                              |
| $ GODEBUG=x509roots=1 gotip test crypto/x509 -v -run TestSystemRoots |
+----------------------------------------------------------------------+

We currently have two code paths to extract system roots on macOS: one
uses cgo to invoke a maze of Security.framework APIs; the other is a
horrible fallback that runs "/usr/bin/security verify-cert" on every
root that has custom policies to check if it's trusted for SSL.

The fallback is not only terrifying because it shells out to a binary,
but also because it lets in certificates that are not trusted roots but
are signed by trusted roots, and because it applies some filters (EKUs
and expiration) only to roots with custom policies, as the others are
not passed to verify-cert. The other code path, of course, requires cgo,
so can't be used when cross-compiling and involves a large ball of C.

It's all a mess, and it broke oh-so-many times (#14514, #16532, #19436,
 #20990, #21416, #24437, #24652, #25649, #26073, #27958, #28025, #28092,
 #29497, #30471, #30672, #30763, #30889, #32891, #38215, #38365, ...).

Since macOS does not have a stable syscall ABI, we already dynamically
link and invoke libSystem.dylib regardless of cgo availability (#17490).

How that works is that functions in package syscall (like syscall.Open)
take the address of assembly trampolines (like libc_open_trampoline)
that jump to symbols imported with cgo_import_dynamic (like libc_open),
and pass them along with arguments to syscall.syscall (which is
implemented as runtime.syscall_syscall). syscall_syscall informs the
scheduler and profiler, and then uses asmcgocall to switch to a system
stack and invoke runtime.syscall. The latter is an assembly trampoline
that unpacks the Go ABI arguments passed to syscall.syscall, finally
calls the remote function, and puts the return value on the Go stack.
(This last bit is the part that cgo compiles from a C wrapper.)

We can do something similar to link and invoke Security.framework!

The one difference is that runtime.syscall and friends check errors
based on the errno convention, which Security doesn't follow, so I added
runtime.syscallNoErr which just skips interpreting the return value.
We only need a variant with six arguments because the calling convention
is register-based, and extra arguments simply zero out some registers.

That's plumbed through as crypto/x509/internal/macOS.syscall. The rest
of that package is a set of wrappers for Security.framework and Core
Foundation functions, like syscall is for libSystem. In theory, as long
as macOS respects ABI backwards compatibility (a.k.a. as long as
binaries built for a previous OS version keep running) this should be
stable, as the final result is not different from what a C compiler
would make. (One exception might be dictionary key strings, which we
make our own copy of instead of using the dynamic symbol. If they change
the value of those strings things might break. But why would they.)

Finally, I rewrote the crypto/x509 cgo logic in Go using those wrappers.
It works! I tried to make it match 1:1 the old logic, so that
root_darwin_amd64.go can be reviewed by comparing it to
root_cgo_darwin_amd64.go. The only difference is that we do proper error
handling now, and assume that if there is no error the return values are
there, while before we'd just check for nil pointers and move on.

I kept the cgo logic to help with review and testing, but we should
delete it once we are confident the new code works.

The nocgo logic is gone and we shall never speak of it again.

Fixes #32604
Fixes #19561
Fixes #38365
Awakens Cthulhu

Change-Id: Id850962bad667f71e3af594bdfebbbb1edfbcbb4
Reviewed-on: https://go-review.googlesource.com/c/go/+/227037
Reviewed-by: Katie Hockman <katie@golang.org>